This time we’ll dig into the common misconception that heat pumps don’t work well in cold climates, and then finish out our technical discussion by tackling the 6th Function of HVAC: Humidification.
A quick review, we spent parts 1-5 working through the importance of the 6 Functions of HVAC:
Last time we finally started digging into what BAD ASS HVAC actually looks like:
Here’s the equipment that’s involved perform the 6 Functions:
We dug into reheat dehumidification pretty hard in parts 2 and 5. It requires a heat pump to do it since you can’t pump heat into an air conditioner coil like a furnace would do.
This is when I hear the usual objection, “you can’t use heat pumps in cold climates!” Let’s dig into that.
Heat Pumps in Cold Climates
While heat pumps may have been a bad idea as recently as 10 years ago in colder climates (US climate zones 5-8), the latest generation of heat pumps, aka cold climate heat pumps or inverter heat pumps, can work in a surprisingly large number of homes.
I live in Cleveland Ohio, which is climate zone 5 with about 6300 heating degree days per year. It’s not a warm place in winter!
In our practice we usually get basketcase homes that are very old and/or leaky. Just like Freud must of thought most people wanted to have an inappropriate relationship with their mom, we thought that houses were more screwed up than they are. The truth is that most people and houses aren’t that messed up!
We’re now alpha testing the HVAC 2.0 software, which is essentially a new way to sell HVAC that helps solve client comfort and moisture problems. Our alpha testers have found that about half of homes can simply switch a furnace out for a heat pump without major modifications.
This matches what we’ve found in our projects, which is that if a house is reasonably tight and insulated, a 3 ton (36,000 BTU per hour) heat pump is more than enough to do the job for houses 2000 square feet and less. That’s a huge swath of existing homes in the US!
For reference, the smallest commonly available furnace is 60,000 BTUs, and most HVAC contractors will get nervous if you ask for one that “small”, when in fact about half that will usually work just fine.
All it takes is doing a little homework and math up front.
We’ve done all of this math and homework. We’ve completed 13 electrifications in Cleveland including removing 10 gas meters, 1 fuel oil tank, one propane tank, and oddly enough one corn furnace. This just isn’t scary anymore.
The operating costs are roughly equal to gas homes, usually within a few hundred dollars per year. Electricity costs here are close to national averages.
The key to our success and confidence? A good load calculation.
How To Do a Good Load Calculation - With Blower Doors and Energy Bills
If you don’t have a blower door number (leakage in cubic feet per minute at 50 pascals), and prior energy bills, a load calculation is just a guess. Let me explain.
I mentioned that a 3 ton/36,000 BTU heat pump can work for about half of homes under 2000 square feet here in Cleveland (and in most of the country!)
Simply by increasing or decreasing the air leakage of a home to levels that I have actually seen in the field, the correct size HVAC equipment for heating a home is +/- 30,000 BTUs. This is not adjusting windows or insulation levels, just air leakage.
I’ve consistently seen load calcs houses come in between 24,000 and 80,000 BTUs by only manipulating air leakage.
That means the margin of error is an entire piece of equipment!
We find that when we have the blower door number, which is how leaky the house is, and prior energy use, we can figure out what size HVAC a house needs +/- 3,000 BTUs. That’s a 10X improvement in accuracy with just a few pieces of information.
3,000 BTUs is also more than good enough to decide what piece of HVAC is best. Heat pumps come in 6,000 or 12,000 BTU increments, furnaces generally come in 20,000 BTU increments. This level of accuracy is more than sufficient.
The REAL problem with heat pumps in cold climates
Sorry HVAC contractor readers in northern climates, but you’re the real problem. You’re petrified of heat pumps. (If you’re the rare exception, please tell us about it in the Electrify Everything or HVAC 2.0 Advanced Discussion Facebook groups!)
Not that I blame you, with load calcs varying by an entire piece of equipment, you don’t want a callback on a cold day when a house won’t heat because the HVAC is too small. It would be your responsibility to replace that equipment with something more powerful, completely at your expense.
The solution is technically pretty simple - run your load calcs using a blower door reading and then true them to prior energy use. Your load calcs will give you courage to size much smaller than you normally would.
The sales and marketing reality is much harsher though - you can’t give blower door tests and load calcs away for free and still run a profitable business.
The solution? Offer your clients a service that includes a load calc and blower door test, which is part of what we do in HVAC 2.0. It’s particularly important to offer to clients that have comfort or moisture problems, a lot of questions and a blower door test are the best way to begin to figure out what the root causes of their problems are.
That’s it, that’s the secret to selling heat pumps in a cold climate and sleeping at night, even on those below zero nights: run your load calculations based on air leakage and energy use to make them accurate. Come join us in the HVAC 2.0 Advanced Discussion group if you’re a contractor and would like to learn more about how to do this.
Heat pumps work!
Suffice it to say, we’ve done a lot of work with heat pumps in a pretty cold climate, and they work just fine if you do a little homework and math up front.
Which means we can use reheat dehumidification to get much better humidity control in shoulder seasons when it’s humid but not hot.
Better humidity control leads to better comfort, and also to better air quality and health.
Speaking of health, keeping humidity at reasonable levels in cold weather is helpful on a number of fronts, including getting sick less often and even reducing stress by reducing cortisol production.
On the flip side, if humidity levels get too high in a cold climate, there’s a high risk of mold, mildew, and/or rot. Problematic levels aren’t all that high. For example, 30% relative humidity at 70 degrees is a 37 degree dew point. If that indoor air touches any surface below 37 degrees (like a window or inside a wall), you’ll get condensation on that surface, which is the precursor to those moisture problems.
Another major risk of over-humidification is mold in attics. Water vapor is buoyant because it’s lighter than air. If your house is leaky that vapor will find its way through leaks in your ceiling and into the attic. If your attic insulation is on the ceiling of the space right below the attic, your roof deck is cold, and that vapor will condense there. This is a huge problem with bath fans that dump into attics, but I’ve also routinely seen humidifiers set to high levels that are creating mold problems in attics.
These problems have a few different solutions.
The third is a building performance solution and is outside the scope of this series, read Comfort 101 if you want to learn more. The first two are related to humidifiers, let’s look at them.
Don’t Put Too Much Water Into Your Air
Traditional humidifiers run as much as 10-15 gallons of water per day through the pad. Most of this goes down the drain, but if you run your fan often or the furnace runs a lot, a big chunk of that humidity will be pushed into your house. We’ve seen this cause attic mold.
We’ve found a simple solution to putting too much water in your air: a water saver humidifier.
Water saver humidifiers have little “tanks” in them that the pad wicks water from, and they only ask for water when those tanks get low. They use much less water and limit how much water you can put into your home.
Our advice is that if you need more water than a water saver humidifier can put into your air to keep your winter humidity in the 25-40% relative humidity range, your house is too leaky.
They cost about the same as standard humidifiers, you just have to ask for one next time you buy a whole home humidifier.
Control Your Humidity With Outdoor Temperature
Like I mentioned earlier, 70 degree air at 30% relative humidity has a 37 degree dew point. If the outdoor temperature is below 37, there’s a chance that condensation could be occurring.
Realistically, this condition is probably fine down to 20 degrees or so, as most parts of your house have at least a little insulation value so surfaces don’t get below 37 F. However, when it gets very cold you should reduce the humidity setting in your home.
If you have a manual humidistat on your furnace, the odds are you are not going to change it as the outdoor temperature goes up and down.
The only realistic solution is to have humidity levels be controlled automatically using an outdoor temperature sensor. There are humidifier controls that include this option, but the better way is to hook your humidifier to a thermostat that detects the outdoor temperature and controls humidity based on it. Nest and Ecobee do it by using conditions at your nearest airport, most high end HVAC equipment has a temperature sensor in the outdoor unit, which is our preferred method.
This way, your humidity levels are controlled automatically, you don’t have to think about them, and your odds of having condensation and mold problems are greatly reduced. Yet humidity levels will likely stay high enough that your skin won’t be as dry, you won’t get sick as much, and your stress levels may even be reduced!
Those are the last technical pieces of BAD ASS HVAC. Now we’ve covered why the 6 Functions of HVAC are important, and how BAD ASS HVAC delivers all of them.
Summing up this part, heat pumps definitely work in cold climates if used correctly. Secondly, humidification is important in cold climates, but some care needs to be used. Be sure to limit how much moisture you add to your air, and use a control that reduces humidity levels as the outdoor temperature drops.
There’s one last part of this series - it’s time to take a big step back and look at how this (fairly) simple system can help drive us towards using more clean energy and unlock The Corsi Code! What’s the Corsi Code? Come back next time!
Nate Adams is fiercely determined to get feedback on every project to learn more about what works and what doesn't. This blog shows that learning process.