The term heat pump is used a lot these days with many misnomers. Under the umbrella of heat pumps is a niche type that are quickly gaining momentum in both the commercial and residential sectors as a decarbonization tool for hydronic systems — air-to-water heat pumps. Unlike mini-split heat pumps or central air-source rooftop heat pumps, air-to-water heat pumps extract heat from the outside air and heat water, which is then utilized as the distribution medium throughout the building to heat indoor spaces in the winter or even heat domestic hot water year-round.
It has been known for a long time that hydronics is a more comfortable and efficient way to transport energy and condition buildings compared to air-distribution systems, especially in spaces featuring high ceilings. As the marketplace rapidly transitions to net-zero, air-to-water heat pumps are often not considered as a viable solution for building electrification. This is oftentimes due to many other competing technologies that are widely available. They often get overlooked.
Geothermal heat pumps, or GSHP (using water as a heat source via geothermal wells), have gained market adoption over the last decade and can offer very high efficiencies and significant greenhouse gas (GHG) reduction, but they can be cost-prohibitive to install.
Furthermore, GSHPs may not offer a scalable solution to tackle the magnitude of retrofit buildings that will be required to achieve net zero in existing buildings by 2050. Which brings us back to hydronic heat pumps.
If we want to continue using hydronics in our HVAC designs but electrify and decarbonize those systems, then an electric boiler, a ground source or an air-to-water heat pump are essentially the only options. Advancements in technology and availability from more manufacturers in recent years for air-to-water heat pumps has brought this technology front and center in the hydronics world.
Today’s equipment offers operation in lower ambient temperatures with modest outlet water temperatures in the medium to high-temperature range, which can still be suitable for many applications.
When considering an air-to-water heat pump, there are three main considerations for the designer and installer. The first is selecting the operating temperature and being aware of the maximum supply temperature that is available according to the ambient temperature. Next, determine the capacity that the heat pump will be able to deliver, and lastly, figure out the efficiency at which the heat pump will operate.
Incoming technology
Conventional air-to-water heat pump equipment available in the market today can typically provide hot water temperatures of approximately 50 C at -5 C ambient temperature.
New equipment coming to the market over the next several years employing newer refrigerants with lower GWP will offer better heating performance, with some manufacturers even starting to offer equipment that can produce 50 C hot water at -15 C or lower with efficiencies above two W/W (200 per cent) or more at very cold temperatures within this range, and operation down to as low as -20 C.
And this is where the challenge comes when comparing air-to-water heat pump systems to other heat pump types with air distribution. Some other heat pump types, such as VRF systems or packaged rooftop heat pumps, can operate with high heating capacities and deliver adequate air temperatures with external air temperatures as low as -30 C, and come as a package. By heating air directly, the system is typically a bit easier to design. It is also worth noting, however, that some of these air-distribution heat pump systems will still utilize auxiliary heat sources like electric baseboards or duct heaters to handle the coldest days in winter.
This is why we need to think more holistically about implementing heat pumps. The market is very binary when it comes to this; it’s all or nothing. Air-to-water heat pumps, as part of a hybrid system, can offer significant energy and greenhouse gas emission savings, even when only used during a portion of the heating season with natural gas supplementing the few hours where the heat pump is not able to provide adequate performance for space heating.
While air-to-water heat pumps may not be the silver bullet, if we consider a hybrid system with auxiliary fossil fuel boiler to handle the peak winter days where higher water temperatures and more capacity are needed, we can find a happy medium where a substantial period of the year the building can be heated by the heat pump leading to GHG emission reductions and operating cost savings.
Managing supply temperature
Firstly, we need to utilize lower temperatures in hydronic systems. For example, with typical condensing boiler design where the heat emitters in the building are selected for 140 F (60 C), it is important to consider that this is the temperature and capacity needed on a peak winter day at the design temperature (for Toronto this is around -23 C). Usually, with gas systems, we will put our supply temperature on an outdoor-reset curve where the supply temperature is decreased as it gets warmer outside.
This allows us to keep the buildings comfortable and maintain higher efficiencies. This type of strategy works well with heat pumps since the heat pump is able to deliver decent supply temperatures down to about -10 C to -5 C (or -15 C with next-generation equipment).
GHG emissions and energy efficiency
Depending on the electricity generation type by region, air-to-water heat pumps can have significant GHG emission savings for providing the same amount of energy to the building compared to natural gas boilers.
The changeover point from switching between an air-to-water heat pump to an auxiliary boiler could be determined by a couple of factors. It could be determined by the temperature that the heat pump can supply. If the hydronic system cannot operate according to the design with a supply temperature below 45 C, then switch over to the boiler at the ambient temperature of the heat pump that corresponds to this. Alternatively, depending on the ratio of costs between natural gas or electricity, it may be more economical to operate the heat pump only when the COP is above 2.5, in which case you could switch over to the auxiliary heat source according to the COP curve for the heat pump.
Consider the weather data
In today’s HVAC marketplace, we tend to seek a one-size-fits-all solution. The result? We tend to have over-engineered systems but may not realize the full benefit. When we consider the weather data for various climates, we can design heat pump systems to cover 90 per cent or more of the winter hours, and supplement with fossil fuels for only the peak days. The result is a much more flexible system, with balanced operating costs and built-in resiliency.
The chart shows weather BIN hours over the last four years for various Canadian climates and shows the numbers of hours spent on average below -10 C, between -10C to 10 C, and above 10 C where not much heating is required in buildings. Above this point, we tend to move into the cooling season.