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Community and Q&A

Common for heating loads to be MUCH higher than cooling in Zone 4A?

whitenack | Posted in General Questions on

I have done some of my own manual J calculations for a super insulated house I am building. The heating and cooling loads are very close to the same…heating a little higher than cooling.

However, I got a manual J calc from a local HVAC guy who I really like, and his heating loads were double the cooling loads. His cooling loads were very close to the loads I had calculated (within the margin of error), but his heating loads were twice what I had figured.

Is it normal to have heating loads double the cooling loads for zone 4A?

What variables could he be getting incorrect to come up with those results? Air infiltration? Missing attic insulation somewhere (or would that show itself on cooling loads too?)? Window specs? I’m trying to think of a heating specific variable that wouldn’t also increase the cooling loads.


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  1. GBA Editor
    Martin Holladay | | #1

    Heating loads and cooling loads are driven by different variables.

    Lots of north-facing glass increases heating loads, but doesn't affect cooling loads much.

    Thin wall insulation increases heating loads, but doesn't affect cooling loads much.

    Very large amounts of west-facing glass are likely to cause a higher percentage increase in cooling loads than heating loads.

    You should ask your HVAC guy to show you his Manual J inputs, so that you can verify whether they were logical or illogical.

  2. whitenack | | #2

    Thanks Martin,

    I have a copy of his results, and I think I may have found a problem. But let me first say I am WAY over my head in terms of truly understanding what I am doing and I can't thank you guys enough for all the help you guys have given over time. No telling where I would be if you guys weren't offering your guidance. Oh yeah, I'd be in a code-minimum house with a huge HVAC system.

    Anyway, see below for what I think tells the story. The second story results aren't too far out of whack. His heat/cool loads are higher than mine (15.9k/10.6k for him vs 9.5k/6.8k for me), but they are still pretty close considering all the variables and unknowns. For example, my Man.J. calculator didn't have an option for my wall construction, so to get the targeting R value I had to fudge the construction and choose ICF to get to R35. It looks like his software had the same issue and he used just the 2x6 stud and one R6 board. Mine may be a little low and his may be a little high. That's not a big deal to me. The big deal is the 1st floor.

    On my calculations, the first floor load is 15k/9.6k, but their calculations are 27.5k/15.8k. That is a huge difference. But when I think about it further, the ratio between heat/cool is about the same. Almost double. So it may not be so much of an issue of the heat load is way off, as much as something else.

    So I look at the inputs below. Looks like there is a ton of heat loss on ductwork and infiltration. I have no way of knowing how much heat loss you should expect from those two factors, but his results show about 12k BTUs. That seems excessive. I could eliminate half of that loss by going with ductless units, and surely I am going to have a very tight home with all the taping and air sealing we will be doing. So I doubt I could fully eliminate all of the losses from those two factors, but if I could reduce the loss by more than half, the heating load drops down to closer to my aggressive estimates. And then you have the same issues with the lower R walls as above.

    Am I off base here?

    ETA: On another sheet, he has Winter Infiltration as "0.28 AC/hr (96 CFM), Construction: Semi- Tight" Not sure what I should expect on my infiltration, other than very tight, and something less than 3 ACH50. Not sure how that translates.

  3. charlie_sullivan | | #3

    It's pretty common for an HVAC company's manual J calculation to include lots of little fudges making the result larger. Their incentives are all to err in that direction when there's any doubt. It can be worthwhile to hire a third party to do the manual J calculation.

    I would hope that if they design a good system and install it well, they wouldn't have much duct loss, but if you go with minisplits, you can eliminate that loss, whether it's small or large.

    For infiltration, 3 ACH50 and 0.28 ACH actual in the winter are in the same ballpark, but you might do significantly better than 3 ACH50 if you are doing stuff carefully. Does your builder have experience in achieving low ACH50 numbers?

    Also, are you planning a ventilation system, and if so, what?

  4. whitenack | | #4

    Thanks Charile. Yes, my builder has experience building homes that are very tight. With this home, we are taping all the OSB, plus running two layers of polyiso foam on the outside and taping the outside layer of that. We are hoping it is a very tight home, but we will have to wait a while to know for use.

    I haven't completely decided about a ventilation system, but I am currently leaning towards an exhaust-only strategy.

  5. charlie_sullivan | | #5

    If you use exhaust only ventilation, you'll need to count that ventilation rate in your infiltration number for manual J. If you set that up for 96 CFM, that will make your HVAC contractor's number for infiltration correct.

  6. whitenack | | #6

    Thanks for the reply Charilie. I realize that there will be some infiltration, but I wasn't expecting that much. Am I understanding that correctly that it is saying that a single fan running 96 CFM of exhaust would add 6.5k BTU of heating load for 2200 sq ft? Does that sound right?

  7. Expert Member
    Dana Dorsett | | #7

    The square footage doesn't matter- it's all about the 96cfm, and the difference in indoor & outdoor design temperatures.

    The thermal mass of air by volume is about 0.018 BTU per cubic foot per degree F.

    96cfm x 60 minutes/hr is 5760 cubic feet per hour. For that to add up to 6500 BTU/hr means there is 6500/5760 = 1.13 BTU per cubic foot of ventilation flow, which implies in indoor to outdoor temperature difference of about 1.13 / 0.018= ~63F temperature difference, which seems high for a zone 4 location.

    Typical 99% outside design temps are in the ~15-25F range, and at a code-min +68F at +15F outdoors you only have a 53F temperature difference.

    What were the inside and outside temperatures used (both heating & cooling) for the Manual-J?

    Got a ZIP code (for estimating the true 99th percentile temperature bin in your location)?

  8. whitenack | | #8

    Dana, you rock! Yeah, I was thinking that was high...and that's just for the first floor. The 2nd floor shows another 5.7k BTU heat loss.

    The reference city is Lexington, KY, (40508).

    The report shows (Winter/ Summer):
    Outdoor Dry Bulb - 8 / 95
    Outdoor Wet Bulb - 7.04 / 73
    Indoor Driy Bulb - 72 / 74
    Grains Difference - n/a / 26

    These figures seem to be more extreme design temps compared to what coolcalc uses (not that coolcalc should be trusted either).

  9. Expert Member
    Dana Dorsett | | #9

    According to ACCA datasets the 99% and 1% outside design temps for Lexington are +10F and 89F respectively:

    According to AHRAE it's +12.7F and 89.3%

    But note, the 99.6th percentile temperature bin is +6.0F in that reference document- designing to +8F isn't necessariliy insane, even though it adds a bias to the numbers, and would not be required by codes.

    The 72F indoors/ 8F outdoors design temps they used is a 64F F delta which is about where my napkin-math came in (probably rounding errors from using only 2 significant digits on the napkin. :-) )

    Using 72F/+8F as the heating design temps rather than the code-min 68F and a 99% temp of +10F (a 58F delta) exaggerates the total heating load by about 10%. That is a fairly common thumb on the scale to see in HVAC contractor Manual-Js, and by no means the most egregious (it's often 15-20%, and sometimes more.)

  10. whitenack | | #10

    Thanks again Dana! You rock, again!

    The next question is whether 96 CFM is an appropriate estimation for infiltration. I am guessing not, because it looks like he is using an infiltration rate of 0.28 AC/hr, which I have read elsewhere is equivalent to very basic air sealing. With all the air sealing we are doing, I am hoping for an infiltration rate of at most half of that amount.

    On the other hand, I see that there is no calculations for ventilation, so the lack of ventilation loss probably offsets the leaky infiltration assumption?

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