Inverter air-to-water heat pumps

By now, most of you have either installed inverter-driven variable speed heat pumps that can deliver their nominal rating output at temperatures well below 0° F without any backup heat source, or at least have become familiar with the technology. The most efficient models rival the COP (Coefficient of Performance) ratings of geothermal heat pumps, but without the expense of installing the boreholes, or wells, for the heat exchanger.

With tax credits now reduced from 30% to 22% in 2021, and now 0% in 2022, geothermal systems will have to compete against inverter air-to-water systems installed cost, which were already less expensive back when geothermal systems enjoyed the full 30% federal tax credit. Based on my own cost analysis between inverter-driven air-to-water and geothermal heat pump systems, customers were easily swayed to select the air-to-water systems. Fortunately, John Siegenthaler’s “Picking Winners” column has done the heavy lifting where numbers crunching is concerned with well-defined costs for both systems along with extended payback.

During the last live and in-person AHR show in Atlanta before the COVID-19 pandemic, it was abundantly apparent that inverter air-to-water heat pumps are on their way to capturing an increasing share of the HVAC and hydronic markets. Geothermal contractors, myself included, have been keeping a keen eye on strategies to adopt once the tax credits expire because the requests for geothermal systems essentially evaporated in 2017 after geothermal tax credits expired the first time.  

Where the rubber meets the road

Communication is key. Know your customers’ expectations in advance. What indoor air temperatures are desired for heating and cooling? My brother is an example of why you need to know in advance because he wants 68° during summer cooling! My mother-in-law thinks 84° is best during the winter months!

Use a top-notch radiant design program that allows you to switch between installation methods to know what water delivery temperatures are required on a design day as well as what total Btu/h is needed. Then add a cushion to allow for unexpected weather events that will exceed design day conditions. I always ran a Manual-J calculation and a geothermal design program, too.

Domestic hot water load will need to be determined as well. Will the DHW tank have electric backup in case the air-to-water heat pump cannot keep up during the 10% of winter when design-day conditions are present or the heat pump breaks down?

Get training from the manufacturer of the air-to-water heat pump. Most require programming the internal controller for optimum performance. Make absolutely sure the output is a match for your design-day loads.

Pay attention to what the builder and other trades are doing! If the insulation is not being installed to specifications, or windows/doors differ causing increased heat loss/gain, you’ll need to have a meeting to ensure you cover your ass-sets! For example: Who is responsible to reimburse you if another contractor drills through your radiant tubing? Will the owner demand the loop be replaced or allow it to be repaired? In my personal experience, owners have almost always required the entire loop be replaced. Our contracts always included a clause that provided we bill the general contractor because they can withdraw that cost from the subcontractor’s charges.

Under promise and over-deliver. I always strive to keep my design day water temperatures under 100° because that drives the efficiency bus and provides me with a comfortable cushion to raise water temperature if something is amiss with the building envelope rendering a higher demand for output or the owners added throw rugs that dampened the radiant energy output. Never promise specific energy savings. Instead, project operating costs on absolute conditions, such as run hours, water temps, COP and cost for electricity with a notice those are strictly speculation and not the actual projected operating costs.

In the real world

Where hydronic radiant heating performance is concerned, there are multiple methods of installing your radiant panels for floors, walls and ceilings. Keep in mind that the lower the required water temperature for a design day (coldest weather on average for your area), the better the operating efficiency, which translates into reduced energy costs for your customers. This pretty much rules out suspended tubing and staple-up installation methods for new installations as stand-alone methods.  

If, as I have, you find yourself confronted with an underperforming hydronic radiant system where your accurate detailed heat loss calculation and cross-comparison of the installed system’s Btu/h output reveal the available geothermal or inverter air-to-water heat pump cannot meet the room’s desired comfort-level, you can simply incorporate supplemental heating. My go-to resolution is flat-panel radiant radiators because you can custom tailor the Btu/h output by sizing the radiator to the Btu/h shortfall based on the available water temperature. In most cases, you can interrupt the PEX loop to include the flat panel radiator(s) in that zone to greatly simplify your resolution strategy.

If zoning is to be incorporated, you’ll most likely need to incorporate a buffer tank, and if you are utilizing zoned cooling, a second chilled-water buffer tank will be needed. Case in point: An actual installation in a multi-million-dollar home with hydronic in-floor radiant heating and multiple chilled-water air handlers had just one 80-gallon buffer tank. During the dead of winter and dog days of summer, there was no problem, but when shoulder seasons were present, the owner had to manually changeover from heating to cooling or cooling to heating each day/night, which was a huge PITA. Not to mention, it wasted tons of heat pump energy by chilling an already hot 80-gallons or heating a mass of already chilled water.