What is Back Up Heat in a Heat Pump?
Air Conditioners become less efficient the hotter it is outside because they have to work harder to move the heat out of your house. Similarly heat pumps become less efficient the cooler it is outside.
The graph on the left shows the amount of heat a heat pump can deliver at various temperatures. The amount of heat (capacity) drops as the outside air is colder.
For this particular heat pump, it can deliver about 5 kW of heat at -12 C and 12 kW of heat at 15 C. Our houses usually don't require heating about 15 C. The internal loads (TV, fridge Stove even people, etc.) act as the heating above 15 C.
The efficiency of a heat pump can be expressed as the amount of heat delivered divided by the amount of energy used to deliver that heat. This term is called the Coefficient of Efficiency (COP). See the graph above. When the amount of heat delivered and the amount of energy consumed are are expressed in the same units, the COP shows how productive a heat pump can be. Any COP over over 1.0 and the heat pump is delivering more energy than it consumes. This does not defy the conservation of energy law. That extra heat is actually coming from the outdoor air (environment). Just like a refrigerator compressor pulls heat from the ice box and rejects it to the back of the fridge (yes it does check how warm the back of the fridge is), a heat pump gathers environmental heat from the outdoors even in cold winter weather (not extremely cold winter weather). In the graph above the heat pump delivers more energy than it consumes over the entire range of temperatures.
When the COP drops below 1.0 electricity is used as the back up heat (electric resistance heat). This is manufacturer and model dependent but for most occurs around -20 C. This temperature is called the "Cut Off" temperature. See Point B in the graph below. Natural gas can be used as the back up heat but it has a different operational requirement. See below for it's operation.
Note that there are relatively few hours in a winter (even a Canadian winter) below -20 C. Sure lots of days where the minimum temperature drops to below -20 C but that is just for a few hours in the late night, daytime temperatures are generally warmer than -20 C. This means that heat pumps generally run much more efficiently than electric resistance heating most of the time.
Balance Point and Back Up Heat
The graph on the left shows a house's heating energy requirement plotted against the outdoor temperature (orange line). It is highest at the coldest weather and drops off to nothing as the weather warms. The ability of the heat pump to deliver heat (it's capacity) does the opposite and drops as the temperature drops (blue line). This creates a temperature point (D) where the heating capacity of the heat pump matches the house's heating requirement. This is the balance point. For temperatures colder than the balance point, additional or back up heat is required.
In the temperature range between B and D both the heat pump and the back up heat contribute to the house's heating requirements. This is the partial backup heating range.
Heat Pumps with Natural Gas Back Up Heat Have an Economical Balance Point
In Ontario at the moment electrical energy is $.128/kWh. Natural gas energy is about $.045/kWh. So at the moment (March 2020) the ratio of electrical energy cost to natural gas energy cost is 2.8. This means any time a heat pump has a COP of lower than 2.8 it is more cost effective to operate the natural gas backup heating (many jurisdictions have a ratio in a similar range). The point at which a heat pump's COP is the same as the ratio of the heat pump's electrical energy cost to natural gas energy cost ratio is the economical balance point for natural gas back up heat (again 2.8 in Ontario at the moment). Each heat pump model is different but a typical one we studied has an economical balance point at -2 C. So a cost effective way of heating is to use the heat pump in temperatures down to -2 C, then use natural gas backup heating below that temperature. In Ottawa, that amounts to about 60% of the annual heating season energy that can be run on the more cost effective and lower polluting GHG emissions with heat pumps than caused by natural gas furnaces. See this webpage.
A Second Safety Reason for a Different Natural Gas Back Up Temperature Switch Point
Ideally a heat pump coil is placed before the backup heater (furnace) as the cool air returning from the house should be warmed by the heat pump then the backup heater to "top up" any additional heat required as back up heat. If the heat pump coil were placed after the back up heater and both were run at once the heat pump would tend to cool the pre-heated air from the backup heater and efficiency would be lost. In electric furnaces the back up heater can be placed before the heat pump coil. Unfortunately if a natural gas furnace is used the heat pump coil must go after the natural gas furnace. This is because the heat pump is used as an air conditioner in the summer. Air conditioners create condensate which is corrosive to the furnace's heat ex-changer's plates. If the heat pump coil is placed in front of the furnace, the heat exchanger plates could corrode and fail. These plates are safety equipment that separate potentially toxic combustion gases in the furnace from the healthy warm air circulated through the house by the furnace fan. This means the heat pump coil must be placed after a natural gas furnace and both the heat pump and natural gas furnace can not run simultaneously to assist each other (don't worry HVAC contractors with licensed techs are very familiar with this issue and won't install a heat pump unsafely). A little backup heat is required starting at temperatures below around -4 C and designers typically choose to run a heat pump at temperatures at or above -2 C to leave a little safety margin (yes the same temperature as the economical balance point). This arrangement means that when natural gas backup heat is used the heat pump is usually only operational down to about -2 C where as a heat pump with an electric furnace is typically used to -20C.
A Note About GHGs
In Ontario the electrical grid has very little electrical generation from fossil fuels. This means that using an electrically driven heat pump has significantly lower GHG emissions than using a natural gas furnace, Furthermore using electricity as the back up heat also has dramatically lower GHG emissions than using natural gas as the back up heat. To see that difference for Ontario (Ottawa) look at this graph.
Since solar power has no GHG emissions of course a heat pump powered with or offset by solar produced electricity has no net GHG emissions and as site supplied solar power is cheaper than utility supplied electricity it is also the cheapest option in the long run!
If you live in another jurisdiction we can provide your area's graph.
NG - Natural Gas
HP NG - Heat pump with Natural Gas Back Up Heat
HP Electric - Heat Pump with Electric Backup Heat
HP Electric/Solar - Heat Pump with Electric Backup Heat Offset by Onsite Net Metered Solar Powered Electricity
Clear as mud?
Don't worry. We do the energy studies to figure which way is best for you. See our fees webpage for the Space Heating Fuel Transition to Heat Pump Study fees.