idronics #27:  Air-to-Water Heat Pumps


The system shown in Figure 8-3 supplies multiple zones of space heating with a mixture of heat emitters. It also provides domestic hot water.

A split system air-to-water heat pump is the primary heat source. An electric boiler, piped in parallel with the heat pump’s condenser, provides a second stage of heat input if needed. It also provides a backup to the heat pump should it be down for service. Each heat source is equipped with a pressure relief valve and can be fully isolated if necessary. Depending on local codes, the electric boiler may require a low water cutoff and a manual reset high limit controller.

Both heat sources supply a “tank-in-tank” buffer. The inner tank is constructed of stainless steel and holds 40 gallons of domestic water. It is surrounded by an outer tank that receives heat from the heat pump or the electric boiler. Heat flows from the system water in the outer tank to domestic water in the inner tank whenever the former is at a higher temperature than the latter. Domestic cold water enters the inner tank whenever there is a draw from a hot water fixture. The temperature of the domestic water leaving the inner tank depends on the temperature maintained in the tank shell. A  thermostatically controlled tankless electric water heater boosts the domestic water to the desired supply temperature.

The buffer tank has a 3-pipe configuration. Heated water from the heat pump or electric boiler can flow directly to one or more of the space heating zones when either heat source is on at the same time as one or more of the zones.

Any difference in flow rates between the heat source(s) and zones passes through the buffer tank. All return flow passes into the lower portion of the buffer tank, and thus keeps its thermal mass well engaged. A spring-loaded check valve is installed in the piping leaving the heat pump and the electric boiler. These valves prevent reverse thermosiphon flow from the heated tank through either heat source when they are off.

The water temperature in the tank is controlled by a 2-stage outdoor reset controller. At design load conditions, the target water temperature in the tank is 120ºF. The target temperature decreases as the outdoor temperature increases. The minimum target temperature is 100ºF. Maintaining the tank temperature in this range allows the heat pump to operate with relatively good COPs. It also allows the heat pump to provide the majority of the “temperature lift” required for domestic hot water. If the heat pump is not able to maintain the necessary target temperature, the controller operates the electric boiler for supplemental heat input. There is a 5-minute interstage time delay to allow time for the heat pump to stabilize its operation before turning on the electric boiler.

Space heating is supplied through several types of heat emitters. The home’s main floor uses a combination of panel radiators, a towel warmer, and two areas of tube & plate underfloor heating. All of these emitters have been sized for design load output at 120ºF, and thus they can all be supplied as parallel circuits. The towel warmer is combined in series with a short tubing circuit that provides a small area of floor heating in the master bathroom. The three panel radiators each have integral thermostatic radiator valves and operate independently. The master bathroom and one other area of floor heating are equipped with non-electric modulating valves that are coupled to remote setting dials by capillary tubes.

Flow to all the main floor circuits is provided by a variable speed pressure-regulated circulator operating in constant differential pressure mode. All  circuits begin and end at a common manifold station that is equipped with three “extra” connections. Those connections are initially capped but allow additional panel radiators or other emitters to be easily added in the future.

The home’s basement slab is also heated. The slab circuits require lower water temperature compared to the main floor circuits (100ºF water at design load). This water temperature is provided by a 3-way motorized mixing valve, also operated using outdoor reset control.


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.