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Community and Q&A

Heat pump air distribution noise

NearCoastalBC | Posted in Expert Exchange Q&A on

Given that when I move/rebuild I probably will be required to use a heat pump (as opposed to a condensing boiler and baseboards), what is the air noise going to be like in the house? (I’m not concerned about exterior noise from the unit as this can be attenuated with a suitable enclosure). Do new installations use the same big fan feeding a plenum with enough velocity to feed the zones like forced-air furnaces, or can there be a smaller fan feeding the plenum and small “muffin fans” in the zone ducts? Reducing air noise as much as possible will be important, and it seems that low air velocity would be a major factor in this, along with proper duct design and installation.

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  1. paul_wiedefeld | | #1

    A variable speed blower will be very quiet. Mine is inaudible without well designed ducts - yours should be even quieter.

  2. aunsafe2015 | | #2

    With proper duct design, even single stage units shouldn't be very loud. But yeah, a variable speed unit would be even quieter because it would theoretically be running at a much lower fan speed the vast majority of the time, whereas the single stage would be running full speed whenever its on.

  3. walta100 | | #3

    You question is much like how noisy is a car.

    Some cars have no muffler and make lots of noise some cars have diesel motors and make lots of noise with good mufflers.

    If you purchase the lowest cost system it is likely to be noisy.

    If you want a quiet system tell the installer you want one and will pay the extra cost to get it.

    Lower the air speed in the ducts keep it below 200 feet per minute modeled in a computer. This is about half the normal speed. Your ductwork will take up twice the space and cost much more.

    Locate the outdoor unit as far as possible from any bedroom. Most people will not notice the sound from the outdoor unit during the day.

    Buy the top of the line equipment to get extra sound dampeners.



    1. NearCoastalBC | | #5

      I would think 200 CFM would be more than plenty; 400 would almost certainly be unacceptably loud! The house would not be all that big, so the cost of larger diameter ducting to allow lower velocity may not be prohibitive. This is why I was wondering about "muffin fans" in the individual ducts as it would remove the need for one big fan to push the air through the duct system. I put in a boiler and rads in my current house as it is very quiet!

      As I said above, I'm not worried about external noise. That can be dealt with relatively easily.

      As others have suggested, a varable speed fan should help. And certainly paying for a good system and (hopefully) a fully qualified installer. I can help to ensure the latter if I ask enough "awkward" questions :)

  4. Expert Member
    BILL WICHERS | | #4

    If you install a conventional heat pump, the kind that uses an indoor air handler and regular metal ductwork like a convention furnace uses, then the heat pump will sound the same as the furnace would on the inside of the home. The outdoor unit on a heat pump, the part that would be the "air conditioner" with a conventional furnace with a cooling function, would run during the cooling AND heating cycles with a heat pump though.

    If you use minisplits, the outdoor unit runs whenever the indoor unit runs, but my experience is that the indoor units are usually very nearly silent. I'm sure there are some units out there that make more noise than others, and if you crank up the airflow to max you'll hear more air noise, but the units while running in normal operation are very, very quiet.

    Most of the time, when someone complains about their minisplit system being noisy, it's because the OUTDOOR unit was mounted on the exterior of a wood framed house, somewhere about halfway up a wall. That's the worst spot to mount the exterior unit, because the sound couples through the wall! If you mount the outdoor unit on a small concrete pad, or a large precast paver, on the ground outdoors, and route the lines with a loop on their way into the home, you won't have those noise issues. Even if you have to mount the outdoor unit on a wall, there are ways to keep it from being too noisy. There are some other threads on GBA about how to mount these units to keep them quiet, but the basics are to mount to a "hard point" (the edge of a floor, not the middle of a wall), use vibration isolators, and don't run the lineset straight through the wall -- put a "U" loop first.


    1. NearCoastalBC | | #6

      Not concerned about noise from the external unit, which would definitely NOT be mounted on a wall in any case. If the internal air handler is likely to be as noisy as a traditional forced-air furnace, that WILL be a problem, although I've not heard a "modern" forced-air furnace so I may be a bit out of date. Mechanical noise from the internal unit may be possible to attenuate, as it would likely be in a basement primarily used for storage. I'm thinking more of low velocity air (therefore quieter) delivery into the room(s)

      I hadn't seriously considered minisplits so might give them a look.

      By the time I'm ready to do this, much may have changed but it's good to start asking questions early. I'll certainly be going into this project better informed than I was whem I built the current house.

      1. paul_wiedefeld | | #7

        The rub is that existing furnaces are almost always oversized with huge blowers to match, so you’ve only been exposed to poor installation. My furnace was 90kbtu input and extremely loud - when replaced with a 24kbtu heat pump the noise disappeared, along with the drafts.

  5. Expert Member
    Akos | | #8

    Generally any modulating unit with an ECM blower will be reasonably quiet. The rest is ducting as Walta suggested, keep the FPM in your ducts down and they will be silent.

    Generally the low and mid static ducted mini split indoor units are pretty much silent after you connect reasonable sized ducting. About the only way to even know that it is on is by putting your hand in front of a register.

    A 3' or so length of flex between the main supply and any registers also makes a big difference in sound attenuation and reduce cross talk from other rooms. If you want even better, you can get in-line duct silencers.

    From my experience, sound from a ducted mini split is much more pleasant than a muffin fan, closer to true white noise without any of the peaks of the blade pass harmonics.

  6. NearCoastalBC | | #9

    Thanks, everyone. Lots of good information here for someone pretty much at "beginner level" with these things. The principle of decoupling with flex and duct silencers I'm aware of. relatively small

    When the time comes I want to be "fully armed" with as much information as possible so I know what questions to ask of a potential installer.

  7. Expert Member
    BILL WICHERS | | #10

    Another thing to help reduce noise with a conventional air handler (I say "air handler" here because the same info applies to a furnace or a conventional heat pump), is to make sure there is never a straight duct path between any noise source and a register. That means no straight baths between registers in different rooms, and no straight paths between the air handler itself and any vent or register. A common thing I see is to have a single, central return grate somewhere very close the air handler, and a relatively straight, large duct between the two. This lets the fan noise from the air handler right out into the living space. With a duct that makes at least two bends, you will have a good amount of sound reduction. Small flex sections (which can be the little canvas-style flex coplers, you don't always need full sections of flex duct) to isolate the air handler from the rest of the ductwork is also helpful.

    Sound carries best through rigid connections and straight pathways through air. If you if break up the rigid connections with flexible couplers and flex duct, and you make sure the sound has to go around "corners" on the way to any vent or register, you'll make a big improvement in noise. Think about how when you enter a movie theater, the first thing you do is walk around that loop-shaped passageway between the open doorway into the theater and the theater itself. All that pathway is is a few "corners", and they line the walls with squishy sound absorber material. Those two things make the difference between the loud interior of the theater and the outside hallway where you can barely tell anything is going on inside the theater itself.


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