By Tom Heckbert
View the full presentation on “The Reason for Backup Heat Systems in Canada” here.
This may be a controversial opinion, but I don’t think it will ever be a good idea to install a heat pump in Canada without some form of back-up heat in place. This is for a couple of reasons.
First, it’s important to note that, while we’re talking about heat pumps far more now than we were even five years ago, they’re by no means new. A refrigerator is a heat pump system, it just exclusively pumps heat from inside the box to outside. An air conditioner is a heat pump, again pumping heat from inside a structure to outside. When we say heat pumps today, we’re really referring to those that have a special reversing valve inside, so they can change which direction the pumping goes. Pump heat out in the summer, pump it in during the winter.
But we don’t have back-up for cooling systems, so why is it necessary for heating? The trouble is that the ability of a heat pump to extract heat from a source, like the air outside, reduces as the temperature of that source drops. In other words, as it gets colder outside, the heat pump can deliver less and less heat, and eventually the heat pump can provide less than what the building needs.
Once the balance point is reached, the heat pump simply can’t keep up with demand, and a back-up source, like a furnace or boiler, must step in to help. There is a balance point for cooling loads as well, but generally it’s higher than most systems will experience.
Canadian cold climate
There might be some that wonder, “Maybe I can just size my heat pump plant to provide more heat? I could use bigger units, but that might cause the unit to cycle on and off too often in milder weather. I could use additional heat pumps, but this adds considerable costs.”
In theory, it can be done. But there’s other problems that can occur, and that is that most heat pumps on the market today won’t run below -30C (-22F). For all Canadian climate regions, this would not be an unprecedented event and, in many of these regions, it would qualify as a regular occurrence. If a cold snap drives the ambient temperature to somewhere below a heat pump’s operational limit, it’s definitely going to be cold enough to be glad you have a back-up system in place.
Even if you can use a heat pump exclusively, it may not be a desirable operation. For heat pumps to be able to extract heat from the air, the surfaces of the coil have to be colder than the ambient air. This means that water vapour in the air will condense out, which is fine in warmer seasons when it drains away, but in cold winter weather, this means the unit will start to form ice. This ice must be removed for the unit to operate. Periodically, the heat pump will move the reversing valve and switch from heating to cooling mode, extracting heat from inside the structure, using it to melt the ice off.
In very cold weather, this can happen every hour, potentially more than once. During these times, you can activate a back-up heat source to keep the system happy, but now you are introducing a considerable amount of cycling into the system, as well as the potential of temperature swings in the occupied spaces, which could make the occupants uncomfortable. As a result, it may be better to switch to a back-up heat source before it’s necessary to avoid these performance challenges. It may also not be desirable depending on utility costs, especially if time of use rates make it cheaper to heat with gas under certain conditions.
Air-to-air
In fairness, most of these challenges are specific to air-source heat pumps, which add or remove heat from the ambient air outside the structure. Water-source heat pumps, like those used in geothermal or hydronic systems, don’t typically have these problems unless the system is designed incorrectly, and the water source is inadequate to cover the load. It is possible to freeze an underground geo heat exchanger if it’s not designed correctly! But these systems typically have a greater level of complexity than air-source systems, and it’s still best practice to include a back-up heat source, just in case.
So, if the recommendation is not to replace a heating system with a heat pump, but rather to layer the heat pump on top of a conventional system, is it still worth doing? For one thing, because heat pumps can fully replace traditional air conditioners, there is some offset in material cost by replacing that equipment. For another, with efficiencies as high as 500 per cent under the right conditions, heat pumps will always be the most efficient means of heating any space, whether or not that’s the most economical or preferable option at all times. If we are to hit our climate goals, it’s simply not possible to get to the system efficiencies we need without deploying heat pumps.
Installing heat pumps will always be recommended when the opportunity presents itself, but designers should always be mindful about the fact that they’re almost certainly going to want a back-up option. Especially on retrofit applications, leave the existing plant in place. There’s no reason to remove perfectly functional equipment when the function it’s doing is still needed anyway. The energy required to manufacture that equipment is wasted if the back-up system is disposed of before necessary.