Apr 7, 2023
It’s the Order of Operations with Heat Pumps: PART 2
In part one, guest contributor Jeremy Begley talked about why equipment replacement without any kind of thermal measures was rarely done. In part two, he details why that's the case.
By: Jeremy Begley
Let’s talk about the need to have a “pretty good house” when installing a heat pump. Are there heat pumps that will operate at full capacity at temperatures below 0° F in outdoor temperatures? Yes. And, increasingly, heat pumps that will maintain full capacity at lower and lower temperatures are becoming available every day. The race to the new bottom outdoor temperature at full capacity is in full swing. Can you always size one of these heat pumps to cover your entire heat load? Again, technically, yes. Without getting into a topic for an entirely different blog post, I say “technically” because when sizing a heat pump, the heating load is not the only thing (or even the main thing) you’re concerned about.
Sizing for the Cooling Load
Even in heating-dominated climates, you are primarily sizing for the cooling load because of dehumidification concerns. But with the advent of inverter technology that allows the heat pump to cover the full load at much lower outdoor temperatures and auxiliary dehumidification that can be used to supplement short cycling, you can size more towards the heating load without suffering a dehumidification penalty. You can strike a balance point of sorts between the heating and cooling loads.
This balance is a lot harder (if not impossible) to achieve in a home that has not been air sealed nor insulated in a heating-dominated climate because of the disparity between the heating and cooling loads. That leaves us with a choice between adding very energy-intensive resistance heat or adding a dual fuel option like a high-efficiency boiler or furnace.
These are considerations for inverter pumps made to operate at very cold temperatures. For a regular run-of-the-mill heat pump, you are likely to need quite a bit of help heating a home in a heating-dominated climate, whether the home is sealed and insulated or not. The real question then becomes, will you be comfortable and healthy in an unsealed, uninsulated home (especially in heating-dominated climates) with just a heat pump installed? Can the indoor environmental quality of the home be made better just by installing a heat pump and doing nothing else?
It’s a Comfort Issue
As far as comfort goes, there are a few considerations. These considerations apply, for the most part, unilaterally to any heating plant. Although depending on the type of heat pump, it may take a bit longer to overcome extreme cold than with combustion heat. This is because of the discharge temperature (how hot the air is coming directly out of the unit) of each type of heating plant. A gas furnace has a discharge temperature of around 140°F. A regular heat pump has a discharge temperature of just 90°F. A “cold weather” inverter heat pump has a discharge temperature of around 120°F. It stands to reason that the hotter the air you treat the cold air with, the quicker it will heat up.
This is why in the heyday of “set your thermostat back when you sleep or aren’t at home to save money,” homeowners with heat pumps that had only resistance backup heat as their primary heating plant were cautioned not to use setbacks. In the event of sustained extreme cold, it could take a long time to bring your home back up to a reasonable temperature. From some of the comments in Allison Bailes’ article on a similar topic, “Will a Heat Pump Work in an Old House,” it appears this is still somewhat of a concern. When heating with any “forced air” system, you are heating the air in the space. If the home is unsealed, there will be consistent convective currents (outside air coming in and moving around), which automatically lowers the mixed air temperature.
Then there is the matter of something called the “mean radiant temperature” of surfaces in the home. Dr. Bailes does an excellent job with a simple explanation of this complicated topic in his article, “Naked People Need Building Science.” When surfaces are cold (due to lack of air sealing and insulation), the heat from your body leaves your body and goes to the cold surfaces, the same thing that happens when you walk through the freezer aisle at the grocery store.
It’s the Right Equipment for the Right Situation
As HVAC designers, we look for the right equipment for the right situation. Taking into consideration everything I’ve mentioned and adding the fact that in heating-dominated climates when trying to size a heat pump correctly in a home with little or no insulation, the amount of equipment you need increases rapidly. A heat pump of any kind is rarely the right equipment selection. You should first address the air sealing and insulation.
In conclusion, the order of operations still matters. As Dr. Bailes says in his book, echoing the sentiment of another highly esteemed building science practitioner, Robert Bean, “design for people and a good building will follow.” Taking this approach will rarely lead your design to specify a heat pump for a home with a below-average thermal profile unless your only goal is decarbonization.
Even if your goal is only decarbonization, why not get the best bang for your buck and make your home a nicer place to live than if you just slap lipstick on a pig and call it a day?
So, to the question of can a heat pump work on a non- or poorly air-sealed and insulated building? Sure. It can. But in the words of the late great Jack Rise of ACCA, “The problem with residential HVAC is any damn thing works,” and of the great Chris Rock, “you can drive a car with your feet if you want to, but that don’t make it a good *&%$# idea!”
This article originally appeared on the Insulation Institute blog and is republished with permission.
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