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Will my small HVAC [heat pump] move enough air?

dhammond | Posted in General Questions on

Hey guys, I’m completing a new build in Chattanooga, TN. (South end of CZ 4) Using coolcalc and getting as detailed as I can I’ve come up with a sensible load of 28,000. 2300 sq ft single level. Should be pretty tight. R20 walls and R45 sealed roof assembly with continuous polyiso on both. My HVAC guy predictably shot down my suggestion of a 2 or 2.5 ton heat pump and wants to install a 4 ton. His reasoning has nothing to do with the load calc. I have high ceilings. A lot of the house ranges from 10-14 feet. He argues that a smaller unit won’t move enough air to comfortably cool the house. All of my ductwork will be in the basement underneath with ducts coming up through the floor. I’ll have an ERV with its own duct system.

I’m specifically wondering about the logic of using a much larger Hvac system just to move air around. Is that true? Everything I’m reading on this site is pushing toward 1000 or even 1500 SQ per ton. How much does air volume affect that (due to high ceilings) in the real world?

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  1. Jon_R | | #1

    Combine a proper Manual J with S, T & D and it will work well.

    If the concern is stratification, that is solved by having proper register throw (which is duct air flow related, but can still be done correctly at the lower airflow of the 2.5 ton unit).

  2. _Stephen_ | | #2

    Air flow has nothing to do with it, except in really extreme situations, like a 700' long hallway with a vent at one end or something...

    If your house needs 18k BTU of heat, you just don't need massive volumes of air to deliver it. I experience this in my own house. My heat pump will come on at minimum, and the air flow through the vents is so weak, you can't hear it, and you can barely feel it if you put your hand over top of the vent.

    However, the thermal comfort is unparalleled. You're so much better off delivering a tiny amount of heat, with a small amount of air flow over long periods of time, instead of delivering a blast furnace worth of heat for tiny periods of time. The temperature in the house is even, and consistent.

    Don't forget, as you increase your levels of insulation, your internal convection currents become significantly stronger thermal transfers relative to the transfer of heat in and out of the building envelope, and this will act to equalize the temperatures within the structure.

  3. 300TTto545 | | #3

    I've gotten this argument before.
    You are talking cooling load, correct? You mention a sensible load but what about the latent load?
    Advice here is mainly focused on heating. Not that the answers are significantly different but just want to be 100% clear. Especially since the above post is about heat.
    Ok - so when discussing cooling, the windows is your number one issue and you didn't even mention them.
    Next - give us an example of the run length. Presuming a central basement location and a rectangular house, they should be plenty short.
    Load calcs are rarely an exact science and it would seem like 3 tons would be reasonable and acceptable. And you would also probably be fine with 2.5 tons. But I am just guessing.

    I am in Raleigh. My coldest room (in the winter) has a 14 foot cathedral ceiling. It is furthest from the 2 ton HVAC. That floor is 2300 square feet - with nothing above the tall room. It also has the most windows. Now it isn't the hottest. That is a second floor room with some south and west windows. My point here is just that sizing is of course not by cooling alone ... unless it is? So clarify that also.

    And if you aren't doing a heat pump, I would be curious why?

  4. dhammond | | #4

    Yes, I do intend to install a heat pump system. If I can make the budget work, I'd like to use a Trane XV 18 as it has some modulating capability.

    My duct system was installed by a highly qualified installer and it is well sealed. Although he said he did a manual D he hasn't yet produced the results. The air handler is centrally located in the basement and biased toward the SW corner of the house. Because the house is one level, the duct system is way more spread out than I wish. There are 3 trunk runs - each about 30 feet long with short take-offs from there.

    In a perfect world I would have gone with a handful of slim duct units but I couldn't find a qualified installer out here in time to execute the design. And, most of the people I contacted threw out massive numbers to build such a system. My hope is that there will be more available should I choose to retrofit the system down the road.

    I do have a fair amount of glass. My windows are Anderson 100's with Low E. Most of them are covered by a 10-12' porch overhang. A few on the SW side will have some afternoon sun before the trees shade them. Otherwise, sun exposure is well managed. My biggest windows on the SW side should give me some passive solar in the winter when the sun drops below the overhangs.

  5. Expert Member
    Akos | | #5


    Ceiling height doesn't matter much in a well sealed house. I have some tall celings with heated floors, pretty much as low air flow as you can get, never an issue.

    You will get some stratification, so best to have some supply and return registers up high or a ceiling fan to get some mixing.

    There is no excuse for over-sizing equipment. More money and less comfort.

  6. Expert Member
    Dana Dorsett | | #6

    Oversizing by 2x CREATES more comfort problems than it cures. The turn-down ratio of an XV18 is about 2.5: 1. With a 2 ton load and a 4 ton XV18 you've pretty much re-invented the 1-speed heat pump, which only needs a higher output level than it's minimum ~10% of the time, running at max speed only a few minutes at a time (if ever.)

    In Cleveland Nate Adams makes a business out of diagnosing and fixing comfort issues, and is constantly having to inform clients that the comfort issues can't be fully fixed until/unless they down-size their brand new oversized furnace or heat pump to something right-sized. (He's partial to the Carrier Infinity/w GreenSpeed heat pumps, comparable to the XV18.) Modulation range notwithstanding, on a project when he specified a 3 ton and the contractor showed up with a 4 ton he made them take it back. On the heating end it's more comfortable & efficient to undersize it slightly and burn some heat strip to cover the last few percent of load.

    You can read some of his blogs and short videos here:

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