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Community and Q&A

HVAC setup in small basement

JRoyal | Posted in Mechanicals on

Hey folks,
I am finishing my basement and am looking for some feedback on the HVAC selection. I’ve posted about the basement in a prior post but some of the pertinent information is: climate zone 3A, 1500 sqft space, walls and floor will be insulated with 1” rigid foam with the walls having R13-R15 fiberous insulation installed in the cavities. The basement will be moderately air-sealed (as in I’ll seal it the best I can but I’m not a pro). The current HVAC setup has a heat pump and air handler system for each of the finished floors (3 ton main fl, 1.5 ton 2nd story).

I performed a rough manual J calculation using Cool Calc software and got latent and sensible cooling load of 9,500 btu’s. So about ¾ of a ton.

Since the smallest heat pump system is 1.5 tons, I’m concerned that a traditional system would short cycle and not handle the humidity very well in the Georgia summers. I think the normal suggestions I’ve read in this situation is to put in a mini-split. Other than the ascetics concern I have with the wall mounted heads, the main issue I have is my basement essentially has 4 separate areas where the doors are expected to be closed a large portion of the time.

The only other approach I can think of is a variable speed 1.5 ton system with a separated dehumidifying unit. I’m not sure if this setup would be much cheaper than the 4-head mini-split system, plus the energy penalty for the dehumidifier isn’t idea.

So looking for any thoughts or suggestions on how to proceed. Heating loads are not a concern…the one week a year the basement gets down below 60 degrees I can use a space heater.


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  1. user-2310254 | | #1


    Are the ducts for the first floor in the basement ceiling? I suspect that 3 tons for the first floor is way oversized and the load in the basement is probably even smaller than your estimated 9,500 btus. It just seems probable that you could tap into the existing first floor system. Maybe Dana will chime in and tell us what he thinks.

  2. Expert Member
    Dana Dorsett | | #2

    9500 BTU/hr is a lot of load for a 1500' basement, unless it has a lot of unshaded window area, (a west-facing walk out with some big glass sliders?) high occupancy (say, 20 people doing a spin exercise class) or an unusual amount of electrical plug loads.

    How aggressive were you on the input assumptions?

    That said, a 3/4 ton Fujitsu mini-ducted unit can probably deal with it in an aesthetic manner:

    This unit can be mounted either vertically or horizontally, but you have to keep the duct runs short and hard-piped (minimal or no flex). With 4 doored off rooms it's better to install it somewhat centrally to shorten the duct runs. Use only radiused ells, no sharp-throated versions and install the registers either in the ceiling or the wall nearest the mini-duct cassette, not the exterior walls or by windows as is usually done with bigger deal air handlers. Use register grilles with unidirectional throw to accomplish the desired distribution across the individual rooms.

    It can deliver as much as 12,000 BTU/hr of cooling, but it can also throttle back to 3100 BTU/hr, so short-cycling won't be a huge problem. But you may need to run it in "Dehumidify" mode to get the latent cooling your looking for. It's somewhat less efficient running in that mode, but still WAY more efficient and more effective than a short cycling 1.5 1-speed.

    Daikin and Mitsubishi have similar mini-duct cassettes that could be made to work, but they're a bit wimpier than the Fujitsu units.

    Almost all mini-ducted mini-splits are heat pumps, not AC-only, so the heating function is there, even if rarely used.

  3. GBA Editor
    Martin Holladay | | #3

    I'm with Steve. I bet all you need is one or two registers cut into the ducts that now serve the first floor.

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