Last month, we discussed the role of buffer tanks in combination with air-to-water and water-to-water heat pumps that operate as “on/off” devices. The buffer tank allows the heat pump to operate with reasonably long cycles (e.g., it prevents short cycling) when the hydronic distribution system is heavily zoned, and thus subject to “micro-loading” at times. We also discussed how some buffer tanks can do double duty as both buffers for space heating loads and domestic water heaters. The key concept was to turn the heat pump on and off based on changes in the buffer tank temperatures and not based on thermostats calling for heat.

Just as modulating brought variable heat output to gas-fired boilers, air-to-water heat pumps and water-to-water heat pumps are now available with “inverter” driven (e.g., variable speed) compressors that provide a range of heat output from some maximum rated value down to approximately 35% and 40% of that rated value. They also modulate to maintain a set chilled water temperature when operating in cooling mode.