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Building Science

Replacing an Air Conditioner With a Same-Size Heat Pump

When replacing a furnace with a heat pump, it's critical to determine the heating and cooling loads; do not assume the current system is sized correctly

Sizing a heat pump when you replace an air conditioner and furnace

I recently got a couple of questions from someone who’s interested in changing out his air conditioner and furnace with a heat pump. With the Inflation Reduction Act and talk of electrification, the question is becoming more common. Let’s talk about it.

The questions

Here are the questions I received:

If I replace a 5-ton AC + 100,000 BTU/hr furnace with a 5-ton heat pump:
1.) Will my home heat up more slowly?
2.) Is the total heat generated sufficient heat to the desired temperature of 70 degrees (say, from 45 degrees to 70 degrees)?

I’m sure many homeowners have the same or similar questions about this kind of heating and cooling system replacement. The answer to the first question is: yes. A 5-ton heat pump has a capacity of 60,000 BTU/hr, so it will heat up the home more slowly. Let’s focus on the second question, though.

With an AC and furnace, you’re heating from the furnace, but a heat pump is more like an air conditioner. In fact, a heat pump is really an air conditioner that can operate in both directions. In air-conditioning mode, it pumps heat from indoors to outdoors. In heating mode, it pumps heat from outdoors to indoors. The reversing valve (shown in the photo above) is the component that makes that happen. So how do you figure out if you can replace a 5-ton air conditioner with a 5-ton heat pump?

One way to approach it

The person asking the questions knew that the heating capacity of a 5-ton heat pump doesn’t match that of a 100,000 BTU per hour furnace. He did the calculation to come up with a (nominal) heating capacity of 60,000 BTU/hr for the heat pump and then asked if he should add 5 kilowatts (kW) of electric resistance heat strips for more capacity.

That still wouldn’t get the system to match the 100,000 BTU/hr furnace, so then he asked about the possibility of going with a dual-fuel system. Instead of going heat pump only, a dual-fuel system pairs a heat pump with a furnace. That system uses the heat pump down to a certain outdoor temperature. Then it switches to the furnace. The heat pump can be set to switch to the furnace at whatever temperature makes the most sense.

The answer

Now, many of you reading this already know why that line of thought is taking us down the wrong road. There’s an assumption built into that is not justified. Trying to get the heating capacity of the new heating system (heat pump or dual fuel) to match heating capacity of the existing system assumes that the existing system is sized properly. And that’s almost always not true.

So the first thing to do if you want to change out an air conditioner and furnace with a heat pump is to find out what the heating load is. If the existing system still works properly, one way to do this is to see how long it runs when the outdoor temperature is close to your local outdoor design temperature.

Here in Atlanta, for example, our winter design temperature is 23°F. If a 100,000 BTU/hr furnace runs for 30 minutes an hour when the outdoor temperature is about 23°F, the heating load is about half the capacity of the furnace. That means the heating load in this case would be about 50,000 BTU/hr, so you could put in a 5-ton (60,000 BTU/hr) heat pump and probably be OK.

Keep in mind this method is an approximation. The result you get will depend not only on the outdoor temperature when you time the furnace operation, but also what the outdoor temperature was before you started timing. If it had been 10°F for three days and then warmed up to 23°F, you’ll get one answer (more heating load). If it had been 40°F for a week and then dropped to 23°F over a few hours, you’ll get a different answer (less heating load).

Load calculation and equipment selection

Even accounting for the previous weather, though, is a simplification. Better would be to have long-term runtime data. Few people except energy and HVAC geeks like the people I hang out with have those data. What you can do instead is hire someone to do a heating-and-cooling load calculation. They’ll need a lot of information about the house for the results to be accurate. But with an experienced HVAC designer, you’ll also get a lot of useful information.

And the process includes more than just finding the heating and cooling loads. You really should have them do the equipment selection process as well. Why? Well, especially if you’re going with a heat pump, you’ll need to know that the system can meet the heating load at the design conditions. And if it can’t, you’ll need some kind of auxiliary heat.

I’ve written about these topics in many ways here in this blog, so if you follow the links in this article, you can find out a lot more about this topic. But to go back to the question at hand, when replacing a furnace with a heat pump, don’t assume your current system is sized correctly.


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 writes the Energy Vanguard Blog. He also has written a book on building science. You can follow him on Twitter at @EnergyVanguard. Image courtesy of author.


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