Buffer tanks solve the issue of backup heat

One of the most asked questions is “Do I need a buffer tank with a hydronic heat pump?” Despite trying to save some dollars by not purchasing a buffer tank, it is always recommended and should be included in any hydronic heat pump system including both geothermal and low temperature heat pumps. There are several reasons why a buffer tank is used.

Hydronic heat pumps or geothermal have high flow rate requirements typically about three gallons per ton. This means a four-ton heat pump will need 12 gallons per minute flow. However, hydronic heating requires a much lower flow rate of typically .75 GPM per loop. In order to accommodate two different flow rates, you require a buffer tank or some other type of hydronic separator.

Depending on the size of the hydronic loop, the heat pump will quickly heat the volume of liquid and then shut down. This means frequent on/off starts on the compressor and heat pump. The frequent starting and stopping can cause early failure on the units, especially with single- and twostage compressors. The DC variable speed inverters are less prone to this problem, but it can still add more stress on the system.

The buffer tank stores BTUs of energy. Depending on the size of the buffer tank you can store more energy. 1 BTU is the amount of energy required to heat up 1 lb. of water. An 80-gallon storage tank can store 668 lbs. of water (1 gallon = 8.36 lbs.) So, if we heat a tank to 160°F and use the stored energy up to 90°F we have 70 degrees of energy we can use (160-90). Therefore, in an 80-gallon storage tank we can store 46,760 BTU based on our example (668 lbs. x 70°F). If we raise the water temperature, we can store more and if we increase the size we can store more. So, this storage battery can store 46,760 BTU or 13.7 Kw hrs. This is important especially with an air source heat pump as the demand and production is offset. What this means is normally during the day time it is warmer, so the tank can be loaded during optimal hours and during the night there is more of a demand that can be supplied in part by the stored energy in the buffer tank. By storing the energy, we can increase the efficiency of an air-to-water heat pump.