Could we reuse our AC ducts for heat pump heating?
March 20, 2024 9:04 AM Subscribe
We've been looking into replacing our aged central AC and our gas-powered hydronic heat with an air-source heat pump. We already have ducts for the AC, but they are in the ceiling and cooled air flows down from them into the house. The HVAC guy pointed out that if the heat pump pumped heat through those ducts, it would be inefficient because heat rises, so he suggested either building floor ducts (which is expensive) or going with a ductless system. It makes sense, but I just want to check: How inefficient is it? Is there really no way to use these ducts we already have?
Also, cold air sinks, so wouldn't floor ducts also be inefficient when being used for AC?
I know I may have to just write off these existing ducts, but I'd like to know a little more before I do.
Also, cold air sinks, so wouldn't floor ducts also be inefficient when being used for AC?
I know I may have to just write off these existing ducts, but I'd like to know a little more before I do.
Yes, (assuming yours are in the attic) attic ducts are less efficient than floor ducts, but they are used regularly because there are cost and space limitations. If you have the money (and high ceilings) another option is moving them into conditioned space which may mean sealing your attic, or putting them in chases that are sealed in the attic or in living space. Most attics don't have space to even consider that and most people don't want exposed ducting or boxed soffits everywhere, so they deal with the less efficiency.
So what the guy says is technically true, but regular duct runs in the attic are insulated, so are they totally inefficient? No. So going from marginally inefficient to very efficient has costs that may not ever pay back.
posted by The_Vegetables at 9:29 AM on March 20
So what the guy says is technically true, but regular duct runs in the attic are insulated, so are they totally inefficient? No. So going from marginally inefficient to very efficient has costs that may not ever pay back.
posted by The_Vegetables at 9:29 AM on March 20
So what the guy says is technically true, but regular duct runs in the attic are insulated, so are they totally inefficient? No. So going from marginally inefficient to very efficient has costs that may not ever pay back.
Exactly. I don't know which is "better" to the threshold that it makes a difference for you, but it's fairly common for buildings to have high supply for heating or cooling, but it generally works best if you can pair it with a low return vent. So, yes, the heating supply ducts up high will put the heat at the ceiling and since heat rises it'll kind of stay there, but a low return will suck it down and mix the air in the room.
Also, cold air sinks, so wouldn't floor ducts also be inefficient when being used for AC?
No, most people are walking around on or sitting near the floor of their spaces, so putting the cold air in at the bottom of the room is fine.
posted by LionIndex at 9:33 AM on March 20
Exactly. I don't know which is "better" to the threshold that it makes a difference for you, but it's fairly common for buildings to have high supply for heating or cooling, but it generally works best if you can pair it with a low return vent. So, yes, the heating supply ducts up high will put the heat at the ceiling and since heat rises it'll kind of stay there, but a low return will suck it down and mix the air in the room.
Also, cold air sinks, so wouldn't floor ducts also be inefficient when being used for AC?
No, most people are walking around on or sitting near the floor of their spaces, so putting the cold air in at the bottom of the room is fine.
posted by LionIndex at 9:33 AM on March 20
Best answer: You could add a couple ceiling fans for far less than the cost of redoing ducting. We moved to a hybrid heat pump system last year using existing ducts and it's fine. Am intrigued by the water to air system mentioned upthread - didn't know those existed.
posted by leslies at 9:41 AM on March 20 [6 favorites]
posted by leslies at 9:41 AM on March 20 [6 favorites]
Best answer: Most commercial buildings are heated through ceiling vents, including offices, retail, etc. If it didn't work, they wouldn't do it. It might not be ideal, but it is workable. As noted above, having returns near the floor helps the air mix. You may be able to add a few large returns in the bottom of walls if you don't have them already.
posted by ssg at 9:41 AM on March 20 [1 favorite]
posted by ssg at 9:41 AM on March 20 [1 favorite]
Best answer: Hey, HVAC pro here. Your guy isn’t necessarily wrong. He could be right for the wrong reason, or he may be taking a much easier tack to explain what’s going on in the bigger picture. Everyone pretty much gets that heat rises. Getting into static pressure requirements and CFM requirements can get into the weeds and lose people quickly.
Depending on the age of your house, it’s a very real possibility that your ductwork isn’t sized properly for cooling. Forced air gas/propane heating doesn’t require ductwork as large as ductwork for cooling. Lots of houses (especially true of the housing stock in my geographical area) may have ductwork that’s appropriate for and designed for forced air heating. It’s pretty common to find houses where AC was added on later, and the ductwork wasn’t sized properly for it. This can run the gamut of “man the AC struggles when it’s really hot” to “this room/rooms never cool off”. Thing about using a heat pump, is that in terms of airflow, it wants AC CFM more than combustion heating CFM. What you need to figure out is if the AC seems pretty satisfactory in the summer (your subjective opinion), along with your HVAC person doing duct sizing calculations and with a Manual J heat/cool load calculation (the objective opinion). If he can show you the numbers for the load requirement on your house, along with duct sizing requirements, then long term, upgrading your ductwork is not a bad idea.
As to the original question re:floor vs ceiling? Really won’t make much difference. Convection currents will mix the air when the system isn’t running, and a heat pump will be running much longer cycles than a conventional furnace, forcing the air to mix. If he only wants to change duct work for the fact that it’s high, that’s quite a bit of side eye from me.
Obvious disclaimer as I haven’t seen the space in question etc, but redoing ducts may not be a bad idea. Ask to see evidence that new duct work is needed, in terms of sizing calculations and heat/cool load requirements. If he can’t provide those, he’s really taking a big shortcut and I’d be very hesitant to hire him. Most of us are really happy to geek out about our trade if prompted :).
posted by HVACDC_Bag at 10:18 AM on March 20 [9 favorites]
Depending on the age of your house, it’s a very real possibility that your ductwork isn’t sized properly for cooling. Forced air gas/propane heating doesn’t require ductwork as large as ductwork for cooling. Lots of houses (especially true of the housing stock in my geographical area) may have ductwork that’s appropriate for and designed for forced air heating. It’s pretty common to find houses where AC was added on later, and the ductwork wasn’t sized properly for it. This can run the gamut of “man the AC struggles when it’s really hot” to “this room/rooms never cool off”. Thing about using a heat pump, is that in terms of airflow, it wants AC CFM more than combustion heating CFM. What you need to figure out is if the AC seems pretty satisfactory in the summer (your subjective opinion), along with your HVAC person doing duct sizing calculations and with a Manual J heat/cool load calculation (the objective opinion). If he can show you the numbers for the load requirement on your house, along with duct sizing requirements, then long term, upgrading your ductwork is not a bad idea.
As to the original question re:floor vs ceiling? Really won’t make much difference. Convection currents will mix the air when the system isn’t running, and a heat pump will be running much longer cycles than a conventional furnace, forcing the air to mix. If he only wants to change duct work for the fact that it’s high, that’s quite a bit of side eye from me.
Obvious disclaimer as I haven’t seen the space in question etc, but redoing ducts may not be a bad idea. Ask to see evidence that new duct work is needed, in terms of sizing calculations and heat/cool load requirements. If he can’t provide those, he’s really taking a big shortcut and I’d be very hesitant to hire him. Most of us are really happy to geek out about our trade if prompted :).
posted by HVACDC_Bag at 10:18 AM on March 20 [9 favorites]
Best answer: 1. Don’t be afraid to get more quotes. I work with home improvement contractors and homeowners will often call multiple HVAC contractors for a quote.
2. Given HVACDC_Bag’s great response up there, if it turns out your existing ductwork is indeed too small, I will say that everyone I know with a ductless system has been very happy with them. I’m in Arizona and those things get a major workout for five hot months a year.
posted by azpenguin at 1:17 PM on March 20
2. Given HVACDC_Bag’s great response up there, if it turns out your existing ductwork is indeed too small, I will say that everyone I know with a ductless system has been very happy with them. I’m in Arizona and those things get a major workout for five hot months a year.
posted by azpenguin at 1:17 PM on March 20
Best answer: When we had a house with a ductless heat pump, the air handling unit was mounted high on the wall and provided heat and ac, so I'm not seeing why it would be an issue with ducted heat pumps (unless of course the problem is with the ducts themselves).
posted by oneirodynia at 5:25 PM on March 20
posted by oneirodynia at 5:25 PM on March 20
Back in the day of heavy cigarette smoking, I worked in an office with forced hot air heat from ceiling vents. It wasn't very satisfactory. In the office of a smoker, you might find a layer warm smokey air resting on clear cold air.
The fix was to keep the circulation fans running even when the air was not being heated.
HVAC in office buildings is usually not very even. There are hot spots and cold spots because mixing the circulating air is hard.
posted by SemiSalt at 5:14 AM on March 21
The fix was to keep the circulation fans running even when the air was not being heated.
HVAC in office buildings is usually not very even. There are hot spots and cold spots because mixing the circulating air is hard.
posted by SemiSalt at 5:14 AM on March 21
ducted heat pumps (unless of course the problem is with the ducts themselves).
Yes, there are problems with ducts when you are talking a maximally efficient system.
First is that ducts in an unconditioned attic are subject to the external temperature + any radiant heating or snow-based cooling effects when they are not running. So the duct is filling with hot air in the summer (attics can be 150F+ in the summer) so depending on many factors, you are essentially heating up your house (or cooling it in the winter) when you first turn on your system.
Secondly, ducts are in attics and subject to degradation over time, so the insulation or joints fail and unless some AC guy (or you) is up there checking them regularly, you wouldn't know you are heating or cooling your attic.
Low vents actually work better because they are not impacted by radiant heating effects nearly as much, and the ground stays a lot more consistent in temperature. Vents and systems mostly in your your house (if you have a basement) are by far the best because leaky vents are leaking into conditioned space and not quite as subject to the wild temperature swings the outdoors are.
For a regular system, these are relatively minor, but if your state requires high efficiency ratings for new systems (or you do) then they have to be considered.
Office buildings mostly have ducts (mostly) in conditioned space - those drop ceiling panels are not sealed in the same way an attic should be.
posted by The_Vegetables at 7:19 AM on March 21
Yes, there are problems with ducts when you are talking a maximally efficient system.
First is that ducts in an unconditioned attic are subject to the external temperature + any radiant heating or snow-based cooling effects when they are not running. So the duct is filling with hot air in the summer (attics can be 150F+ in the summer) so depending on many factors, you are essentially heating up your house (or cooling it in the winter) when you first turn on your system.
Secondly, ducts are in attics and subject to degradation over time, so the insulation or joints fail and unless some AC guy (or you) is up there checking them regularly, you wouldn't know you are heating or cooling your attic.
Low vents actually work better because they are not impacted by radiant heating effects nearly as much, and the ground stays a lot more consistent in temperature. Vents and systems mostly in your your house (if you have a basement) are by far the best because leaky vents are leaking into conditioned space and not quite as subject to the wild temperature swings the outdoors are.
For a regular system, these are relatively minor, but if your state requires high efficiency ratings for new systems (or you do) then they have to be considered.
Office buildings mostly have ducts (mostly) in conditioned space - those drop ceiling panels are not sealed in the same way an attic should be.
posted by The_Vegetables at 7:19 AM on March 21
also there are now air to water heat pumps specifically for hydronic heating now. not sure where you live but that may be an option for you.
In fact, in places with hydronic heating and no A/C like The Netherlands / Germany / UK, these are the most common heat pumps by far. Note that you may have to get bigger radiators to get good performance from the heat pump, heat pumps like outputting low temperature and if your radiators are sized for high temperature water (which they will be if not very new) then they may be too small.
A similar principle applies to ducted air: The smaller the difference between the air coming out of the unit and the room, the bigger the ducts need to be. Furnaces produce smaller amounts of very hot air (high delta T) and therefore usually need smaller ducts than air conditioners which produce larger amounts of moderately cool air (low delta T). An air conditioner is in fact a heat pump (thermodynamically, I know they're not labelled that way for consumer purposes) and has the same issue: it is very efficient at producing large volumes of moderately cool air, the way a heat pump for heating is very efficient at producing large volumes of moderately warm air.
The thing is, in Boston you have a much bigger heating peak load than cooling peak load so your ducts may well be sized for the max cooling load but not for the peak heating load.
posted by atrazine at 11:27 AM on March 21
In fact, in places with hydronic heating and no A/C like The Netherlands / Germany / UK, these are the most common heat pumps by far. Note that you may have to get bigger radiators to get good performance from the heat pump, heat pumps like outputting low temperature and if your radiators are sized for high temperature water (which they will be if not very new) then they may be too small.
A similar principle applies to ducted air: The smaller the difference between the air coming out of the unit and the room, the bigger the ducts need to be. Furnaces produce smaller amounts of very hot air (high delta T) and therefore usually need smaller ducts than air conditioners which produce larger amounts of moderately cool air (low delta T). An air conditioner is in fact a heat pump (thermodynamically, I know they're not labelled that way for consumer purposes) and has the same issue: it is very efficient at producing large volumes of moderately cool air, the way a heat pump for heating is very efficient at producing large volumes of moderately warm air.
The thing is, in Boston you have a much bigger heating peak load than cooling peak load so your ducts may well be sized for the max cooling load but not for the peak heating load.
posted by atrazine at 11:27 AM on March 21
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also there are now air to water heat pumps specifically for hydronic heating now. not sure where you live but that may be an option for you.
posted by Dr. Twist at 9:24 AM on March 20 [2 favorites]