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Load calculation questions

FrankFulton | Posted in General Questions on

We have just hired a ResNet rater to complete room by room load calculations in our new home (CZ4, humid), prior to interviewing HVAC contractors.

The first load calculations are about 73k BTU peak heating for the whole house. Our goals are thus 1) confirm model inputs and 2) identify envelope upgrades to lower peak heating load to 60k, so we can pursue a 5 ton air source heat pump. We’d consider zoning but are not sure hoe much this would increase efficiency or comfort with ASHP.

We like and trust the rater, just confirming a few things with the experts here:

1. What is R value of thick (12-18″), below grade poured concrete basement walls? (We estimated R7 due to insulating dirt and reduced delta t below grade.)
2. What is R value of this wall assembly: 12″ stone and mortar exterior + cinder block + furring strip/drywall? (We estimated R4.5, with 1.5 for the stone, 2 for the cinder block, and 1 for the drywall, but the whole assembly is leaky.)
3. Do we really need to plan for heating closed crawl spaces connected by vent opening to conditioned basement space? (Current calculations treat this space just as any other space within the envelope.)
4. If we go with an ASHP in the basement, will we be able to cool the second floor? Heating this winter was no problem (hot air rises), but I’d like to hear from others who have successfully cooled from 2 floors below, before adopting the approach.


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  1. GBA Editor
    Martin Holladay | | #1

    The R-value of concrete is about R-0.16 per inch, so the R-value of a concrete wall that is 12 inches to 18 inches thick is about R-2 to R-3. You don't consider the R-value of the dirt in these circumstances, but the software program should include an input that requests information on the percentage of the wall that is below grade.

    When it comes to the above-grade wall assembly (12" stone and mortar exterior + cinder block + furring strip/drywall), I would estimate about R-1.5 or R-2 for the stone and mortar, about R-1 for the cinder block, and about R-1 for the airspace between the furring strips -- so call it about R-3.5 or R-4. Your calculation of R-4.5 is close enough to mine.

    The fact that an assembly is "leaky" is irrelevant. R-values aren't determined based on air leakage rates. The software has different inputs for air leakage.

    Most software will require inputs on conditioned basements and conditioned crawl spaces. I have no idea wither the software used by your rater was sophisticated or crude.

    If you install a ductless minisplit in your basement, you can't expect the ductless minisplit to cool the floors above.

  2. FrankFulton | | #2

    Thanks Martin.

    The ASHP in the basement would be ducted, not ductless, replacing an old oil furnace/AC combination. We have improved the envelope such that the second floor was warm this winter. But I want to be sure that the ASHP in the basement will be able to cool the second floor (ie, drive cold air up two stories). Is this possible/have others done this successfully?

    We are also unsure about zoning - because the old oil furnace is so inefficient, replacing it with an ASHP (use same old ducts, perhaps add returns where needed) is predicted to reduce heating/cooling bills by about 50%.

  3. Reid Baldwin | | #3

    Since you are replacing an existing system, a good sanity check on your manual J would be the method described by Dana Dorsett at .

  4. GBA Editor
    Martin Holladay | | #4

    A heat pump in the basement connected to forced-air ductwork can easily cool the first and second floor, assuming: (a) the loads have been calculated correctly, and (b) the ducts are properly sized.

    Problems occur when the second floor has a high cooling load (for example, because of unshaded windows or thin ceiling insulation) but a low heating load (due to air leaks through the ceiling and the stack effect). Under those circumstances, it can be hard to tweak the air flows (from the air handler) to work in both summer and winter. That's why lots of HVAC contractors recommend separate heating and cooling appliances for each floor.

    Cue my usual comment: "But if you use a ductless minisplit for each floor, you'll solve this problem..."

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