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OK, you’re thinking of replacing that old furnace in your home with a new heat pump but you have doubts. You’ve heard that heat pumps don’t heat as well as furnaces (wrong), or that they’ll jack up your electricity bills because of the auxiliary heat (maybe). And recently you’ve learned that you can’t just run a heat pump using the ducts that were installed for your furnace.
Let’s explore that last claim.
How much heat do you need?
Here’s the reasoning behind someone saying the ducts for your furnace won’t work for a heat pump:
- A furnace makes hotter air than a heat pump.
- To get the same amount of heat from a heat pump, you need a higher airflow rate.
- Higher airflow requires bigger ducts.
Sounds reasonable, right? But here’s the first thing you need to know to keep your heat pump dreams alive: Your furnace is almost certainly oversized. It’s extremely rare to find a right-sized heating and cooling system, whether it be a furnace, air conditioner, heat pump, or boiler.
The condo I used to live in, for example, had a furnace rated to provide 66,000 BTU/hour of heat. When I calculated the heating and cooling loads to size the new air conditioner we needed, I found that the actual heating load was only 23,000 BTU/hour. So the furnace was about THREE TIMES LARGER than it needed to be.
The first step, then, is to get a load calculation done and find out how much heat you actually need. That should allow you to downsize the total heating capacity as you transition from a furnace to a heat pump.
More mileage from old ducts
Let’s say you’ve got the size of the heat pump you need, but it still appears that the required airflow is more than the furnace had. Don’t give up. Even without a full duct replacement, a knowledgeable duct pro can reduce the resistance to airflow and make it work better. Here are four ways to squeeze more performance out of those old ducts:
- Repair disconnected ducts.
- Reduce excessive duct length.
- Tighten up the flex duct.
- Re-insulate to prevent heat loss and gain in the duct system.
I wrote an article about these four things, “Fixing Problem Ductwork,” so be sure to read that.
Should you do it?
As someone who has replaced a furnace with heat pumps in my house, I’m a big proponent of going in this direction. Heat pumps are often better for comfort, can improve your indoor air quality, and help reduce emissions of carbon dioxide and other pollutants. Electricity keeps getting cleaner every year, you know.
Of course, one way to ease into heat pump world would be to keep the furnace (assuming it’s still got years life left in it) and pair it with a heat pump. This is called a dual-fuel system. It relieves you of having to worry about the ducts because the furnace takes over when you need more heat. But since the majority of the heating hours occur when it’s not too cold for the heat pump, you’d still get a lot of benefit from this setup.
In the end, you have to get a good analysis from an HVAC pro who understands heat pumps. Or study up enough yourself so that you understand the pros and cons, benefits and pitfalls. There’s a lot of good info about heat pumps available these days.
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Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel. Photo courtesy of author.
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9 Comments
Timely article, as my oil furnace has been on the fritz for two weeks, waiting for a technician to fix it. We have a wood stove but it's not big enough to keep the house comfortable.
I've been planning on adding mini-split heat pumps soon but now I'm thinking of just tuning up the existing ductwork and getting a central heat pump/air handler. It's good to know that you think that's a good idea, at least in concept.
Mike: As with everything like this, it depends. And you know that as well as anyone.
Allison inspired me to purchase my own copy of WrightSoft core. Getting ready to have Energy Vanguard right size the heat pump and ductwork. But wanted to take a stab at the heat transfer for my PGH.
BirchwoodBill: We're happy to work with you on your HVAC design.
For anyone else looking for help with that, check out our website:
https://www.energyvanguard.com/hvac-design/
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Dear Allison: Thanks for another great article! I agree completely with your observation that existing HVAC systems are typically WAY oversized - thanks to the "rule of thumb" used by many HVAC contractors instead of Manual J calculations. I ran a Manual J analysis on our home in Massachusetts for an AC system - which was well insulated and air sealed - and came up with 24,000 BTUs or 2 tons. The HVAC contractor choked when he heard this number, for our 1,700 SF home. The smallest 'brand name' condenser we could get was 2.5 tons, with the efficiency rating we wanted. Running a Manual J report for your home isn't difficult with ACCA approved solutions, e.g. Cool Calc. (See: http://www.acca.org/standards/approved-software) You will need to spend some time measuring windows, and checking details on your home, e.g. insulation.
I’m a homeowner who has replaced a natural gas furnace with a Mitsubishi heat pump, and it has worked for me without a hitch. My heat pump has performed well during unseasonably cold weather. It had no problem handling temperatures below 15 degrees. It also cooled during temperatures above 100. Both these temperature events were outside the standard temperatures expected in my Pacific Northwest climate. I am very happy with the experience of using the heat pump during average temperatures as well. I later replaced a gas heater in an extension in the house with a Mitsubishi ductlesss heat pump. Bills have not been a problem. My gas bill virtually disappeared and my electric bill went up less than my gas bill declined. My house has fewer cold and hot spots.
I had some years before installation upgraded my insulation and had my ducts insulated and retuned, which helped I guess.
Overall a great experience. I have lived with the heat pump for more than four years.
Dear PESC: Thanks for your post. We are also planning to install a heat pump for our home along the shore in Massachusetts - for all of the reasons you highlighted. Our existing natural gas furnace can remain in place, as backup. Plus, we are planning to install some solar panels on the roof, now that a big tree had to removed next door - due to rot.
We’re currently renovating, and replacing the old furnace with a heat pump system was one of the best decisions we made! Using existing ducts can work well, but make sure they’re properly inspected for leaks or sizing issues, heat pumps can require better airflow. Sealing and insulating the ducts saved us from losing efficiency. If you’re upgrading to something more energy-efficient, I’d also recommend checking out smart thermostats. Also, when choosing new fixtures during the renovation, I found Modern Forms customer service https://www.pissedconsumer.com/company/modern-forms/customer-service.html super helpful for lighting solutions.
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