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Will a Heat Pump Work With Your Existing Ducts?

Will Your Existing Ducts Work With A Heat Pump?

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.  (Could happen.)  And recently you heard that you can’t just put a heat pump on the ducts that were installed for your furnace.  Well, let’s explore that last one.

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 air flow rate.
  • Higher air flow requires bigger ducts.

Sounds reasonable, right?  But here’s the first thing that 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 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.

Can you get more oomph out those old ducts?

Let’s say you’ve got the size of the heat pump you need, but it still appears that the required air flow 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 air flow and make it work better.  Here are four ways to squeeze more performance out of those old ducts:

  1. Repair disconnected ducts.
  2. Reduce excessive duct length.
  3. Tighten up the flex duct.
  4. Reinsulate to prevent heat loss and gain in the duct system.

I wrote a whole article about these four things, so go there for the details.

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, so 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.


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 subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn.


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This Post Has 14 Comments

  1. Allison,
    Your comments are very good it may not be practical. After inspecting over 3K-4K homes more than 95% of ducted distribution systems were improperly installed and trying to spend a lot of money trying to make them more efficient may be a loosing proposition. There are dozens of HVAC contractors where I live who think ducts have two legs and swim. There are only around three contractors here that know how to install them properly and two of them actually do testing!!
    It may be cheaper to remove the existing ducts and replace them with new ducts. The problem then is, can you find anyone competent enough to do it correctly.
    BTW, love your post. It clears up a lot of misnomers about Heat Pumps!!

    1. Richard: I also have seen a large number of crappy duct systems and very few done well. And yes, replacing the whole duct system would probably be best, but if the problem of fixing ducts is lack of experienced and knowledgeable installers, that applies also to replacing, as you say. The big issue here is getting HVAC contractors to understand air flow better than they do. A lot of people in the industry are working on that, but it’s going to be a long haul.

  2. Hi Allison!

    I’m going to share this informative article with my provider, NVEnergy in northern Nevada.


  3. So there are two sizing issues, right? Sizing the system for heating and cooling loads (manual J) and then sizing the duct work for the forced air until installed. If you know the CFM output of the existing unit, you can match a heat pump with the same cfm and be fine for duct sizing, assuming the ducts are sized properly, which may (likely) be a poor assumption. Unfortunately many contractors are not going to this level of analysis.

    But the last point is the best point, dual fuel is a great option, maybe the best option.

    1. Scott: Yes, it’s a poor assumption, but that’s where you can interview the homeowners and find out what their current level of comfort is. If they say things are OK, matching a heat pump with the same air flow should work OK if it also meets the heating load for the house.

  4. Going thru this now, connected Heat Pump Air Handler to existing 6″ 1974 square ducts. Prior Air Handler had a very loud 6″ squirrel cage fan. New system has 4, 4″ones. I realize i need more ducts to move sir for sure. Unit runs all the time/ variable 3 speed fan but as I now also have a dishwasher, my power use is down over last year for sure. Now just need to reengineer my duct dynasty for more exchange.

  5. Always had a question about heat pumps. I’m in chilly New England. Most heat pumps that i see draw outside air. Yet our cellars typically are a reservoir of pre-heated air. Why not draw air from them? And pull air into the cellar to pre-heat? Likewise, in summer, cellar air is often cooler, and “redneck AC” pulls this air upward with fans (requires closed windows to augment suction so i don’t use it but a roommate did, quite effectively). Anyway seems a heating/cooling system should use these natural advantages. Am i missing something?

  6. Warning: If you go dual-fuel, don’t let the controls operate the furnace and heat pump simultaneously.

  7. Hello Allison,
    Thanks for the article. I am curious as to the limits of existing ductwork when retrofitted or adapted to low temperature heat sources. I have experience with this using condensing boilers, and now ATW heat pumps.

    When the delivery temperature drops, the output of the system drops along with it resulting in longer run times per thermostat cycle. In the hydronics world this is the natural and desired result of lowering the temperature of the delivery medium.
    In an ATW heat pump set up the coil is decoupled from a direct connection with the compressor by the hydronic loop and the buffer tank. In such a system, the heat pump is controlled by the temperature in the buffer tank. Thermostat(s) call are to the buffer tank and not directly to the compressor.
    I think this decoupling can have positive effects on the operation and performance of the system and compressor and perhaps make is easier to accommodate existing duct work to lower temperature sources, especially heat pumps.
    Thanks again,

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