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TFT: Our Flagship Boiler

from Mechanical Business January/February 2023

from Mechanical Business January/February 2023, page 82

ENGINEERING IN TRANSITION

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The Next Generation

4. Two mediums

Normally the fluid in a cold weather heat pump uses some antifreeze which is typically polypropylene glycol. In order to separate this from potable water for domestic heating we need to use a heat exchanger. To get the maximum exchange it is best to have a large coil heat exchanger. Without the larger storage capacity there would be much less of a heat transfer available, so the buffer tank aids in the transfer of the energy from one medium to another.

5. Back up

A cold weather heat pump has a paradox that the colder the temperature is the less energy it can produce. Proper sizing is important but ultimately a backup heating source should be included in a system design. Sizing a heat pump to the absolute coldest day is not recommended. Instead, it should be sized to the average temperature of the coldest month. This means that there will be short periods where the temperature drops below the average and a backup heating source is needed.

The best and most cost-effective means to supply extra heat is by using a heating element found in some buffer tanks. Until now most contractors and installers used a standard potable water tank that by code was limited to a 4.5 Kw heating element. Unfortunately, this is not sufficient heat back up for some houses. So, by using a buffer tank you can size the back up to suit the total house demand up to the limits of the tank.

This means should there be a complete failure you would still have a full heating solution. Some buffer tanks also have the ability to work in conjunction with the cold weather heat pump and supply only the amount of supplemental energy needed. As an example, if the total heating load is 15 Kw and the HP at the outdoor temp can only supply 12 Kw, the electric back up will only add 3 Kw to the system even though it may be sized for a total of 15 Kw.

In summary, a buffer tank is a key part to any hydronic heat pump system. If a customer has an existing hydronic system, the buffer tank can utilize this as a second source for the backup even if it is a gas heater. When looking at the size of a buffer tank we recommend a minimum of 15 gallons per ton of buffer, but more is better.