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Design Guide

SAV Systems FlatStation Design Guide

www.sav-systems.com Rev: 4.0 06/2013


About SAV Systems SAV has been operating in the UK since 1988. SAV Systems is a leading provider of innovative building services solutions designed to boost energy efficiency and cut carbon emissions. Products vary from sophisticated mini-CHP systems to simple water meters, but all are purpose-developed to serve a better internal environment and a greener world. Used as individual products, or integrated into custom-designed systems, the SAV range makes a perfect partner for low energy technologies from renewables to central plant and even district heating systems. It’s a full family of choice sourced from some of the world’s leading specialist companies - our Partners in Technology. Product groups cover the full spectrum of modern building services.

www.sav-systems.com

UK CUSTOMER SUPPORT CENTRE SAV Systems, Scandia House, Boundary Road, Woking, Surrey GU21 5BX Telephone: +44 (0)1483 771910 EMAIL: info@sav-systems.com GENERAL INFORMATION: Office Hours: 9.00am - 5.00pm Monday to Friday.


SAV FLATSTATIONS - DESIGN GUIDE

Contents 1. INTRODUCTION................................................................................................................................4 2.

CLIENT BENEFITS...............................................................................................................................4

3.

THE 70/40 QUESTION.......................................................................................................................5

4.

PRODUCT OPTIONS..........................................................................................................................5

4.1. 1 Series FlatStations..........................................................................................................................6 4.2. 3 Series FlatStations..........................................................................................................................7 4.3. 5 Series FlatStations..........................................................................................................................8 4.4. 7 Series FlatStations..........................................................................................................................9 5.

VALVE OPTIONS..............................................................................................................................10

5.1. AVTB valve and sensor accelerator..................................................................................................10 5.2. TPV valve........................................................................................................................................12 6.

FLATSTATION SELECTION................................................................................................................15

7.

SYSTEM LAYOUT............................................................................................................................16

7.1. Mains cold water supply (A)............................................................................................................17 7.2. Heat source (B)...............................................................................................................................17 7.3. Primary pump (C)............................................................................................................................17 7.4. Buffer tank (D)................................................................................................................................17 7.5. Secondary pump (E)........................................................................................................................18 7.6. Differential pressure sensor (F)........................................................................................................18 7.7. Constant flow by-pass (G)...............................................................................................................18 7.8. Flushing and commissioning provisions...........................................................................................19 8.

SYSTEM SIZING...............................................................................................................................19

8.1. Module selection............................................................................................................................19 8.2. District heating pipe sizes................................................................................................................19 8.3. Sizing of heat source......................................................................................................................22 8.4. Buffer tank sizing............................................................................................................................23 8.5. Sizing example................................................................................................................................23 9. COMMISSIONING...........................................................................................................................25 9.1. Pre-commissioning checks..............................................................................................................25 9.2. Flow balancing in district heating circuits and radiator circuits.........................................................25 9.3. Capacity testing – individual FlatStations.........................................................................................26 9.4. Capacity testing - district heating system........................................................................................26 10.0 Specification...................................................................................................................................26

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SAV FLATSTATIONS - DESIGN GUIDE 1. INTRODUCTION This guide explains how to design and commission heating systems for apartment blocks and district heating schemes incorporating SAV FlatStations. These are heat interface units (HIUs) that incorporate plate heat exchangers for transferring heat from a piped distribution main to localised heating and hot water systems. SAV FlatStations significantly outperform alternative products due to their patented valve technology and energy saving features, including: • specialised valves for accurate control of hot water pressure and temperature • energy efficient idling feature for periods of zero demand • thermally insulated casings to minimise heat losses.

To maximise the energy saving benefits of the units, proper system design is essential. This guide provides recommendations for: • unit sizing • heating system layout • integration of low carbon heat sources • prediction of hot water simultaneous demands

2. CLIENT BENEFITS For client organisations, the particular benefits of SAV FlatStations are as follows: Minimal space requirement – SAV FlatStations are provided in compact, well designed, casings. They therefore take up far less room than an equivalent thermal store or an equal capacity combi-boiler. Low maintenance – Unlike combi-boilers, SAV FlatStations do not require regular servicing and maintenance. Easy to incorporate individual metering and billing services – In a sub-tenanted apartment block that requires separate metering and billing of energy used, SAV FlatStations can be integrated with intelligent heat metering that automatically monitors and records energy consumption and enables automatic billing of tenants based on energy used. (Refer to SAV Watchman service for more details). Experience shows that individual billing, based on actual consumption, also leads to behavioral changes resulting in lower consumption and reduced energy costs. Improved SAP ratings - The SAP rating achievable by using SAV FlatStations in dwellings, fed from a community heating system with low carbon heat source, will be significantly better than for systems with distributed combi-boilers or hot water cylinders. This will also help to achieve target ratings under the Code for Sustainable Homes. Ease of integrating with low or zero carbon technologies – The centralisation of the heat source, and inclusion of a thermal store, makes it easier to incorporate a low or zero carbon technology such as combined heat and power (CHP), solar heating or biomass boilers. Improved efficiency of heat sources – SAV FlatStations enable low heating return water temperatures which are crucial to maximising the energy efficiency of heat sources such as combined heat and power, or gas fired condensing boilers. It is recommended that the return water temperature from a community heating scheme should not exceed 25˚C for hot water systems and 40˚C for radiator systems. During hot water generation, SAV FlatStations typically return heating water around 20˚C easily complying with the Part L recommendations.

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SAV FLATSTATIONS - DESIGN GUIDE Minimised risk of legionella – Because there is no stored hot water there is no risk of legionella bacteria multiplying in the system. Thermostatic temperature control – SAV FlatStations provide thermostatic control of hot water temperature at varying inlet pressures. Hence, hot water can be supplied at whatever temperature setting is required, and this temperature is unaffected by the subsequent opening or closing of additional taps off the same system. Low heat losses from casings – SAV FlatStations can be provided in thermally insulated casings in order to minimise the uncontrolled loss of heat. This ensures that all of the heat delivered to each apartment is used for useful heat production and that there are no uncontrolled heat emissions during summer months. Lower energy bills – Correct system design and high efficiency FlatStations ensures energy bills are kept to a minimum.

3. THE 70/40 QUESTION District heating or centralised systems utilising low carbon heat sources (such as condensing boilers, CHP units, heat pumps, etc.) should be designed to achieve the lowest possible return temperatures. This is so that heat source outputs and efficiencies are maximised. As a general rule, the aim should be to design on the basis of heating flow and return temperatures of 70°C and 40°C respectively. An essential element in achieving such a low return temperature is to incorporate a properly controlled heat interface unit for production of hot water. Heat interface units incorporate plate heat exchangers to heat hot water instantaneously resulting in heating water return temperatures in the range 15-30°C. This enables compliance with the latest industry guidance for district heating systems. For example, Greater London Authority’s “District Heating Manual for London” (2013) specifically requires that primary return temperatures from hot water generating heat interface units do not exceed 25°C. Similarly, CIBSE’s AM12/2013 “Combined heat and power for buildings” section 9.16, Design of district heating states: “It is recommended that, for new systems, radiator circuit temperatures of 70°C (flow) and 40°C (return) are used with a maximum return temperature of 25°C from instantaneous domestic hot water heat exchangers.” These requirements make hot water cylinders unacceptable since, due to the legionella risk associated with hot water storage, return temperatures have to be kept above 60°C. The only feasible solution is the utilisation of heat interface units incorporating accurate temperature and pressure control valves. Accurate control of the heat transfer across the plate heat exchanger is the key to achieving uninterrupted hot water supplies and consistently low heating return temperatures.

4. PRODUCT OPTIONS SAV FlatStations can be connected from a heating distribution system to provide localised heating, domestic hot water or both. The alternative product options vary depending on their functions, valve types and insulation:

Type

Functions

1 Series BS DHW only 3 Series BS Indirect heating 5 Series BS DHW + direct heating 7 Series BS DHW + indirect heating 1 Series DS DHW only 5 Series DS DHW + direct heating 7 Series DS DHW + indirect heating *See section 5 for valve details

DHW Valve type* AVTB AVTB AVTB TPV TPV TPV 5

Insulated casing No No No No Yes Yes Yes


SAV FLATSTATIONS - DESIGN GUIDE 4.1. 1 Series FlatStations

1 Series FlatStations provide instantaneous hot water only. When there is a demand for hot water from taps, water from the heating system distribution mains is automatically introduced to the plate heat exchanger thereby heating incoming mains cold water passing through on the other side. The temperature of the hot water produced is thermostatically controlled and can be set by the user. During hot water draw-off, the water returned to the heating system is typically at or below 20째C. Operating limits Nominal pressure (bar) Max heating supply temp Tmax (째C) Min DCW pressure Pmin (bar) Max DHW capacity (kW)

1 Series BS PN16 120 0.5 90

DHW

1 Series DS PN16 120 1.5 59

DH supply

B Water heater 9 Strainer 72 IHPT/TPV

B 72

CWM

9

Technical parameters:

DH return

Connections:

6

Connections sizes:


SAV FLATSTATIONS - DESIGN GUIDE 4.2. 3 Series FlatStations

3 Series FlatStations provide heated water for the supply of heating only with the possibility of the addition of a cylinder feed. Water from the primary heating system distribution mains is introduced to the plate heat exchanger thereby heating water for distribution to a secondary heating system. The primary (district heating) side of the FlatStation incorporates a strainer, differential pressure control valve and spacer piece for optional heat meter. The secondary side of the heat exchanger incorporates safety valve, expansion vessel and circulating pump. The unit can incorporate two secondary circuit feeds each with their own zone valve. These can be used to feed separate hot water cylinder and radiator circuits. This type of FlatStation is therefore ideal for connecting traditional cylinder/radiator systems to district heating mains. The 3 Series BS includes a cylinder feed, whereas the 3 Series BS-B provides a hydraulic break for heating alone. Operating limits Nominal pressure (bar) Heating supply temp Tmax (°C) Heating capacity (kW)

Series 3 BS-B PN10 120 40

3 Series BS PN10 120 25

Circuit diagram - example A M 2 4 7 9

Heat exchanger Electrical wiring box Single check valve Safety valve Thermostatic valve Strainer

10 14 18 20 21 24

Circulation pump Sensor pocket, energy meter Thermometer Filling/drain valve To be ordered separately Delivered loose with unit

26 31 38 41 48 69

Manometer Differential pressure controller Expansion tank Fitting piece, energy meter Air escape, manual On/off valve Cylinder FL

DHF

HFL Cylinder RL

DHR

HRL

Border of delivery Technical parameters: Nominal pressure:

7

PN 10*

Connections sizes: DH + HE:

G ¾” (int. thread)


SAV FLATSTATIONS - DESIGN GUIDE 4.3. 5 Series FlatStations

5 Series FlatStations provide instantaneous hot water and direct fed heating. The hot water generation is achieved in the same way as for 1 Series FlatStations (see section 4.1 above). Heating water is also fed directly into a heating circuit piped from the FlatStation. The primary (district heating) connection to the FlatStation incorporates a strainer, ball valves, and spacer piece for optional heat meter. An adjustable differential pressure control valve enables the pressure across the connected heating system to be set and controlled. An actuated zone valve is also provided so that the heating circuit can be time clock and thermostatically controlled. Operating limits Nominal pressure (bar) Max heating supply temp Tmax (°C) Min DCW pressure Pmin (bar) Max DHW capacity (kW) Max primary pressure differential (bar)

5 Series BS PN10 120 0.5 90 4.5

5 Series DS PN10 120 1.0 59 4.5

Circuit diagram - example Mounting rail DHW Option

B M 2A 2C 9 14 21 31 41 69 72

HWC

DCW

Heat exchanger Electrical wiring box Single check valve, WRAS Single check valve incl. circulation pipe Strainer Sensor pocket, energy meter To be ordered separately Differential pressure controller Fitting piece, energy meter ¾” x 110 mm On/off valve TPV

DH supply

HE supply

DH return

HE return

8

Technical parameters: Nominal pressure:

PN 10

Connections: 1 District heating (DH) supply

Connections sizes: DH + HE:

G ¾” (int. thread)


SAV FLATSTATIONS - DESIGN GUIDE 4.4. 7 Series FlatStations

7 Series FlatStations provide instantaneous hot water and indirect fed heating. The hot water generation is achieved in the same way as for 1 Series FlatStations (see section 4.1 above). Heating is also indirectly supplied to a heating circuit piped from the station i.e. by means of an additional plate heat exchanger. The primary (district heating) side of the FlatStation incorporates a strainer, differential pressure control valve, thermostatic control valve and spacer piece for optional heat meter. The secondary circuit feeding to the heating system incorporates a safety valve, expansion vessel and circulating pump. The plate heat exchanger can be dimensioned to suit either radiators or underfloor heating flow temperatures. Operating limits Nominal pressure (bar) Max heating supply temp Tmax (°C) Min DCW pressure Pmin (bar) Max DHW capacity (kW) Max heating capacity (kW) Max primary pressure differential (bar)

7 Series BS PN10 120 0.5 90 40 4.5

7 Series DS PN10 120 1.0 59 35 4.5

Circuit diagram - example Mounting rail Option

DH supply

7 Thermostatic valve 9 Strainer 10 Circulation pump 14 Sensor pocket, heat meter 20 Filling/drain valve 21 To be ordered separately 24 Delivered loose with unit 26 Manometer 31 Differential pressure controller 38 Expansion vessel 41 Fitting piece, energy meter 48 Air escape, manual 69 On/off valve 72 TPV valve HE supply

DH return

HE return

A B M 2A 2B 2C

DHW

Circ.

4

DCW

Technical parameters: Nominal pressure: PN 10*

9 Connections: 1 District heating (DH) supply

Plate heat exchanger HE Plate heat exchanger DHW Electrical wiring box Single check valve, WRAS Double check valve, WRAS Single check valve incl. circulation pipe Safety valve

Connections sizes: All connections:

G ¾” (int. thread)


SAV FLATSTATIONS - DESIGN GUIDE 5. VALVE OPTIONS A unique feature of SAV FlatStations is the inclusion of patented Danfoss thermostatic and pressure control valves. These specially designed valves provide the following significant performance benefits relative to alternative products: • faster response time • more accurate control of hot water flow temperature • reduced heat losses during idle periods.

5.1. AVTB valve and sensor accelerator

The AVTB valve, integral to the BS range, is incorporated on the return side of the district/community heating supply. The valve varies the heating water flow rate in response to changes in the hot water supply temperature, as detected by a sensor (which forms part of the sensor accelerator) in the hot water supply connection. When hot water taps are opened, cold water is initially drawn through the heat exchanger into the sensor accelerator. The temperature of the water is detected by the sensor and transmitted to the AVTB valve causing it to open until the set hot water flow temperature is achieved. The AVTB’s ability to modulate flow results in accurate control of hot water flow temperature.

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SAV FLATSTATIONS - DESIGN GUIDE The below stress test results show just how accurate the SAV FlatStation is controlling the DHW temperature for end-user comfort and the primary return temperature for system and central plant efficiency. The bottom graph shows fluctuations in DHW demand (changes in flow rate). The top graph shows primary flow (brown), primary return (green), cold water mains (blue) and DHW (red). It can clearly be seen that even at sudden demand changes the DHW temperature is kept constant providing optimal comfort for the end user. Also, the green line shows how the primary return temperature is kept low, ensuring the highest efficiency and the best conditions for the central plant. The primary return temperatures are well below the maximum value of 25 °C required by GLA’s District Heating Manual for London (2013) and CIBSE AM12:2013.

AVTB Performance documentation for SAV FlatStation BS range (AVTB valve): District heating supply:

70°C

District heating supply:

70°C

Differential pressure:

0.5 bar

Differential pressure:

1.0 bar

°C 110 100 90 80 70 60 50 40 30 20 10 0 0

°C 110 100 90 80 70 60 50 40 30 20 10 0

Temperature

100

200

300

400 DHF

l/h 1000 900 800 700 600 500 400 300 200 100 0 0

0

DHR DHW CW

Time [sec]

200

100

200

Time [sec]

400

400

0

DHW

100

200

300

Time [sec]

Pri. flow = 70°C Dp = 0.5 bar

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DHF DHR DHW CW

500

Flow

l/h 1000 900 800 700 600 500 400 300 200 100 0 300

300

Time [sec]

Flow

100

Temperature

400

500 DHW


SAV FLATSTATIONS - DESIGN GUIDE 5.2. TPV valve The new TPV valve, integral to the DS range, achieves highly accurate temperature and pressure control from a single valve.

2

3

4

TPV VALVE

1 The valve actually consists of four separate components all housed in a single valve body: (1) The pilot valve (2) Thermostatic control valve with remote sensor (3) Differential pressure controller (4) Thermostat with sensor.

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SAV FLATSTATIONS - DESIGN GUIDE A cross section of the valve is shown below.

Operation: When a hot water outlet is opened, the pressure in the cold water supply pipe reduces. This reduction in pressure causes the pilot valve (1) (located in the cold feed pipe) to open to allow flow. The pilot valve is mechanically linked to a thermostatic control valve (2) located in the district heating supply pipe to the plate heat exchanger. As a result, the action of the pilot valve opening causes the thermostatic control valve to open as well allowing heating water into the other side of the plate heat exchanger. The degree of opening of the thermostatic control valve is governable by a thermostat and remote sensor (4) which measures the temperature in the hot water supply pipe from the plate heat exchanger. This allows the hot water supply temperature to be set at the required value. An additional differential pressure control valve located in the district heating supply pipe maintains a constant differential pressure across the thermostatic control valve. This ensures accurate control of flow through the thermostatic control valve regardless of pressure changes at the pump or in other circuits. When the DHW tap is closed, the pilot valve simultaneously closes the district heating flow.

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SAV FLATSTATIONS - DESIGN GUIDE As with the AVTB valve for the BS range, the TPV valve for the DS range is a made-for-purpose DHW control valve designed to keep the FlatStation primed to deliver and maintain an accurate set DHW temperature at the taps and low system return temperatures for optimal efficiency. Again, it can be seen how the primary return temperature is kept well below the maximum value of 25 °C required by GLA’s District Heating Manual for London (2013) and CIBSE AM12:2013.

Performance documentation for SAV FlatStation DS range (TPV valve): District heating supply:

70°C

District heating supply:

70°C

Differential pressure:

0.5 bar

Differential pressure:

1.0 bar

°C 110 100 90 80 70 60 50 40 30 20 10 0

°C 110 100 90 80 70 60 50 40 30 20 10 0

Temperature

0

100

200

300

400

Time [sec]

DHF DHR DHW CW

0

500

0

100

200

300

Time [sec]

100

300

400

0

500 DHW

100

200

300

Time [sec]

14

DHF DHR DHW CW

500

Flow

l/h 1000 900 800 700 600 500 400 300 200 100 0 400

200

Time [sec]

Flow

l/h 1000 900 800 700 600 500 400 300 200 100 0

Temperature

400 DHW

500


SAV FLATSTATIONS - DESIGN GUIDE 6. FLATSTATION SELECTION The appropriate SAV FlatStation is the one that can meet the anticipated kW heating and hot water requirements for the project at the design district heating circuit operating temperatures. Each FlatStation has a specific product brochure which gives sizing examples based on different district heating operating temperatures.

Heating demand The estimation of heat losses and consequent heating loads for dwellings is explained in BS EN 12831:2003 Heating Systems in Buildings – Method for calculation of the design heat load. Similar advice is provided in the CIBSE Domestic Heating Design Guide. Peak heating demands should be calculated based on this guidance.

Hot water demand The peak simultaneous demand from the hot water outlets to be served by the FlatStation must be estimated. For many system designers BS 6700 used to be the usual source of simultaneous demand values for multiple fittings. However, in 2012 BS 6700 was superseded by BS 8558 together with BS EN 806 part 1-5 as the new standard for DHW systems. The new standard states “the designer is free to use a nationally approved detailed calculation method for pipe sizing”, such as the Danish Standard DS 439, which is also the standard recommended in CIBSE AM12:2013. Section 8 “System sizing” of this Design Guide will show how to apply the Danish Standard DS 439.

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SAV FLATSTATIONS - DESIGN GUIDE 7. SYSTEM LAYOUT A typical layout for a complete heating and hot water system incorporating SAV FlatStations is shown below. Valves have been omitted for clarity. The following sections 7.1 to 7.7 describe the main design issues which need to be considered to ensure effective and efficient performance of both heating and hot water supplies.

Further advice on the design of variable flow systems with low carbon heat sources is provided in BSRIA Guide BS 12/2011 Energy Efficient Pumping Systems.

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SAV FLATSTATIONS - DESIGN GUIDE 7.1. Mains cold water supply (A) A minimum mains cold water supply of at least 0.5 bar is required for BS series FlatStations whereas a minimum pressure of 1.5 bar is required for the DS range of FlatStations. However, the actual pressure provided should also depend on the requirements of the hot water outlets fittings which may require higher pressures to operate correctly. In tall buildings the required cold water pressure will typically be achieved by provision of a boosted main with pressure reducing valves set to the required pressure on each floor branch.

7.2. Heat source (B) To minimise carbon dioxide emissions, heat should ideally be provided from a low carbon heat source such as a combined heat and power (CHP) unit. The low carbon heat source should be configured such that it is the lead heat source and is given priority over any secondary or back-up heat source. Back-up boilers should only operate if the temperature of the water leaving the buffer tank falls below the district heating set point flow temperature. Low carbon heat sources such as CHP units are more efficient when operated with low return water temperatures. This condition can be achieved by proper dimensioning of the heat emitters fed from the FlatStation to achieve design temperature drops in the range 20-30˚C. Furthermore, the draw-off of hot water via the FlatStations causes a large drop in the heating system return temperature (typically to a value in the range 15-30°C). This further reduces the temperature of water returning to the heat source. When selecting the FlatStation, the intended heat source and primary flow/return operating temperatures must be taken into account. SAV FlatStations will work with heating water flow temperatures in the range 60-90˚C. However, the lower the heating flow temperature, the lower the heating and hot water capacity of the FlatStation. The individual datasheets for each product show typical heating and hot water capacities depending on district heating circuit flow and return temperatures.

7.3. Primary pump (C) A dedicated constant flow primary pump can be used to circulate water between the lead heat source and buffer tank. Similarly, shunt pumps can be used. The shunt pumps should usually operate at constant speed to ensure that the boilers always have sufficient water flow when in operation, and are therefore not at risk of over-heating. The flow rate in the primary circuit must not be less than the total flow in all secondary circuits fed from the buffer tank.

7.4. Buffer tank (D) Suitably sized buffer tanks can provide a thermal store of water enabling low carbon heat sources to run for longer periods. The buffer tank is also important to the operation of the system since it serves as an energy store, providing for short term, high load demands for hot water. Without this store, the heat source may be unable to react with sufficient speed to the heating load imposed by high temporary hot water demands.

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SAV FLATSTATIONS - DESIGN GUIDE To minimise return water temperatures and therefore maximise heat source efficiency, the buffer tank should be dimensioned such that temperature stratification is encouraged. Hence, an elongated vertical cylinder is appropriate with primary and secondary flow pipes located near the top of the tank, whilst primary and secondary return pipes are located near the bottom. The operation of the lead heat source should be controlled so as to maintain the required heating water flow temperature at a point two thirds of the way down the height of the buffer tank. This creates an adequate store of heating water for high demand periods, whilst allowing space for cooler return water at the base of the tank. Since the buffer tank does not contain hot water but merely the heating water that will be used to heat the hot water, there is no need to worry about anti-stratification measures. The cooler area at the base of the tank can therefore be used to introduce water heated from renewable sources such as heat pumps or solar energy.

7.5. Secondary pump (E) The secondary pump should be variable speed to take advantage of pump energy savings when the heating system is operating at part load. Pump speed should be controlled such that there is always sufficient pressure available to satisfy the most remote SAV FlatStation. The heating side pressure loss value for each SAV FlatStation is provided in the appropriate product brochure.

7.6. Differential pressure sensor (F) A differential pressure sensor installed across the most remote SAV FlatStation will minimise pump energy consumption. Pump speed should be controlled to maintain the required minimum pressure differential across the most remote FlatStation.

7.7. Constant flow by-pass (G) A constant flow by-pass located at the top of the heating system riser will provide a path for flow under minimum load conditions i.e. when all radiator control valves are closed and there is no demand for hot water. Although SAV FlatStations have in-built “idling� by-passes which are open when there is no demand for hot water, this flow may be a very small percentage of the pump maximum design flow rate. Hence, an additional constant flow by-pass set to pass 5-10% of the maximum flow rate should be provided. The by-pass flow should be as close as possible to the minimum limit advised by the pump manufacturer. By locating the by-pass close to the differential pressure sensor, the pressure differential across the by-pass, and hence the flow rate through it, will remain roughly constant under all operating conditions.

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SAV FLATSTATIONS - DESIGN GUIDE 7.8. Flushing and commissioning provisions The features shown are as recommended in BSRIA Application Guide AG 1/2001.1 Pre-commission Cleaning of Pipework Systems. Following the principles set out in the BSRIA Guide AG 1/2001.1 Pre-commission Cleaning of Pipework Systems, each type of water heater should be treated as a terminal unit fed from the main heating system pipework. In accordance with the guide, SAV provide all FlatStations with their own flushing by-pass and flushing drain cock. The flushing bypass will enable the main system pipework to be flushed and cleaned whilst the FlatStation remains isolated. The drain cock will enable the FlatStation to be flushed if required. It is possible that some debris could be carried into the hot water side of the plate heat exchanger with the incoming mains cold water supply. It may therefore also be prudent to install a strainer on the mains cold water supply to the heat exchanger in areas where water is known to contain some level of suspended solids. Pressure test points are required to facilitate commissioning. Each module requires a minimum pressure differential across it in order to function correctly. Pressure tappings across the primary heating circuit flow and return pipes will enable the available pressure to be measured and confirmed as adequate.

8. SYSTEM SIZING 8.1. Module selection As discussed in section 6, individual FlatStations must be selected based on an assessment of the maximum heating demand and maximum simultaneous hot water demand for each dwelling under consideration. It is also necessary to know the intended operating temperatures for the district heating system.

8.2. District heating pipe sizes Each district heating system pipe must be sized to accommodate the maximum heating and hot water demands of the FlatStations served by that pipe. The maximum heating demand is relatively predictable, this being the summation of the calculated heating loads for each of the dwellings served. However, the estimation of maximum hot water demand is less obvious. It is extremely unlikely that all of the hot water taps in all of the dwellings served will be open simultaneously. Therefore, some allowance for the diversity in usage is required.

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SAV FLATSTATIONS - DESIGN GUIDE The degree of diversity for multiple dwellings is expressed as a “coincidence factor� and is defined as:

(1) Where F = coincidence factor DFR = design flow rate for downstream hot water outlets (l/s) MFR = maximum possible flow rate for downstream hot water outlets (l/s)

The diversity factors recommended for sizing supplies to multiple dwellings by CIBSE AM12:2013 are shown in the below graph.

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SAV FLATSTATIONS - DESIGN GUIDE

Coincidence factor  for  DHW   1   0.9   0.8  

Coincidence factor  

0.7 0.6  

Danish Standard  DS  439   0.5   0.4   0.3   0.2   0.1   0   1  

11

21

31

41

51

61

71

81

91

101

111

121

131

141

151

161

171

181

191

Number of  dwellings  

NOTE: The Danish Standard DS 439 calculation method has been approved in BS 8558/BS EN 806-3 stating “the designer is free to use a nationally approved detailed calculation method for pipe sizing”. Furthermore CIBSE AM12:2013 states: “Experience from continental schemes indicates that the BS 6700 (BSI, 2009a) factors are too conservative and Danish Standard DS 439: 2009 (Dansk Standard, 2009) diversity factors are recommended for sizing supplies to multiple dwellings.”

Calculation of flow rates for pipe sizing Using the appropriate coincidence factors estimated from the above graph, the maximum design flow rate for each section of heating pipe can be determined. The flow rate for each pipe must be capable of delivering the peak heating demand for the dwellings served by the pipe, plus the peak simultaneous hot water heating demand for those dwellings. The overall design flow rate for each section of pipe will be:

QT = (FQDHW)+(QHTG)

(2)

Where, QT = total design flow rate in district heating pipe (l/s) F = coincidence factor QDHW = heating water flow rate required to meet peak domestic hot water demand (l/s) QHTG = heating water flow rate required to meet peak heating demand (l/s).

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SAV FLATSTATIONS - DESIGN GUIDE The value QDHW can be calculated from the equation:

QDHW =

PDHW 4.2 x ΔTDH

(3)

Where, PDHW = power requirement (kW) for all downstream FlatStation hot water heaters TDH = design temperature drop across the district heating side of the heat exchanger during hot water production (typically around 60˚C, i.e. 80˚C in, 20˚C out).The value QHTG can be calculated from the equation:

QHTG =

PHTG 4.2 x ΔTHTG

(4)

Where, PHTG = power requirement (kW) for all downstream apartments (typically 3-10kW each) THTG = design temperature drop across the district heating system (typically 10-30˚C).

8.3. Sizing of heat source There is no necessity for the power output of the central heat source to match the calculated peak heating and hot water demand from the district heating system. This is because the peak demand should only occur for relatively short periods, this being when all heating systems are on and there is peak hot water draw-off. This condition is unlikely to be sustained for a prolonged period. On this basis, two factors enable the heat source capacity to be reduced: • When there is a draw-off of hot water, each FlatStation prioritises the hot water circuit, temporarily reducing the flow of water to the heating circuit. Since hot water demand periods are relatively short, this does not affect internal temperatures. • A central buffer tank (see sections 7.4 and 8.4) provides a thermal store to enable the system to cope with large but short term hot water demands. The store empties during peak demand and then re-fills when the demand has passed.

Hence, the heat source power capacity can be sized such that it is sufficient to cope with the entire heating load (PHTG), plus an additional allowance sufficient to re-heat the volume of water in the buffer tank within 1 hour (PBUFFER).

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SAV FLATSTATIONS - DESIGN GUIDE The power required to heat the contents of the buffer tank within 1 hour can be calculated from the equation:

PBUFFER =

V x 4.2 x ΔTHTG 3600

Where, V = heated (and hence useful) volume of buffer tank

8.4. Buffer tank sizing The buffer tank should be sized to deal with the anticipated peak heating and hot water demand sustained over a notional period of 10 minutes (i.e. 600 seconds). Assuming that the boiler is controlled to maintain the required heating flow temperature at a point two thirds of the way down the tank, then the tank will need to accommodate 900 seconds of flow. Hence the equation for sizing the tank is as follows:

V = 900FQDHW Where, V = tank volume (litres).

8.5. Sizing example System design for a development comprising 40 identical 2 bedroom apartments, each with the following appliances requiring hot water: basin, sink, shower, washing machine, and each with a heating load of 5kW. 7 Series FlatStations are required and are to be fed from a district heating system with design temperatures of 70°C flow and 40°C return.

Solution FlatStation selection: Feeding one bath and one shower at the same time requires a mass flow rate of ~0.37 kg/s of water at 50C, using a design temperature differential of 40K (10C – 50C). The power required to heat this volume of water to the required flow temperature (i.e. QDHW) is ~60kW per FlatStation. Therefore this unit should just meet the requirements of each apartment.

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SAV FLATSTATIONS - DESIGN GUIDE Pipe sizing Each pipe must be sized individually. For the main pipe from the secondary pumps serving all 40 apartments the procedure is as follows: From the graph in section 8.2 above (accepting the DS 439 values) the coincidence factor F can be determined as 0.12. From equation (2) the total flow rate for the pipe can be calculated as: QT = (0.12 x QDHW ) + (QHTG)

Given a design temperature drop of 49°C (i.e. from 70˚C to 21˚C) across the heating water side of the heat exchanger, QDHW = (60kW x 40 apartments) / (4.2 x 49˚C) = 11.66 l/s

Assuming a 20˚C temperature drop (i.e. from 80˚C to 60˚C) across the heating system, QHTG = (5kW x 40 apartments) / (4.2 x 20˚C) = 2.38 l/s

Hence, QT = (0.12 x 11.66) + (2.38) = 3.78 l/s

Buffer tank The buffer tank volume will be: 900 x 0.12 x 11.66 = 1259 litres

Heat source sizing The heat source must be sized to meet the peak heating load plus sufficient power to re-heat the hot water buffer contents in 1 hour, i.e. (40 apartments x 5kW) + (1259 x 4.2 x 40)/3600 = 259kW

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SAV FLATSTATIONS - DESIGN GUIDE 9. COMMISSIONING 9.1. Pre-commissioning checks Before system commissioning commences an inspection should be undertaken to ensure that: • the pipework installation is complete, and all components are correctly positioned, correctly installed, easily accessible, properly identified (NB: refer to CIBSE Code W: 2010 Water distribution systems for more comprehensive and detailed installation check-lists) • the system has been filled, thoroughly vented and pressure tested in accordance with HVCA TM6 Pressure Testing of Pipework. • the system has been flushed and chemically cleaned in accordance with BSRIA Guide BG29/2011 Pre-commission Cleaning of Water Systems. • the pumps and associated variable speed drives are installed, inspected and tested in accordance with the manufacturer’s instructions and are ready to operate. • a closed head pump test has been carried out on each pump and the results plotted on the manufacturer’s pump performance graph.

9.2. Flow balancing in district heating circuits and radiator circuits As described in section 5 of this guide, each FlatStation serving either a direct or indirect fed heating system, is fitted with its own differential pressure control valve (DPCV). These DPCVs are pre-set to maintain a fixed pressure differential across the heating circuit (if direct fed) or the plate heat exchanger (if indirect fed). The secondary pumps should be controlled to vary their speed such that the pressure differential across the most remote FlatStation is maintained at a value that is sufficient to operate the DPCV in that FlatStation. (Hence, the recommendation to locate a differential pressure sensor close to the most remote FlatStation for pump speed control, as explained in section 7.6.) If there is sufficient pressure differential across the most remote FlatStation, then all other DPCVs located in FlatStations closer to the pump will also be satisfied. The DPCVs will then effectively balance the flows throughout the system, allowing sufficient flow to pass through when radiators are calling for heat, but closing when room temperatures are satisfied and thermostatic radiator valves begin to close. There will be no need to proportionally balance the flows in the district heating system branches feeding to the FlatStations. The only balancing required will be between radiator branches in the heating circuits fed from each FlatStation. The flows between radiators will need to be balanced by means of a “temperature balance” whereby the lockshield valves are regulated until the return temperature from each radiator is at approximately the same temperature. The only other item that requires flow balancing is the constant flow by-pass (see section 7.7). This can be set by adjusting its flow rate to a value in the range 5-10% of the maximum load flow rate, as recommended by the pump supplier.

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SAV FLATSTATIONS - DESIGN GUIDE 9.3. Capacity testing – individual FlatStations Having established adequate temperature, flow and pressure conditions in the main heating system, the hot water outputs from individual FlatStations can be adjusted and tested as required: 1.

Set the pressure reducing valves on the boosted mains water supply branches to the required value for each apartment (i.e. typically 3bar minimum such that there is sufficient pressure available to satisfy the FlatStation and downstream hot water outlets).

2. In each apartment (or selected representative apartments) open sufficient tap outlets to simulate the anticipated peak simultaneous hot water demand for the dwelling. 3. Measure the hot water temperature from the tap outlets to ensure that the temperature obtained is within expected limits, and that flow rate (and hence pressure) is adequate.

9.4. Capacity testing - district heating system In setting up the pump, it should be possible to establish maximum and minimum load operating conditions for the pump. This test should demonstrate a significant reduction in pump speed at minimum load conditions. With the system operating at its design temperature, the procedure for carrying out these tests is as follows: 1.  Ensure that all radiator circuits are set to full flow i.e. all zone control valves, and radiator valves are fully open. 2.  Open a sufficient number of tap outlets, starting with the most remote outlets and working back towards the pump, until the measured flow rate through the pump is equal to the calculated maximum load flow rate for the system (as calculated following the guidance in section 8.2). 3.  Measure the differential pressure being generated by the pump by reference to inlet and outlet pressure gauges. Confirm and record the total flow rate leaving the pump using the flow measurement device installed on the secondary circuit main return pipe. 4.  Record how long it takes to empty the buffer tank at this condition. This should be a minimum of 10 minutes. 5.  Next, close all tap outlets. Override the controls to force all 2 port heating zone control valves into their fully closed positions. 6.  Measure the differential pressure being generated by the pump as before, and re-measure the total flow rate leaving the pump. If the pump is being controlled properly, the pump pressure value should be close to the controlled value at the differential pressure sensor. Furthermore the flow rate should be close to the flow rate passing through the by-pass at the top of the riser.

10.0. Specification Please contact SAV Systems for further information and specification templates.

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SAV FLATSTATIONS - DESIGN GUIDE NOTES

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Helping Building Services Engineers Apply Low Carbon Technologies

SAV Systems’ technical team can help specifiers to successfully integrate modules into their projects. Specially prepared design guides are also available. SAV Systems modules are delivered pre-packaged and pre-tested to integrate easily into modern, fast-track construction programmes.

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UK CUSTOMER SUPPORT CENTRE SAV Systems, Scandia House, Boundary Road, Woking, Surrey GU21 5BX Telephone: +44 (0)1483 771910 EMAIL: info@sav-systems.com GENERAL INFORMATION: Office Hours: 9.00am - 5.00pm Monday to Friday.

Sav flatstation design guide rev july 2013  
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