idronics #27:  Air-to-Water Heat Pumps


The system shown in Figure 8-5 provides two independently controlled zones of floor heating, as well as two associated zones of chilled water cooling. It also provides domestic hot water.

This system is supplied by a variable-speed monobloc air-to-water heat pump. It’s variable speed capability allows the cooling portion of the system to operate without a buffer tank. In cooling mode, the speed of the heat pump’s compressor automatically adjusts as necessary to maintain a chilled water temperature between 45° and 60ºF when either (or both) of the chilled water air handlers is operating. Note: Zoned cooling distribution systems should not be used with a fixed-speed heat pump unless a buffer tank is provided.

A motorized diverter valve is used to route the heat pump’s output to the balance of the system associated with each mode of operation. It is possible for this system to deliver limited amounts of simultaneous heating and cooling. The buffer tank could deliver some amount of heating to the floor circuits as well as preheat domestic water while the heat pump is operating in cooling mode.

One possible control scenario would be to set up controls to prioritize cooling operation during the cooling season and heating mode operation during the heating season. The heat pump would switch to the lower priority load only after temporarily satisfying the higher priority load.

Another control possibility would be to set the target temperature of the buffer tank using outdoor reset control during the heating season, and switch that target temperature to a higher, narrower range during warm weather when the heat pump’s COP is relatively high. This would allow a higher percentage of the energy needed for domestic hot water to be supplied by the heat pump rather than the resistance heating elements in the supplemental water heater.


21st century refrigeration techniques now make it possible for air-source heat pumps to operate in cold winter climates, with significantly improved performance compared to earlier-generation heat pumps. By combining low-ambient air-source heat pump technology with the versatility and high distribution efficiency of modern hydronics, designers can create systems for unsurpassed heating and cooling comfort, as well as domestic hot water production. Those systems are ideally suited for use with renewably sourced electricity, carbon reduction goals and “net zero” building projects. When properly selected and applied, low-ambient air-to-water heat pumps can approach the performance of geothermal heat pump systems at significantly lower installation cost and complexity. Air-to-water heat pump systems offer hydronic heating professionals a means of responding to evolving market trends and constraints without compromising quality, comfort or efficiency.