America has been heating buildings with water for well over a century. Cast-iron radiators in brownstones and row houses. Steam and hot-water systems in schools, hospitals, and apartment buildings from Boston to Chicago. Radiant floors in mid-century homes, and in millions of houses built since. Hydronic heating is not a European import — it's a mature, proven part of the North American building stock.
What has changed is what sits at the front of the loop.
For most of that history, the heat source was a boiler burning gas, oil, or coal. Today that same hot-water distribution can be driven by an air-to-water heat pump: an outdoor unit that moves heat instead of burning fuel, delivering three or more units of heat for every unit of electricity it uses — and reversing in summer to produce chilled water for cooling.
The plumbing is familiar. The economics are not.
What an air-to-water heat pump actually does
An air-to-water heat pump is a single outdoor unit that pulls thermal energy out of the outside air and transfers it into water. That water then circulates through your building to deliver heating — through radiant floors, panel radiators, or hydronic fan coils — and, in cooling mode, the same unit chills the water and the same distribution system removes heat from the house.
The important word is transfers. A boiler or furnace creates heat by burning fuel, and can never return more energy than the fuel contains. A heat pump moves heat that already exists, which is why it isn't bound by that ceiling — a coefficient of performance (COP) above 3 is routine. Even at outdoor temperatures well below freezing there is usable thermal energy in the air, and a well-designed inverter unit can extract it.
If your building already has a boiler, radiators, or radiant floors, the heat pump is a change of heat source, not a change of building. The distribution you have is the distribution it uses.
The difference between an air-to-water heat pump and the air-source heat pump most Americans know is what happens on the indoor side. A conventional heat pump blows conditioned air through ductwork. An air-to-water heat pump produces hot or chilled water, which is a far denser carrier of energy than air and travels through 1/2" or 3/4" PEX instead of sheet-metal trunks.
Why water beats air as a delivery medium
Water carries roughly 3,500 times more heat per unit volume than air. That single fact drives most of the practical advantages:
Smaller distribution, less lost space. A pair of PEX lines replaces a duct chase. No soffits dropped through finished ceilings, no attic ductwork sitting in unconditioned space, no 20–30% distribution losses through leaky joints.
Genuine zoning. Every loop or fan coil can have its own thermostat and its own valve. Zoning a hydronic system means adding a manifold port. Zoning a ducted system means dampers, bypasses, and static-pressure problems.
Comfort at lower temperatures. Radiant floors operate happily on 90–110°F water. Because the heat pump doesn't have to work as hard to reach that temperature, efficiency climbs — heat pumps get dramatically more efficient the lower the water temperature they're asked to produce. Radiant heat also feels different: warm surfaces rather than warm air blowing across the room.
One system, multiple jobs. The same heat pump that heats the building can produce domestic hot water through an indirect tank, and can switch to chilled water for cooling in summer. One outdoor unit, one refrigerant circuit, one point of maintenance.
Quiet operation. No air handler cycling in the ceiling, no blower noise, no drafts.
What a complete system looks like
An air-to-water system is a set of components that have to be sized to work together. A typical residential installation includes:
- The outdoor heat pump — the compressor, coil, fan, and inverter drive that generate hot or chilled water
- A buffer tank — thermal mass that prevents the compressor from short-cycling under low load and keeps run times long and efficient
- A DHW tank — an indirect tank with a coil, so the heat pump also makes your domestic hot water
- Distribution — radiant floor loops with PEX and manifolds, panel radiators, hydronic fan coils, or a combination
- Circulators, controls, and thermostats — tying it all together and managing changeover between heating, cooling, and hot water
The buffer tank is the piece most often skipped by people new to hydronics, and it's the one that most often causes trouble later. A heat pump that short-cycles wears out its compressor and never reaches its rated efficiency. Sizing it correctly is not optional.
Refrigerants: R290 and R32
Two refrigerants dominate modern air-to-water equipment.
R290 (propane) has a global warming potential of 3 — effectively negligible — and excellent thermodynamic properties, which means high efficiency and the ability to produce higher water temperatures in cold weather. It's flammable, which is why R290 units are built as monobloc systems: the entire refrigerant circuit stays sealed outdoors, and only water enters the building. No refrigerant lines are run through living space, and no field refrigerant work is required at installation.
R32 has a GWP of 675 — far lower than the R410A it replaced — and remains widely used across a broad range of capacities.
Both are a substantial improvement over legacy refrigerants, and US regulation is steadily pushing the market toward lower-GWP options. Buying equipment on a low-GWP refrigerant today is a hedge against being stranded with a system that's expensive to service in ten years.
Is it right for your project?
Air-to-water makes the most sense when:
- You're building new, or doing a renovation deep enough to install radiant floors
- You have existing hydronic distribution — a boiler with radiators or radiant, ready to be swapped
- You want heating and cooling from one system without running ductwork
- You need real room-by-room zoning
- You're replacing propane or oil, where operating-cost savings are largest
It's a harder fit when you have existing ductwork in good condition and no appetite for changing your distribution system. In that case a conventional ducted heat pump may get you there faster and cheaper — though a hydronic air handler is also an option worth pricing.
The honest caveat: hydronic systems reward good design and punish bad design. Load calculations, water temperature targets, buffer sizing, and flow rates all matter. This is not equipment to size by rule of thumb.
Frequently asked questions
Do air-to-water heat pumps work in cold climates? Yes. Modern inverter-driven units maintain useful capacity well below freezing, and R290 units in particular hold higher water temperatures in cold weather. Correct sizing for your design temperature is what determines success, along with a properly specified backup or supplemental element.
Can one unit do both heating and cooling? Yes. A reversible air-to-water heat pump produces hot water in winter and chilled water in summer, using the same distribution. Cooling with radiant floors requires dew-point control; most systems use hydronic fan coils for the cooling side.
Can I use my existing radiators? Often, yes — but existing radiators sized for 180°F boiler water may need to be larger, or supplemented, to deliver the same output at heat-pump water temperatures. This is exactly what a load calculation determines.
How efficient are they? Typical seasonal COP runs between 3 and 4.5 depending on climate, water temperature, and design. Lower water temperature means higher efficiency, which is why radiant floors pair so well with heat pumps.
Do they need refrigerant line sets run into the house? Not with a monobloc design. The refrigerant circuit is sealed outdoors and only water lines enter the building — which simplifies installation and eliminates field refrigerant work.
Where Aquacoils fits
We supply complete hydronic systems for the US market — R290 and R32 air-to-water heat pumps, geothermal units, buffer and DHW tanks, hydronic fan coils, manifolds, PEX, circulators, and controls — so the components arrive already matched to each other.
Our heat pumps are built around a Panasonic compressor, Carel controller, and Grundfos circulator, with stock held domestically so parts are available when you need them. Equipment is covered by a 3-year parts warranty and a 7-year compressor warranty.
And because hydronic design is where these projects are won or lost, our in-house team of hydronic specialists will size and lay out the system with you before you buy anything.
Planning a project? Talk to our design team or browse air-to-water heat pumps.