Air-to-Water Heat Pumps


Inside this Article

Facing page: A Jaga Briza fan-coil emitter provides both heating and cooling from hydronic input. Inset: A SpacePak Solstice monobloc A2W heat pump.
This heating-only emitter has the look of an electric baseboard, but has heat-pump-driven thermal performance.
This wall-mounted air handler can be coupled to an A2W heat pump to provide space heating and cooling.
The outdoor portion of a split-system heat pump by Nordic. The stand elevates the unit above snow level. Refrigerant tubing and electrical cables pass through the wall to the indoor unit.
A cutaway of the Nordic split-system interior unit.
A2W heat pumps can be mounted on concrete pads on the ground but, in deep-snow climates, they should be mounted higher than the anticipated snow level.
Radiant ceiling panels—1/2-inch PEX-AL-PEX tubing spaced 8 inches apart with aluminum heat transfer plates—prior to drywall installation.
A radiant wall panel, also with PEX-AL-PEX on aluminum heat-transfer plates, installed behind drywall.
This layout for a heated slab consists of 1/2-inch PEX-AL-PEX tubing fastened to welded wire reinforcing on top of extruded polystyrene insulation. Tubing and reinforcing will be lifted to mid-slab height when concrete is poured.
A typical manifold station that supplies water to radiant floor-panel zones.
A typical wall-panel radiator with wireless thermostat. Although their origins are in Europe, panel radiators are now readily available in North America.
A typical wall-panel radiator with wireless thermostat. Although their origins are in Europe, panel radiators are now readily available in North America.
For most residential systems, buffer tanks between 40 and 119 gallons are appropriate.
Using pressure-rated hose connections between the heat pump and rigid piping will accommodate thermal expansion and reduce vibration transfer.
Piping through exterior walls should be protected from condensation and insulated. Here, tubing from the heat pump passes through a 2-inch PVC sleeve; the space between is filled with expanding foam insulation.

In 2016, there were approximately 2 million air-to-water (A2W) heat pumps installed globally, with the largest markets in Europe and Asia. The U.S. market is a tiny fraction of the global market, mostly due to its relatively few homes with low-temperature hydronic heating.

With increasing interest in net-zero-energy (NZE) homes, state renewable energy generation requirements, and carbon-dioxide reduction goals, heat pumps of all types will play increasingly important roles in heating and cooling systems. Air-to-water (A2W) heat pumps can compete with geothermal water-to-water heat pumps, especially in an unsubsidized market. They will also compete with air-to-air ductless minisplit heat pumps (MSHPs) based on their versatility and the comfort they offer.

While air-to-air (A2A) heat pumps and forced-air furnaces deliver heating and cooling using a blower, those systems may introduce drafts, noise, and dust. They also contribute to temperature stratification (warm air at the ceiling; cold air at the floor); higher rates of air leakage and heat loss; and lowered humidity. Hydronic distribution systems supplied by A2W heat pumps avoid many of these disadvantages because they distribute heating or cooling using low-temperature (120°F or less) emitters that do not create high air flows. If hydronic heating is a priority, your choices are either an A2W heat pump, a geothermal system, or a boiler-based system. When coupled to low-temperature radiant panels, A2W can provide excellent heating comfort. Unlike boilers, they can also supply hydronic cooling.

A2W Heating & Cooling

An A2W heat pump, in heating mode, absorbs low-temperature heat from the outside air and delivers higher-temperature water to the building’s hydronic distribution system—usually radiant floor, wall, and ceiling panels; panel radiators; or fan-coil convectors. In cooling mode, the same heat pump delivers chilled water (or water/antifreeze solution) to air handlers with blowers to cool and dehumidify the building.


In heating mode, some A2W heat pumps can produce water temperatures up to 130ºF, even when the outdoor temperature is near 0ºF. This allows a wide variety of hydronic heat emitters to be used.  In some systems A2W heat pumps can also provide some or all of a building’s domestic water heating. With a suitable heat exchanger, heat can also be supplied to an ancillary load, such as heating a swimming pool or hot tub.

In heating mode, an A2W’s liquid refrigerant absorbs low-temperature heat from outside air, which is blown across the evaporator coil by a fan. The absorbed heat vaporizes the liquid refrigerant. This refrigerant vapor passes through a reversing valve to an electrical-motor- driven compressor, which increases the vapor’s pressure and temperature. As the now-hot refrigerant vapor exits the compressor, it does a U-turn through the reversing valve, and enters another heat exchanger called the condenser. A stream of water or an antifreeze solution is pumped through the other side of the condenser. The hot refrigerant vapor transfers heat to this fluid, and in the process, condenses back into a liquid. The liquid refrigerant then passes through a thermal expansion valve (TXV), where its pressure and temperature are reduced to the condition where this cycle description began. The refrigerant sealed within the heat pump is constantly flowing through this cycle whenever the heat pump is operating.


In cooling mode, A2W heat pumps produce chilled fluid temperatures as low as 42ºF. That fluid can be routed to one or more air handlers or fan coils in which the chilled fluid absorbs heat from interior air and reduces its humidity.

During cooling, the heat pump’s “source” is inside air. In this case, the functions of the two heat exchangers in the refrigerant circuit are reversed compared to when in heating mode. The heat exchanger that served as the evaporator in the heating mode becomes the condenser in cooling mode; and the condenser becomes the evaporator. This swapping between the heat exchangers is accomplished using an electrically operated reversing valve inside the heat pump.

Heating & Cooling Performance

Heating Performance

Heating capacity and coefficient of performance (COP) are two measures of how efficiently an A2W heat pump moves heat from outside air to interior water. Heating capacity is the rate, expressed in Btu per hour, that a heat pump can deliver heat to a building. A heat pump’s “rated” heating capacity is based on operating conditions that are specified by industry standards.

The heating capability the A2W delivers at any given time could be higher or lower than the rated heating capacity, depending on the operating conditions. The colder the outdoor air temperature and the higher the temperature of the fluid leaving the heat pump, the lower its heating capacity. Hydronic heat emitters like radiant panels, which can operate at relatively low fluid temperatures, are a good match for the performance of A2W heat pumps.

A heat pump’s COP is the number of units of energy output the heat pump delivers for each unit of electrical energy input. For example, a heat pump operating at a COP of 3.0 delivers three units of useful heat output for each unit of electrical energy input. Like heating capacity, the colder the outdoor temperature and the higher the temperature of the fluid leaving the heat pump, the lower its COP. Heat pump manufacturers provide tables or graphs that show heating capacity and COP as a function of outdoor temperature and different fluid temperatures leaving the heat pump.

Cooling Performance

Cooling capacity and energy efficiency ratio (EER) describe the cooling performance of A2W heat pumps. Cooling capacity is the rate at which the A2W heat pump absorbs heat from the fluid stream passing through its evaporator. The rated cooling capacity gives the amount of heat absorption under specific operating conditions, such as 44ºF chilled-water temperature and 95ºF outdoor temperature. The cooling capacity can be higher or lower than rated, based on current operating conditions. Cooling capacity decreases as the temperature of the chilled fluid leaving the heat pump decreases. It also decreases as the outdoor air temperature increases. The higher the chilled-water temperature can be and still meet the building’s cooling and dehumidification requirements, the higher the heat pump’s cooling capacity.

The energy efficiency ratio (EER) is the heat pump’s cooling capacity divided by the electrical power (W) supplied to the heat pump. The higher the EER, the greater the cooling capacity per watt of electrical input power. High EERs are always desirable. Like cooling capacity, the EER of an A2W decreases with increasing outdoor temperature. It also decreases as the chilled fluid temperature leaving the heat pump decreases.

Major Components

Heat Emitters

To achieve the best heating performance, A2W heat pumps need to be combined with hydronic heat emitters that operate at low fluid temperatures. Specific types of radiant panel heating, which includes radiant floor heating as well as radiant wall or radiant ceiling heating, can work well with A2W heat pumps. Some can provide adequate heating at supply water temperatures lower than 100ºF, even on very cold days.

Radiant Tubing Embedded in Concrete

One of the best options is a heated, 4- to 6-inch-thick slab-on-grade concrete floor with no covering (other than a coat of stain). In well-insulated buildings, and using close (6- to 12-inch) tube spacing and adequate edge and underslab insulation (R-10, minimum), heated slabs can maintain excellent interior comfort using water temperatures of 95°F to 105ºF, even when outdoor temperatures are below 0ºF. Heated slabs covered by ceramic or vinyl tile can also be good performers. For optimum performance, avoid carpet, cork, or wood flooring over heated slabs.

The tubing is attached to welded wire reinforcing that’s placed at half the slab’s depth. This allows lower water temperature than systems where the tubing is placed at the bottom of the slab. When the required upward heat flow from a bare slab needs to be 15 Btu/hr./ft.2 (a typical design load condition for a well-insulated home) and the tubing is spaced 12 inches apart and placed midslab, the required average fluid temperature in the floor circuits is about 95ºF. If tubing is placed at the bottom of the slab, the average fluid temperature needs to be higher—102ºF—with all other conditions being equal. That 7ºF higher temperature negatively affects the heat pump’s COP and its heating capacity.

Other emitters

Other options include radiant tubing and aluminum heat spreader plates installed above or below wood subfloors, as well as radiant ceiling and wall panels. The useful heat output of these panels varies based on the average water temperature in the tubing.

The rated heat output of panel radiators is typically based on an average fluid temperature as high as 180ºF. A given panel’s heat output at a lower average fluid temperature, such as 110ºF, is typically 20% to 30% of its output at 180ºF. In standard buildings, this usually translates into using larger radiators to meet the heating load. Panel radiators can be smaller in buildings with lower heating loads.

Thermal storage

A2W heat pumps supplying zoned systems often operate at heating outputs higher than required when only one or two zones need heat. In this case, a buffer tank must be used between the heat pump and the remainder of the system. The thermal mass of the tank absorbs excess heat output from the heat pump, which reduces short cycling. After the heat pump shuts off, active zones draw stored heat from the buffer tank, which allows the heat pump to remain off, often for several minutes. Fewer and longer heat-pump operating cycles decrease wear on heat pump electrics, lengthening its service life.

The greater the buffer tank’s volume, the longer the heat pump’s cycles. For most residential systems, buffer tanks between 40 and 119 gallons are sufficient. The smaller tanks are appropriate for A2W heat pumps with variable-speed compressors. Variable-speed compressors on some A2W heat pumps help match the heat pump’s output to the load. This is desirable when the heating or cooling distribution system is divided into several zones. A larger buffer tank is generally needed for on/off heat pumps. Buffer tanks are available from several companies, such as Caleffi, Flexcon, and Thermo 2000 (TurboMax).

Zoned System Example

The system shown in the schematic uses an A2W heat pump to supply two independently controlled zones of radiant panel heating and two systems of air-handler cooling. The entire hydronic system is filled with a solution of 30% propylene glycol.

Heating Mode

When a zone calls for heat, the associated zone valve opens and the circulator pump turns on. The speed of the circulator automatically adjusts depending on which zone(s) are operating. If zones operate simultaneously, the circulator’s speed automatically increases. Any flow not sent to the distribution system passes into the buffer tank.

The heat pump is turned on and off based on the temperature at the midpoint of the buffer tank. That temperature varies based on outdoor temperature. The colder it is outside, the warmer solution the heat pump sends to the tank. When a zone calls for heat, the heat pump runs until the temperature sensor reaches a target that is continuously adjusted by an outdoor reset controller. This controller helps keep the system operating at the lowest temperature that can maintain comfort in the building, maximizing the heat pump’s COP.

Cooling Mode

Cooling and dehumidification are provided by passing chilled fluid from the buffer tank through independently controlled air handlers. The chilled fluid passes through copper tubing in the air handler’s coil as room air is pulled through the coil by a blower, and is then distributed to rooms, directly or through ducts. Water droplets that form on the coil as the air is dehumidified fall into a drip pan and are drained away.

All piping and components carrying chilled fluid must be insulated and vapor-sealed to prevent condensation. This is a critical detail that is often overlooked or poorly done. The result will be water puddles and stained surfaces under the piping and the components carrying chilled fluid.

Resources & Further Reading

A2W Heat-Pump Suppliers

Aermec •

Chilltrix •

Maritime Geothermal •

SpacePak •

ThermAtlantic Energy Products •

“Renewable Hydronic Heating” by John Siegenthaler in

Modern Hydronic Heating, 3rd edition by J. Siegenthaler, Cengage Learning, 2012.

Heating with Renewable Energy by J .Siegenthaler, Cengage Learning, 2017.

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