Air to Water Heat Pump Technology – GreenBuildingAdvisor

Building Science

Air-to-water heat pumps use water, not air, to deliver heat. They’re an emerging product we’re starting to see more often. Here’s how, and where, they work.

This article explores air-to-water (A2W) heat pumps, an emerging HVAC technology that combines the efficiency of heat pumps with the comfort and flexibility of hydronic heating. Unlike the more common air-to-air systems, A2W heat pumps use outdoor air to heat water that is then circulated through radiators, radiant floors, or other hydronic emitters, offering an all-electric alternative to fossil-fuel boilers.

HVAC consultant Jon Harrod explains how the systems work, where they perform best, and why low water temperatures are key to maximizing efficiency. While A2W systems offer advantages such as quiet operation and zoning flexibility, the article also discusses their higher upfront costs, their design complexity, and the challenges of retrofitting older homes.

A Different Type of Heat Pump

I’ve been an HVAC installer and project manager for the last 10 years. Most of the heat pumps I’ve seen during that time have been air-to-air systems. These are the most common type, and I’ve written about them for both Green Building Advisor and Fine Homebuilding. I’ve also worked on a few ground-source heat pumps, which exchange heat with the ground, rather than the air, using buried loops of pipe. But there’s a third category that opens the door to hydronic heating without the cost and requirements of ground-source heat pumps: air-to-water, or A2W, heat pumps.

A2W heat pumps exchange heat with the outdoor air, just like air-to-air heat pumps. But inside the house, they circulate water to hydronic emitters, similar to those used with boilers and some types of ground-source systems. I’ve written about heat-pump water heaters—which extract heat from indoor air and store it in a tank of domestic hot water (“FAQs on HPWHs,” FHB #330)—but A2W heat pumps are different.

Heat-pump water heaters, though they move…