Most heat pumps blow warm air. An air-to-water heat pump instead heats water, which then feeds radiant floors, hydronic baseboards, or fan coils. It is the electric answer for homes built around water-based heat.
What it is and how it works
Weil-McLain explains that an air-to-water heat pump captures thermal energy from outdoor air and transfers it into water for hydronic heating, differing from mini-splits by delivering heat to water rather than directly into air. Mechanically, the outdoor unit pulls air through a refrigerant heat exchanger, the refrigerant absorbs heat and is compressed to raise its temperature, then transfers that energy to water through an indoor heat exchanger. As Weil-McLain puts it, the system moves existing heat rather than creating it, the same principle as any heat pump, applied to water.
How it distributes heat, and its cooling limit
The water can go many places. Weil-McLain notes an air-to-water system distributes heat via radiant floor tubing, hydronic baseboards, and fan coils, and can contribute to domestic hot water supply. This Old House adds that these systems use heated water pumped through PEX tubing under the floor and can provide both heating and cooling as well as household hot water. Cooling has a constraint, though: Weil-McLain notes cooling requires fan coils or radiant cooling systems and is not baseboard-compatible, so the technology is best suited for homes already using water-based heating.
Efficiency and the all-outdoor refrigerant
Efficiency is strong, like other heat pumps. Chiltrix describes the unit acting as a compressor-powered boiler in winter, delivering 2 to 6 times more heat output per unit of energy input, a COP of 2 to 6. A notable design benefit is refrigerant containment: Chiltrix states 100% of the refrigerant stays outdoors and outside of the building envelope, with no refrigerant ever indoors, because indoors the system only circulates heated or cooled water. That keeps all refrigerant handling at the outdoor unit.
Example equipment specs
Concrete models show the range. Chiltrix rates its CX35 at an SCOP of 4.69 (seasonal average COP for heating) and a COP of 4.9 for heating at W95F/A47F, using R-32 refrigerant and working with in-floor radiant heating or cooling. For cold-climate delivery, SpacePak states its Solstice Inverter Extreme delivers water temperatures of 42 to 130F and provides high-performance heating in cold-climate extremes down to -22F outdoor air. These are specific products, not category guarantees, so confirm the exact model's rated water temperature and low-temperature performance for your climate.
Frequently asked questions
What is an air-to-water heat pump?
Weil-McLain describes it as capturing thermal energy from outdoor air and transferring it into water for hydronic heating, delivering heat to water rather than directly into air like a mini-split.
How does it distribute heat?
Weil-McLain notes it uses radiant floor tubing, hydronic baseboards, and fan coils, and can contribute to domestic hot water. This Old House notes it can provide heating, cooling, and hot water.
Can it cool a home?
Yes, with limits. Weil-McLain notes cooling requires fan coils or radiant cooling systems and is not baseboard-compatible, so it suits homes already using water-based heat.
How efficient is it?
Chiltrix describes delivering 2 to 6 times more heat per unit of energy input (a COP of 2 to 6), and rates its CX35 at an SCOP of 4.69.
Is there refrigerant inside the house?
No. Chiltrix states 100% of the refrigerant stays outdoors, because indoors the system only circulates heated or cooled water.