Technical

Swimming pool boilers

In this article, Gas Safe Register’s Technical Team look at the different methods used to heat privately owned swimming pools, and the considerations that must be taken into account.

Swimming pools may be located in gardens or in purpose-built enclosures in domestic dwellings and can be above-ground or in the ground with a liner, usually made of fibreglass or concrete.

Where the pool is indoors, consideration may be given to providing space heating for all year-round operation, and it may also be necessary to provide adequate ventilation and extraction to maintain comfort.

Pools may be heated directly or indirectly with a water-towater calorifier or heat exchanger.

Direct method

Direct systems are the simplest way to provide heat. However, consideration should be given to the control of both the water flow rate and temperature, to avoid corrosion resulting from too high a temperature, and flue condensation when the temperature is too low. It may be necessary to install thermostatic mixing valves and bypass valves to maintain pool temperatures and to prevent condensation, corrosion and scale formation.

Specially designed swimming pool boilers may also include provision for space heating and domestic hot water. Depending on the manufacturer, they may be installed internally, or externally without the need for additional weatherproofing.

Indirect method

With the application of a water-to-water heat exchanger (calorifier) or indirect heat exchanger, the majority of hot water boilers may be used, including domestic central heating appliances. Consideration should be given to the heating demand: at certain times of year, if the system is also used to provide space heating, the appliances(s) should be adequate to cope with the combined heating role.

Sizing

The size of pool, the type of heating, the heat loss from the surface, sides and bottom of the pool, the climate and whether the pool is sited internally or externally must all be considered when calculating the size of boiler required. Some of the factors that should be taken into consideration when calculating the heat loss of the pool are: • The volume, depth, and surface area of the swimming pool • The required water temperature • Average air temperature during the coldest month • Total hours per day that the swimming pool will be in operation • Heat-up time required to heat the pool during the coldest month of use • Whether the swimming pool is covered and for how long • Whether the pool is above or below ground • Any effect caused by the water table, ie, static or moving • The degree of exposure, ie, prevailing wind direction.

Installation and design

Due to the different characteristics of appliances used for swimming pools compared with boilers installed in domestic properties, the HWB1 ACS qualification is required for gas engineers to work on swimming pool boilers. The qualification covers competence relating to the installation, exchange, disconnection, service, repair, breakdown and commissioning of hot water boilers with heat inputs from 15kW to 140kW, provided they have been fitted with atmospheric burners.

However, guidance may differ from specific manufacturers regarding the qualifications required for their appliances, so always check with the manufacturer that you are qualified to work on their product.

Note: Due to the type of environment and appliances used to heat water for a swimming pool, the installation and maintenance of this equipment is specialist work and the initial design must consider more factors than simply heating large quantities of water, including hygiene requirements, filtration and water chemistry. ■

Chemicals

An additional factor to consider is the presence of chlorine and bromide that are frequently added to kill germs and bacteria in the water. Chlorine and bromide corrode metals, including copper, iron and stainless steel that are found in heat exchangers and circuits. Metal corrosion from chlorine and bromine also increases with temperature.

These additives are either added into the body of the pool or injected into the heating circuit, where they should be added downstream of the heat exchanger to ensure that concentrations do not cause it to corrode.