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Using a heat pump with ceiling ductwork

fBulk | Posted in Energy Efficiency and Durability on

Hello,

I want to install a whole house heat pump because I need to replace my ac condenser anyway and I would like a green option for supplementing my home’s heating needs (we will be getting solar panels).  However, I am concerned about running heat through my ceiling ductwork since heat rises and I’m worried it won’t effectively heat the space.  Any insight?

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Replies

  1. Expert Member
    AKOS TOTH | | #1

    The problem with ceiling ductwork isn't that heat rises, the issue is that if it isn't properly insulated and air sealed in an open attic, you can loose a fair bit of heat. Depending on the house construction, lot of times this is not a hard fix. Air sealing and insulating the ductwork is something worth doing as it can make a big difference also on your AC use as well.

    Ceiling supplies can certainly work for heat, the registers need to be directed towards the ground to get good mixing. If you have high wall registers, you might need to put adjustable grills on them for this.

    This is a good resource on the topic:

    https://www.priceindustries.com/content/uploads/assets/literature/engineering-guides/air-distribution-engineering-guide.pdf

  2. jberger | | #2

    Great Link, thanks for posting!

  3. fBulk | | #3

    Thank you so much for your reply! I am happy to know that this is possible. I am hoping that having warm air from the duct work will provide enough heat to eliminate the need for my oil burner. Is it possible to have the heat pump paired with my hot water baseboards as well? I will also mention I live on Long Island. I am planning on having 40 solar panels installed to generate enough power for my heating and cooling needs. Thanks a lot!

  4. walta100 | | #4

    In my opinion the duct work is the most over looked part of a heat pump install. A poorly designed and installed duct work can and will have a large impact on your comfort and operating costs when using a heat pump.

    In many locations the heating loads are larger than the cooling load so you need larger equipment and larger ducts. Many times when AC unit gets replaced the contractor up sells the home owner offering to replace the old 4 ton with a 5 ton unit for a few dollars more but they almost never change the ducts. It is not uncommon the find undersized ductwork is choking the equipment.

    You should be aware that almost all older duct work is very leaky by today’s standards. Leaky ductwork in an unconditioned attic is a huge waste of energy. Because every bit of conditioned air the escapes the ducts in the attic is lost to the outdoors and depressurizes your house that air is replaced by unconditioned air entering your home thru every gap bringing who knows what else in with the air.

    If you select the right size heat pump with a variable speed compressor and 2 electronic expansion valves that heat pump should heat your home without oil whenever it is over 5°F Assuming you have air sealed and insulated the house to today’s model code requirements.

    It is hard to tell from you question but if the plan is to attach a new 5 ton heat pump to the leaky 3 ton 1950s duct work in the almost uninsulated attic over a drafty old house then slap a few solar panels on the roof and call it green my guess is you will not be happy in the end.

    Walta

  5. fBulk | | #5

    My house was built in 1995 so I'm pretty sure it's not too leaky and it's a pretty open layout. The condenser we would be replacing is a 5 ton. I'd prefer not to use the duct work for heating since I don't love the feeling of forced hot air. I'd prefer if the heat pump could hook up to my existing hot water baseboards. Any one know if this is possible? Thanks!!

  6. Expert Member
    AKOS TOTH | | #6

    I don't know what you local heating oil/electricity costs are, but here it is about the same as electric resistance heat. If you use heat pumps which have 2 to 3 efficiency ratio, heating with a heat pump would be significantly cheaper than oil.

    If you are already replacing the AC unit, going with a heat pump is a no brainer. You can use the heat pump to do bulk of you heating, with the baseboards a backup heat.

    As the folks above said, there is a lot more design that needs to go into the system. Without doing that, is how you end up with comfort issues, which is what actually meant when they don't like the "feel" of duct heat. A properly designed heat pump setup is as comfortable as hydronic heat, but it takes some work to get it right. This also means sizing the heat pump properly, 5 ton AC is quite large for new construction but depends on house size and climate.

    Heating water with heat pumps is generally a very expensive proposition, very limited selection of equipment for it, even fewer installers that can work with it. If you want to keep hydronic and want to get rid of your oil burner, your best bet is an electric boiler.

  7. Expert Member
    Dana Dorsett | | #7

    Since you have a heating history on the place, run a fuel-use based load calculation using the oil-burner as the measuring instrument for the load:

    https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

    A 5 ton condenser would be seriously oversized for the air conditioning loads of "normal" sized houses (with a few rare exceptions) but could be right-sized for ~7000' house. Now is the opportunity moment that only comes along every 20-25 years to right-size it.

    As much as I hate rules of thumb, a ton per 1400 is about the middle range when looking at careful Manual-J calculations. Most HVAC hacks will use a ton per 750' or even a ton per 500', so I'm going to go out on a limb and hazard that your house is between 2500- 3800' of conditioned space (?), and could be cooled adequately with a 2-3 ton heat pump, even when allowing another half-ton for duct losses (assuming insulated and sealed ducts.)

    But that range around the "typical" ratio is pretty large as Allison Bailes demonstrated in this graphic plotting house size against square feet per ton ratio:

    https://beta.greenbuildingadvisor.com/app/uploads/sites/default/files/images/Bailes_graph_for_Manual_J_blog.preview.png

    https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb

    FWIW: As it happens, prior to my moving in at some point in the early to mid 1990s somebody installed a 5 ton central air in my 2400' house, using the same circa 1923 uninsulated hard piped duct system used by the original heating system. Even on days when temperatures are 5F or more over the 1% design temperature it's duty cycle is well under 40%, closer to 30%. Assuming a 35% duty cycle that means the actual load is at or under (0.25 x 5 =) 1.75 tons even when it's hotter than the 1% design temp. My house would be much better served with a 1.5 ton or 2 ton AC. As it is now the duty cycle is so low that the kneewalled head-banger ceiling half-story upper floor cooks between cycles, and the only way to keep it comfortable up there is a half-ton window-shaker. Using zoning dampers and serving a zone with a half-ton or so of load with a 5 ton AC wouldn't cut it either.

    It's probably too late in the season to measure the duty cycle- there probably aren't any 1% outside design temp days left in this season, but that would have been a sure-fire way to measure both the combined whole-house load + parasitic duct loss load.

  8. fBulk | | #8

    I had no idea a 5 ton unit would be so oversized! My house is 3,00 sq. ft. How do I get an HVAC professional to agree to swap out a 2 or 3 ton unit? It seems that each one I have spoken with makes it seem that a 5 ton is a given. We actually just moved in so I don't have my home's energy history, but the boiler and condenser are 25 years old and look like they're about to go so this is why we are heavily considering this revamp.

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