By Gord Cooke
Professional contractors will recognize the importance of proper sizing to realize the full value of heat pump technology. Oddly, the challenge starts with the fact that heat pumps supply both space heating and space cooling and historically heating loads in Canadian homes have been significantly higher than cooling loads.
If you size for the greater of the two loads, then the cooling capacity would be far greater than needed and this can lead to poor humidity control in humid summer months in most parts of Canada. This can be overcome with specific dehumidification technology, but generally it is better to adjust the sizing of the heat pump to avoid humidity issues associated with oversized cooling capacity.
What is the appropriate size?
There are three primary factors in determining the appropriate size for both new and existing houses and a number of tools and techniques to assist in the development of a decision matrix. To optimize greenhouse gas emission reductions the goal has to be to provide the highest percentage of heating load with the heat pump without compromising comfort, including humidity control, noise and costs of operation.
For many clients, especially at this time of year, the prompt for considering a new HVAC system will be for comfortable and quiet cooling. Whether for a new application or replacement of an existing air conditioner, a client’s focus on cooling performance provides a useful start to heat pump sizing. In new homes there can be an expectation that a comprehensive heat loss and heat gain calculation has been done for the building permit application. Even if it hasn’t been done, house plans and specifications will be available such that a comprehensive calculation using the latest version of the CSA F280-12 (R2021) Standard, Determining the Required Capacity of Residential Space Heating and Cooling Appliances, can be done.
This same methodology could be applied to existing houses with some measurements and assumptions but there are other techniques as well. Given that the federal grant process requires an EnerGuide Energy Evaluation and that process includes some basic load calculations, ask for a copy of the EnerGuide report, which will indicate a design heating and cooling load.
Another option, if there is an existing air conditioner installed, is to use its capacity as a starting point and ask your client how the current system is meeting their comfort expectations.
A word of advice
Heat pump media coverage and a raft of incentives all provide a marketing boost to selling heat pump technology to customers. In the coming months confidently switch your sales efforts away from traditional air conditioners over to heat pumps, or as one insightful contractor suggested, promote them as dual-function air conditioners.
I can say with confidence, in 2023 and beyond, stop selling or installing air conditioners. Switch to offering heat pumps, slightly bigger than the cooling load and check the airflow capacity of the supply duct system. Think creatively as to how to minimize fossil fuel appliances as the back-up heating sources. Encourage other energy efficiency upgrades and provide controls that match and optimize the operation of the heat pump heating function. Even if they don’t use the heat pump during the heating season right now, it will be a win-win for everyone as we strive for 40 per cent greenhouse gas emissions by 2030.
Ensure proper dehumidification
Technology advancements allow more flexibility in cooling capacity sizing. Oversizing the cooling capacity of a heat pump by 25 per cent in order to provide more heating capacity from the system should be possible, as long as appropriate dehumidification strategies are considered in the final system design and installation.
Dealing with existing systems
The maximum size of a heat pump to be retrofitted into existing centrally-ducted systems may be determined by the maximum airflow capacity of the existing supply ductwork. Take simple total static pressure readings across the existing furnace or air handler and measure or estimate the fan flow at the highest fan speed setting.
For over 40 years in Canada the most commonly used total static design pressure across the air handler has been 0.5 in. of water column (w.c.). New furnaces and air handlers are capable of delivering their design airflows at static pressures approaching 0.7-in. w.c. Thus, if the measured static pressure across the duct work of the existing furnace or air handler is less than 0.7-in. w.c. then the higher airflows required of larger capacity heat pumps can be accommodated without major duct modifications.
An even simpler approach, although less accurate, is to recall that the maximum recommended velocity of air in the main plenums of residential systems is 900 feet per minute (FPM) to optimize sound and pressure levels. Therefore, the maximum airflow capacity of the supply ducts can be estimated by measuring the cross-sectional area of the supply trunks near the equipment, before any branch take-offs and multiplying it by 900 FPM. For example, consider an existing T-type supply duct system, where each side of the main ducts is 8 in. by 14 in. The cross-sectional area of each trunk would be 112 square inches or 0.78 ft2, for a combined area of 1.56 ft. Multiplied by 900 FPM, this would allow for a total airflow capacity of 1,404 cubic feet per minute. This supply duct system could accommodate a heat pump with a nominal capacity of 3.5 tons.
I emphasize the supply side ducting because the return air duct side has been traditionally oversized by at least 50 per cent and opening it up to provide for more return air somewhere in a home is much easier than enlarging or adding to the supply side.
How much is enough?
The last major consideration for heat pump sizing is how much back-up heat is needed and what are the options for providing that back-up. Since a heat pump provides both space heating and space cooling, and considering the capacity of air source heat pumps change as the outside temperature changes, sizing is more challenging than for a fuel-fired furnace and air conditioner. Ground source heat pumps aren’t as sensitive to outdoor conditions, since the ground temperature is more consistent than air temperatures throughout the year.
In all cases though the goal should be to maximize the number of hours of heat pump operation, yet still recognize the back-up heating options.
Real world example
Let’s look at an existing home. My son bought a very old home with a 10-year-old high efficiency gas furnace and no central air conditioner. The furnace had an output capacity of 76,000 BTU/h. In a heat pump context that would be just over 6 tons (12,000 BTUs per ton). There is no practical way to put 6 tons into an old home with small duct work. A comprehensive cooling load, accounting for the new low E glass he was planning to install would be 22,000 BTU/h. Applying the 125 per cent rule, that suggested a heat pump size of 27,500 or 2.5 tons. Clearly that isn’t enough to heat the house throughout the coldest part of the winter, even with a cold climate heat pump.
However, with the window upgrades, converting the vented crawlspace to a conditioned space and doing air sealing work, the heat loss calculation showed the new peak winter load would be just 54,000 BTU/h or a 4.5-ton capacity at winter design temperature. This is still not low enough for a heat pump to practically carry the load. There was a need for at least 26,500 BTUs of back-up heat.
With a full electrification goal, that would suggest a 2.5-ton cold climate heat pump to maintain a 27,500 BTU capacity even at design temperatures of -25⁰C and then add the remaining required heat with electrical resistance elements.
That would require a capacity of at least 8 kW. Even if my son wanted to do that, the electrical service wasn’t big enough to support that amount of electric resistance heat running with the heat pump. In his case, the relatively new gas furnace provided a convenient, reliable back-up heat. The goal is to run the heat pump as much as possible and then switch to the natural gas furnace when the heat pump can’t meet 100 per cent of the load. It is important to note that in a traditional furnace with an add-on heat pump, the coil for the heat pump is above or after the furnace heat exchanger (as the photo above shows). As a result, the heat pump can’t be operating when the furnace is on.
Another back-up heat option employs a true heat pump air handler (like the one on the right) and an add-on back up coil. That coil could be electric resistance heat or, in what is referred to as dual fuel or hybrid heating, a hot water coil fed from a gas boiler or tankless water heater could be used. In these cases, the heat pump can be left to operate down to colder temperatures if it is rated to do so. The back-up coil can be operated simultaneously to provide more comfortable discharge temperatures and better efficiencies.
The use of a dual fuel approach can be optimized with the use of thermostats that are able in real time to look at current electricity and natural gas prices as well as outdoor temperatures (for calculating heat pump time-variant efficiency and capacity) to make hour-by-hour decisions on whether to run the heat pump or the back-up heat.
What about new homes?
These same strategies are applicable in new homes. Although the advancements in energy efficiency in most new homes, coupled with rising design day cooling loads associated with ever increasing glazing areas and internal occupancy loads, means that cooling loads are much closer to heating loads. Take for example, a 2,250 sq.ft Net Zero Ready home we recently did calculations for.
The design day heating load was just 31,000 BTU/h and the cooling load was 24,000 BTU/h. In this application a 2.5-ton cold climate heat pump would only need a couple of kW of electric heat to meet all loads. An alternative would again be to employ a hybrid approach with some hot water back-up and still safely use a more commonly available heat pump even if it can’t operate at the coldest temperatures.