Heat pump equipment has become a ubiquitous part of the HVAC landscape in northern climates over the past decade. Here in the Northeast, the demand for heat pump equipment in new and existing homes has been outstripping the capacity of the contractor base to keep up in recent years. Equipment options and efficiency have continued to expand year over year, to the point that there seems to be a excellent heat pump system for just about every building type or application, so long as the appropriate considerations are made for the application. Here in the heating-season-dominated Northeast where I work, we have a high percentage of hydronic heated homes and an experienced contractor and supply-chain base that supports this type of heating distribution. In this article, I will look at some of the considerations for new and retrofit residential applications using air-to-water heat pumps.
The expansion of options for heat pump equipment for cold climates over the past decade has primarily been focused on air-to-air equipment, most often with ductless systems. But as more homeowners seek to take advantage of the opportunities that heat pumps provide, the demand for whole-house solutions is increasing. At the same time, state and federal policies pushing away from fossil-fuel equipment and focusing on efficient electric heating and cooling have driven the demand for practical and efficient whole-house heat pump systems.
Evolution of Heat Pumps
Early adopters of heat pump equipment in cold climates were mostly limited to ductless mini-splits. Over the years, cold climate inverter-driven ducted heat pump equipment came into play that could be added to existing ducted heating and cooling distribution systems and air handlers or could be ducted independently. Geothermal or ground-source heat pump systems have been a part of the picture for all climates for decades and offer the benefit of being able to work with both ducted and hydronic distribution, though the upfront cost to install the ground-loop side of the system has limited geothermal’s market reach. But in the background, there was buzz about air-source heat pump equipment that could be used to make hot water for hydronic distributed heating systems. Rumors? Hearsay? Just something those Europeans across the pond were messing around with?
Truth is, these air-to-water heat pump systems have been around the whole time, finding their way into effective applications residentially and commercially. But just as the other air-source heat pump equipment was being adapted to applications in cold climates, air-to-water heat pumps also evolved to become viable options for integrating as whole-house systems in heating-dominated climates.
Air-to-water heat pumps use the same refrigerant process and cycle and have the same functional components as air-source heat pump equipment. One primary difference is that there is a heat exchange from the refrigerant cycle to a water loop. Air-to-water equipment is used extensively in mixed heating and cooling climates in Europe and Asia and elsewhere, with millions of units being installed annually across the globe. Air-to-water equipment has been sneakily showing up in applications in cold climates as well over the past decades, primarily for use as chillers for high-velocity ducted AC systems. Ducted AC with hydronic air coils is still a common application for air-to-water heat pump systems today, though air-to-water equipment is coming into its own as a whole-house heating option equivalent to other fuel-fired systems commonly in use.
Air-to-Water System Applications
The refrigerant cycle is highly efficient at transferring heat energy between its source heat sink and heat rejection mediums. For air-source equipment, this source is obviously the air. Because the boiling point of modern refrigerants is so low, there are still plenty of valuable Btus to be harvested from air that is well below 0°F.
While heat pumps are highly efficient at transferring Btus between two sources, there are limitations on the intensity and volume of Btus the heat pump can move. As a result, heat pump equipment is primarily a low-temperature delivery mechanism. For hydronic heating, this is a beneficial match for need.
Historically, hydronic heating distribution systems relied on high-temperature water generated by a fuel-fired boiler distributed through high-Btu output emitters in the home—think fin-tube baseboard and cast-iron radiators. This was a good match when buildings needed high Btu output to overcome the substantial heat loss occurring from building envelopes that left a lot to be desired. High heat loss and large swings in temperature were a fact of life for these buildings. But as our housing stock has improved (and is being retrofit through weatherization) and energy codes continue to push the performance of the building envelopes to low convective and conductive losses, our buildings are trending toward needing less Btu input to keep occupants comfortable. Consequently, the demand for output on the heating equipment for these buildings is becoming lower, and the divergence in the capacity of traditional fuel-fired high-temperature distribution systems from what the building actually needs has become greater. We are at a point that whole-house heat pump systems are becoming a practical and logical match for the demands and loads these buildings are seeing.
There are two types of equipment for air-to-water heat pump systems, monobloc and split-refrigerant. There are about eight manufacturers actively supplying equipment options for cold climates: Aermec, Arctic Heat Pumps, Chiltrix, Enertech, Nordic, SpacePak, Stiebel Eltron, and Taco Comfort Solutions. Nominal capacities for these manufacturers range between 2 and 6 tons. These cold-climate-rated models can maintain varying levels of capacity and varying coefficients of performance (COP, a measure of heat pump efficiency) below 0°F, but all are designed for cold ambient temperature applications.
Monobloc systems package all the components of the system in a single outdoor unit that includes the fully charged refrigerant system components, electronics, and heat exchanger from refrigerant to water. Because these systems package everything in the outdoor unit, they require a glycol-mix supply-and-return loop for the water from the storage tank within the house out to the heat pump.
Split-refrigerant systems look and function similarly to other split-system air-source heat pump equipment, with an outdoor compressor/evaporator unit that pipes the compressed refrigerant to an indoor condensing unit. The indoor unit is about the size of a typical mod-con wall-hung boiler that contains the refrigerant-to-water heat exchanger and all of the electronics and controls. The outdoor units for both systems typically have two fans moving air across the refrigerant coils and are about the size of a standard multi-zone ductless unit. Outdoor equipment needs to be set on a frost-protected foundation and with appropriate clearances and considerations for snow and ice as with other air-source heat pump equipment.
The heart of an air-to-water system, though, is the buffer tank. Buffer tanks are thermal storage tanks that are sized in capacity based on the design load for the house and demands on the system. The buffer tank is a thermal battery, storing Btus generated by the heat pump for distribution as needed to the building zones. A buffer tank allows the heat pump (or any other heat input) to run at a steady state with a stable load to attain the highest efficiency. Typically, these buffer tanks also include one or two electric resistance elements within the tank that can add Btus or take up the load of the heat pump if it is struggling to maintain the tank temperature. A well-designed and commissioned system, however, will likely never need to rely on the resistance elements.
Given that the heat pump is simply maintaining water temperature in a storage tank for distribution, domestic hot water can also be provided by an air-to-water setup, as well as cooling so long as appropriate emitters are used to accommodate the cooling demand. When additional loads such as DHW or cooling are added to the overall systems design, the capacity of the heat pump as it relates to shared demand for Btus needs to be carefully considered. The simpler the distribution design and the fewer the demands on the heat pump’s Btus, the simpler the installation and maintenance and therefore overall cost of the installed system will be.
So what determines if an air-to-water system is a good fit for a building? It all comes down to knowing what the Btu demand of the building and individual rooms are.
The foundation for designing any HVAC system is accurately calculating the heating and cooling demand for the specific building the system is being designed for. There are industry standards and practices referenced in the IECC that specify the procedure for calculating these loads, namely the ACCA Manual J procedure. Manual J considers the specific building envelope dynamics, including the building volume, exposure, climate zone, and defined design temperature. Design temperature varies by climate zone and by what individual jurisdictions define in their local energy codes. Design temperature is typically defined as the temperature that a location stays above a certain percentage of the hours in a year. This is typically defined as the 99% design temperature, meaning the location will stay above this temperature 99% of the hours of the year.
It gets cold in the northern area of the U.S. and in climate zones 5 and above. So, don’t we need to worry about what happens when it starts to dip below zero? Should we worry about the one-percenters?
As noted above, the design temperature typically used to define building loads represents the 99% design temperature. This means that on those nights when the temperature outside drops down to -15° or -25°F, the number of hours actually spent at those temperatures will be few. By the time midmorning comes after those bitter nights, the outdoor air is back up to around zero or above and the heat pump is back in its happy place for efficiency and Btu output. Output and efficiency will suffer when it gets bitter cold, but the duration is minimal and the electric resistance elements in the buffer tank are there to maintain the tank temperature for distribution. And because you have done a bang-up job on lowering the distribution needs of the system down to 100°F delivered water, those electric resistance elements may not need to be doing much if anything at all to maintain the system set points.
Air-to-water heat pump equipment is optimized around delivering water at 120°F or less for distribution. Most air-to-water equipment can provide higher delivered water temperatures, but the lower the outgoing water temperature and the return-side water temperature back to the heat pump (Delta-T), the better the overall efficiency and COP of the system. Most air-to-water equipment on the market can maintain a COP of around 2.0 at 5°F outdoor temperature with varying load maintenance/Btu output at that temperature and below. As with other air-source heat pump equipment, Btu output drops along with COP as the outdoor ambient temperature drops, but as I mentioned above, the buffer tank has electric resistance elements to make up for any shortcoming in Btus from the heat pump.
Water as a medium for moving heat energy is superior to air by a wild percentage. A given volume of water can absorb almost 3,500 times as much heat as the same volume of air. What that translates into for the distribution components of an installed system is a 3/4-inch-diameter copper (or PEX-equivalent) tube carrying the same heat energy as the area of an 8x14-inch sheet-metal duct in a forced-air system. This has a major impact on the feasibility and cost of installing a distribution system for a home and frees up space for other ductwork needed for whole-house ventilation systems in modern homes.
There are a variety of low-temperature hydronic emitters available that can be flexible and intermixed for various application needs in a building. Examples of low-temp emitters include low-temperature baseboard units, wall panel radiators, low-mass radiant floors, ductless wall- or floor-mounted fan-coil units, and even hydronic air-coils for use in fully ducted distribution delivery systems. The shared characteristic of all these emitter types is that they are optimized for low-temperature water by maximizing the surface area that the water “sees” while moving through or across the emitter. The choice of which emitter type to use can depend on the aesthetic choice of the occupants, but also more importantly, the Btu input needs of the individual rooms within a home.
Air-to-water heat pumps can also be used in retrofit applications to replace or supplement existing fuel-fired equipment. But again, the important factor for the viability of such retrofits is predicated on an accurate basis of design load for the total building and the room-by-room loads needed for distribution. An existing hydronic distribution system designed for high-temperature water oftentimes cannot be used as is when homeowners move to low-temperature water delivery.
Knowing what the individual rooms of a building need for delivered Btus to maintain set point defines how much surface area the emitters need to have in each room. Sometimes, the existing fin-tube baseboard in a room is oversized for the need, and in these instances, the existing baseboard can be adapted as is to lower water temperatures. But oftentimes, the room-by-room emitters may need to be replaced with low-temp-specific equipment. An easy win for retrofitting air-to-water equipment is in homes where the existing distribution is radiant floor systems that are already operating at 120°F or lower. This is an easy match for a heat pump to deliver.
Another factor with retrofit applications is that there is less control over the existing building envelope, and thus the existing heating and cooling loads for the building. Weatherization is an ideal match of work scope when considering a heat pump retrofit to lower the existing loads as much as possible before trying to match a system. While that can work wonders, oftentimes it may still be necessary or prudent to leave the existing fuel-fired system as a backup to the new heat pump.
Keep It Simple
First, air-to-water heat pumps are nothing special if we are being honest. This is not some new, unproven, space-age technology; it’s simply a heat pump heating water for distribution. Easy peasy. So, what that boils down to is that this is equipment that is approachable and applicable to just about any well-rounded HVAC professional as a system option like any other for heating and cooling. There is equipment being sold in many regions of the U.S. that is well supported by suppliers and manufacturers helping to design and commission these installations.
When considering using air-to-water heat equipment for a project, keeping it simple is always the foundation for system design. The simpler the system, the lower the cost of installation and the more efficiently the system will run overall. Being accurate and realistic with inputs for block load and room-by-room calculations is crucial for sizing equipment and distribution that will meet the building demands efficiently. Old-school rules of thumb for oversizing or estimating capacity do not apply to modern-day HVAC systems. And this goes for mod-con gas boilers as well; garbage in equals garbage out.
Simple delivery equipment for distribution is also key. Keep zones simple, and design for lowest possible drawdown of Btus per loop and lowest delivered water temperature overall. Right-size the buffer tank to maximize the potential for drawdown of Btus as well, so that the heat pump can run in steady state as often as possible to maximize overall system COP.
That said, you can decide what you want to have the heat pump do for the building. Domestic hot water can be included, as can AC, and mixed distribution types (radiant floors and ducted delivery). This all can be available because again, the heat pump is being used to store energy in a buffer tank for use by the building. To add these other functions, you need to add other tanks to the system. This is all on the table for options, though for each additional output type added to the demand on the heat pump, the complexity of the installation, wiring, piping, sequencing, and overall system operation becomes more complex.
There’s a lot to offer in this brave new world where, for better or worse, we seemingly are heading toward a future of fully electrified buildings. If this is to be successful, a foundation in sound HVAC design is absolutely necessary to make this objective functional. Installing adaptable, low-temperature distribution systems into new homes (and retrofitting into existing homes) offers resiliency for the future. Air-to-water heat pumps are well supported with a long track record of successful applications to be used now. If Elon comes up with some new alien tech that supersedes the current benefits of refrigeration equipment, a well-designed, low-temperature, whole-building distribution system will be adaptable to future technology to heat and cool our buildings.
Be sure to check out the rest of the Radiant Heating Skills Workbook.