I’m a huge advocate for air source heat pumps and electrification but I’ve seen a lot of questions and miss-information online on sizing an air source heat pump for your house.

Rules of thumb are often innaccurate

Part of the issue is that most, if not all, HVAC companies use simple rules of thumb to size air source heat pumps that can be very inaccurate. Typically the only detail they want from you to size your air source heat pump is the area (square footage) of your house but that does not take into account the level of insulation, type and quantity of windows, form/shape of your house, and air tightness. These lookup tables for air source heat pump sizing can be very wrong.

Energy Audit Recommended

Here in Canada one of the best ways to determine how large of an air source heat pump (or furnace!) you need, is to get an energy audit done. During the energy audit the energy advisor will document all of the items above (level of insulation, type and quantity of windows, & form/shape of your house) and also perform a blower door test to determine how air-tight your home is. Based on this information they use a program called HOT2000 created by Natural Resources Canada to calculate the design heating & cooling loads for your house. This design heating load is way more accurate than the rule of thumb area based lookups and I would recommend that anyone thinking of replacing their furnace with an air source heat pump pay the $300-600 to get one of these energy audits done.

Where I live in Hamilton a great company to get an energy audit done is Green Venture.

Design Heating Load

Once you have the energy audit complete, you can find the Design Heating Load in the “Homeowner Information Sheet” under Mechanical Systems > Space Heating > Design heating load. Below is the one from my report:

Air Source Heat Pump Capacity Varies by Outdoor Temperature

Another confusing aspect is that the capacity of an air source heat pump varies with the outdoor temperature, and they all lose capacity as it gets colder outdoors. So to properly size an air source heat pump you need to know how cold it gets in your area, and it is recommended to use the design temperature for your location. The design temperature is based on historical records and is the temperature that 99% of the time the outdoor temperature is higher than that amount.

In Ontario you can look up the design temperature for your location in the tables in MMAH Supplementary Standard SB-1 or the ASHRAE Climatic Design Conditions.

For example, where I live in Hamilton, Ontario the design temperature is -19C / -2.2F (SB-1 2014) or -17.6C / 0.32F (ASHRAE 2021) which means for 99% of the 8,760 hours (24 hours x 365 days) in the average year it is warmer than -19C / -2.2F in the case of the SB-1 and -17.6C / 0.32F in the case of ASHRAE. To be safe I would use whatever temperature is lowest.

Looking back at my design heating load above, if I was sizing a heat pump for my house I would need to get a system that could produce 17.74 kW at the 99% design temperature of -19C / -2.2F.

Backup: Electric vs Gas Furnace

A thing to note when comparing electric resistance to a furnace is that electric resistance can be run concurrently to your air source heat pump so it adds to the capacity of the air source heat pump, but gas furnaces run instead of the air source heat pump so they need to be sized for 100% of the design heat load.

The 99% design temperature is irrelevant when picking a gas furnace or electric heat as they are not affected by the outdoor temperature, but it is very important in picking an air source heat pump as they lose capacity in colder weather, as noted above.

kW vs Tons vs BTU/hr

What can be very confusing is that there are three different units used for the heating capacity of heating systems. Gas furnaces are typically sized by BTU/hr, heat pumps are sized in Tons, and electric backup in kW and it can be very confusing. It is really helpful to convert the design heating load into both BTU/hr and Tons to help understand what is needed to heat your home.

And again, the capacity of an air source heat pump is based on a set outdoor temperature which is typically 8C / 47F and all air source heat pumps lose capacity as it gets colder outside.

How to size your system

So the first thing we should do is to convert the design heating load from kW into both BTU/hr and Tons so that we can easier compare this to what is available.

Conversion	kW	Tons	BTU/hr
KW to Tons or BTU/hr	1.00	0.284345	3,412.142
Tons to KW or BTU/hr	3.52	1.000000	12,000.000
BTU/hr to KW or Tons	2.93	0.833333	10,000.000

For my example design heating load of 17.74 kW this gives:

	kW	Tons	BTU/hr
Example Design Heating Load	17.74	5.04	60,531

Option 1: No backup, heat pump only

If you wanted to heat my house exclusively with a heat pump and avoid any type of backup, I would need to find one that could output a minimum of 5.04 tons / 17.74 kW / 60,531 BTU/hr at your locations 99% design temperature of -19C. If you can find (and afford!) a heat pump that large then you wouldn’t need any backup heat at all.

I did look on the NEEP Air Source Heat Pump list and there is a single heat pump on the list that can meet this criteria, a 6 ton VRF Gree Ultra Heat that appears to typically be used for small commercial buildings. There are likely more heat pumps available on the commercial market but pushing beyond 4 tons is getting beyond the capacity of typical residential systems.

In Hamilton, if your design heating load is less than or equal to 10 kW / 2.84 Tons / 34,534 BTU/hr you likely can find several residential heat pumps available that could take on 100% of your design heating load at the design temperature.

Option 2: Electric backup

For electric backup I’m going to start by assuming a 10 kW electric heat unit as that is a common size and it’s easy to adapt this process to smaller or larger electric heaters.

So based on this assumed 10 kW electric backup, then you can deduct 10 kW from the design heating load, which in my example would give me 7.74 kW (17.74 kW less 10kW). As electric heat elements can run concurrently to the air source heat pump you only need to cover the difference between your design heating load and the output of your electric heat.

	kW	Tons	BTU/hr
Example Design Heating Load	17.74	5.04	60,531
10 kW Electric Heat	10	2.84	34,121
Minimum Heat Pump capacity at Design Temp	7.74	2.20	26,410

So assuming a 10kW electric heat backup, I would need to get at minimum a heat pump that can output 2.2 tons of heat at my locations design temperature.

Option 3: Gas furnace backup

If I wanted to go with a gas furnace backup I would need a gas furnace rated for 100% of my design heating load as a gas furnace cannot be run at the same time as a an air source heat pump. But because your gas furnace is sized for 100% of your design heating load there is no minimum size of heat pump required because your gas furnace can provide 100% backup.

	kW	Tons	BTU/hr
Example Design Heating Load	17.74	5.04	60,531
Gas Furnace Backup	17.74	5.04	60,531
Minimum Heat Pump capacity at Design Temp	0	0	0

In this option I would recommend actually using the square foot / area based rule of thumb to figure out the recommended size of Air Conditioner and using that as the minimum size of air source heat pump to buy. For example if based on the area of my house it is recommended that I have a 2 ton air conditioner, I would use that as the minimum size of an air source heat pump in a gas furnace backup scenario. The maximum size of heat pump recommended would be one that could produce 100% of your design heat load at your design temperature, just like in option 1 above.

What type of backup should I use?

So while I’m hoping the above helps you understand the minimum size of heat pump required for the various backup scenarios it doesn’t really help you pick what size of heat pump to go with as each option has its advantages and disadvantages.

	Advantages	Disadvantages
No Backup, Heat Pump Only	> Lowest electrical usage > No greenhouse gas emissions > Can help avoid electrical panel upgrade > Allows you to get off gas	> Highest cost to install due to very large heat pump required > Even with a variable speed heat pump the unit might not be able to ramp down low enough to avoid short cycling in sholder seasons > Requires very large ductwork that might not exist in older homes > The heat pump may be oversized for the cooling season
Electric Backup	> Lowest cost to install > No greenhouse gas emissions > Allows you to get off of gas	> Electric heat backup pulls a lot of electricity and this typically requires at least a 200 amp service to your home > Electric backup is typically more expensive to operate than gas backup
Gas Furnace Backup	> 100% backup > Depending on the cost of gas & electricity this can be the cheapest to operate	> Requires you to keep a gas service to your house year round even if you only need gas for 1-2 months > Great for single speed or small heat pumps

Note: It is a common misconception that if the power grid goes down you are better off with a gas furnace, but all gas furnaces require electricity to run so if the grid goes down so will your gas furnace. That being said, it doesn’t take a very large generator to power a gas furnace so if you are concerned about the grid going down and you are planning on buying a generator you will need a much smaller generator if you have a gas furnace backup.

Recommended sizing

What is typically recommened is to get a heat pump that can output 80-90% of your design heating load and do the rest with backup. The outdoor temperature varies a lot and for the vast majority of the heating season your heat pump will be able to provide 100% of your heating needs and the backup will only be required on those few days or weeks when it gets really cold.

So again using my example of 17.74 kW design heating load, at 80% of that would give the following heat capacity:

	kW	Tons	BTU/hr
Example Design Heating Load	17.74	5.04	60,531
80% of the design heating load	14.19	4.04	48,425
Remaining heat load	3.55	1	12,106

80% of my example heat load is almost exactly 4 tons which works out nicely as heat pumps are typically sold in 2, 3, and 4 ton sizes.

Warning: while this seems to be saying if I get a 4 ton heat pump I will only need 3.55 kW of electric heat, remember that we need to look at the capacity of the specific air source heat pump we are considering at our location’s design temperature. As stated several times, all air source heat pumps lose capacity in the cold so you need to calculate the output of your air source heat pump at your location’s design temperature.

Note: If you get a gas furnace or you get an electric backup that is sized at 100% of your design heating load then you don’t have to worry about what size of heat pump you have as worst case your backup gas furnace or electric heat can take on 100% of the load.

Minimum Electric Backup Sizing

To correctly determine the minimum size of electric backup required, you need to pick specific make and model of air source heat pump and look up the details on how much heat it produces at your locations design temperature.

For this example I’m going to use the Gree Flexx 4 ton ducted air source heat pump and the NEEP Heat Pump List. This is a great air source heat pump that is very cost effective and while it is rated to work down to -30C / -20F it loses a substantial amount of capacity at lower outdoor temperatures so it is helpful to use it as an example.

Gree Flexx 4 ton central ducted heat pump

What we are looking for is the max heat output at our design temperature. As stated above, the design temperature for Hamilton, Ontario is -19C / -2.2F which falls between the last two rows of this chart, but I’m going to use the last row as a worst case scenario.

Warning: What is very confusing about the above table is that there is a kW number in the max column but that is the input kW not the output kW and we need to ignore that number for load calculations. The input kW is telling you how much electricity the heat pump uses at that outdoor temperature, not how much heat it can putput.

	kW	Tons	BTU/hr
Example Design Heating Load	17.74	5.04	60,531
Output of 4 Ton Gree Flex at -22F / -30C	6.89	1.95	23,500
Remaining heat load	10.85	3.05	37,031

So even though this particular heat pump is technically a “4 ton” unit, at -30C / -22F it puts out less than 2 tons of heat at -30C / -22F so if your design temperature for your location was that cold you would need just over 10 kW of electric backup.

What did I go with?

As I was shopping around and getting quotes I was very impressed with the low temp performance of the Fujitsu XLTH heat pumps and so that is what I went with. As a comparison to the Gree Flexx unit above, here are the performance charactics of my “4” ton Fujitsu central ducted heat pump.

What really blew me away about this Fujitsu unit is that while it is called a “4 ton” air source heat pump it actually produces 4.25 tons / 51,000 BTU/hr / 14.95 kW of heat all the way down to -15C / 5F, and at the bottom end of its operating range it is still producing 81% of its 4 ton rated capacity all the way down to -26C / -15F, which is very impressive. In addition to this impressive heat output across its operating range it also is very efficient with a COP of 2.34 at -15C / 5F and 1.82 at – 26C / -15F.

In comparison the 4 Ton Gree Flexx heat pump mentioned above can only produce 3.25 Tons / 39,000 BTU/hr / 11.43 kw at -15C / 5F and drops to 1.95 tons / 23,500 BTU/Hr / 6.89 kW at -30C / -20F. Also the Gree Flexx is less efficient with a COP of 1.8 and 1.23 respectively. The main advantage of the Gree Flexx is it is less expensive and can operate at lower outdoor temperatures. If the design temperature for your location was close to -26C / -15F and you wanted the Fujitsu you likely would need to get a backup heat source sized at 100% of your design heat load for when it got so cold out that the Fujitsu couldn’t operate, but if you had the Gree Flexx you could still count on its heat output all the way down to -30C / -26F

In terms of backup I went with a 10kW electric heat element so if we calculate my heat output at -15C / 5F we get the numbers below:

	kW	Tons	BTU/hr
Output of “4 Ton” Fujitsu XLTH at -15 C / 5 F	14.95	4.25	51,000
10 kW electric heat	10.00	2.84	34,121
Total system output at -15 C / 5 F	24.95	7.09	85,121
Example Design Heating Load	17.74	5.04	60,531
Surplus heating capacity at -15 C / 5 F	7.21	2.05	24,590

And again for -26C / -15F we get the numbers below:

	kW	Tons	BTU/hr
Output of “4 Ton” Fujitsu XLTH at -26C / -15F	11.43	3.25	39,000
10 kW electric heat	10.00	2.84	34,121
Total system output at -26C / -15F	21.43	6.09	73,121
Example Design Heating Load	17.74	5.04	60,531
Surplus heating capacity at -26C / -15F	3.69	1.05	12590

So as you can see for temperatures all the way down to -26C / -15F my system has more than enough capacity to heat my home.