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HVAC – Smaller HVAC for better than code insulation?

airfix | Posted in Mechanicals on

First off, – New home, climate zone 6A – pretty good home – target <1.5ACH50. My HVAC guy provided me with manual J calculations for sizing the HVAC system and I noticed he just used code insulation values and average house build quality to determine the loads. When I asked him to re-run the numbers for my actual insulation values he said he could do that but it won’t make any difference to the hvac system he installs as it will make no difference to the equipment installed. I find it very surprising that running about 30% better insulation in the walls and 60% better than code in the roof that it has zero effect on my HVAC. Do I need to find another HVAC guy or does this make sense? FYI Roof as planned ~ R-68 Above grade walls as planned ~ R-33 Basement walls as planned ~ R-24 Windows are all about ~U-0.3 Door are all about ~U-0.4 Windows are about 20% of my total wall area (above and below grade). Steve

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  1. Expert Member
    Dana Dorsett | | #1

    That's exactly why you don't let the HVAC guy run the numbers. He has already figure out what HE intends to install, and adjusts the numbers to suit, or runs a pro-forma code minimum Manual-J to make the case ( which is a waste of his time and yours).

    But since you haven't told us what the load numbers are and the equipment proposed it's hard to really say. If it's already the smallest-available equipment it might not matter.

    I've personally given up on letting HVAC contractors run the load numbers & letting them spec the equipment. It's better to have a qualified third party run the load numbers, then YOU figure out the equipment that is the best fit. Then get bids on installing the equipment YOU think works. It's good to listen when they push back and propose something different or bigger, but they really need to make a good & rational case as to why it needs to change. (Some HVAC contractors actually know enough about the equipment to make rational changes to the spec, but sadly that isn't the industry paradigm.)

    The center-cavity R values aren't a particularly good way of evaluating the heat loss through the assembly. It's not hard to come up with examples of an "R33" wall that has nearly twice the heat loss of some other "R33" wall. It's the "whole-wall R" after factoring in all of the thermal bridging and R-valvues of the other materials in the assembly that determines the U-factor.

    So get a real Manual-J (and AGGRESSIVE, per the Manual's instructions) from somebody who makes their living & reputation on the accuracy of their numbers. Present the numbers to the contractor(s) prior to them making a proposal, and let them know you are interested in keeping the oversizing factors as low as possible. If they then propose anything anywhere near 2x oversized you don't need to call them back (unless there isn't more appropriately sized equipment out there.) ASHRAE recommends no more than 1.4x oversizing at the 99% outside design temp, which is reasonable if you would be using overnight setbacks. If you're using modulating or 2-stage equipment and keeping indoor temps steady there isn't a good case for even 1.1x oversizing up from the Manual-J, since there is always some built-in margin, even with fairly aggressive Manual-Js.

  2. jberks | | #2

    I'm no HVAC tech, but I am in the Mechanical phase now with my current build and have done my share of research.

    I think with typical low velocity forced air systems the lowest BTU furnace is something like 40k, and maybe a half tonne for AC. Please correct me if I'm wrong. I think maybe that's what you're HVAC guy was getting at, if the calcs at code insulation values call for the smallest available units, then running high rvalue Calcs is moot (other than bragging rights).

    However, look at some smaller units like mini splits, or in my case I'm installing a couple Dettson dual fuel units. they have some smaller units specifically designed for higher performing houses. I also like it cause it's sort of a "medium velocity" system, so the duct install is much easier cause it's more 2.5" flux duct running off 6" round trunks.

  3. lance_p | | #3

    I'm looking into the same thing right now. Our "very average" townhouse has a 44kBtu furnace that's about 50% oversized. It's the smallest unit that's available in that range, and would be way oversized for our new build.

    If we end up going with gas heat we will likely get a Dettson furnace. They make fully modulating (40-100%) gas furnaces down to 15kBtu, and sell a modulating heat pump system that works with it for AC.

    Someone else here mentioned a ~30kBtu furnace from one of the large manufacturers as well, it may have been Goodman? I don't remember for sure.

    Lance Peters

  4. this_page_left_blank | | #4

    The Dettson seems like a good solution. I hope that it's priced proportionately (i.e. less than a 40kBTU furnace), rather than as a boutique product.

    Their only heat pump unit appears to be a dual fuel unit (electric and gas), which would be of limited application. Maybe I missed something, as their website is kind of sparse.

  5. Yupster | | #5

    Goodman, Keeprite, and others now make 30,000 btu furnaces. In Canada anyway...

  6. Expert Member
    Dana Dorsett | | #6

    Lance: Goodman's pretty good 2-stage GMEC960303ANA condensing gas burner delivers 28,800 BTU/hr output at high fire, 20,160 BTU/hr out at low fire. (I think they have a couple other models in that range too.)

  7. Reid Baldwin | | #7

    Jamie B: After you get your Dettson system installed and use it awhile, please post here to tell us about your experience. I looked into Dettson for my house and was impressed. I had difficulty finding an HVAC contractor willing to install something they were not already familiar with, so I ended up with a conventional system (although I dictated the sizing).

  8. GBA Editor
    Martin Holladay | | #8

    You should read these two articles to learn more about why HVAC contractors do such a bad job when it comes to Manual J calculations, and to determine who you should hire for a good Manual J:

    Saving Energy With Manual J and Manual D

    Who Can Perform My Load Calculations?

    You should read this article to learn more about the Dettson furnace:

    Finally, a Right-Sized Furnace

  9. airfix | | #9

    Thanks everyone for the replies. I was out of town for a while hence my delayed response.

    The values I quoted for R are not center wall numbers but they are not whole values either. I calculated those using an infinitely large wall with double top plates, studs and rim joists doing an analysis for each section and coming up with proportional R values for each section and doing a weighted average. It does not account for window headers or internal/external corners which with more effort I can account for.

    I was planning on doing the calculations myself per the ASHREA 2017 Chapter 17 method. Which is apparently a little different to the Manual J calculations. I had my contractor do them because my county building department needed Manual J and equipment specified for my permit and I didn't have time to run the numbers. I didn't realize I'd need this prior to breaking ground.

    Our home is about 3900 sq ft.
    Wall area above grade is about 3065sq ft
    Wall area below grade is about 715 sq ft
    Garage partition is about 930 sq ft
    Roof area is 2650 sq ft
    Window area is 930 sq ft
    Door area is 80 sq ft

    The HVAC guy came up with a heating load of 28,157 Btuh and a heating average flow rate of 1200cfm. He came up with a cooling load of 19,341 Btuh and a cooling avergae flow rate of 1200cfm.

    He suggested furnace by Tempstar, Comfortmaker, Airquest model N9MSE0601714A with a 58,000 Btuh output and an AC unit Tempstar, ADP, conditioner N4A330A(G)KC and coil H,GE31(6,9)30+ with a total cooling output of 26,800 Btuh.

    Keep in mind his heating and cooling load numbers were using code values of insulation and the equipment is clearly oversized.

    Do you think if I run my own heating cooling load analysis per ASHREA Chapter 17 that it would give me something the county would accept instead of a Manual J method?

    I'll look into having a professional do the analysis though. It's too important to have it messed up because I missed something.

  10. Expert Member
    Dana Dorsett | | #10

    Even for the calculated 28K load specifying a furnace 2x that size isn't doing you any favors. ASHRAE recommends 1.4x to be able to use overnight setback strategies.

    28K x 1.4= 39K, not 58K

    With a single stage unit 39K would be the biggest you'd want, so the N9MSE0601714A is NOT a good being more than 2x oversized even for the HVAC guy's calculated numbers. The REAL numbers are lower still which would put the N9MSE0601714A in 3x oversizing territory.

    Within the same series the Comfortmaker N9MSE0401410A stager would be about right for a single-stager, at 39K-out:

    Ideally with a 2-stager the calculated heat load would somewhere between the high & low fire output. An inexpensive 2-stage 40K furnace such as the Goodman GMEC960403ANA would be about right if that were the actual load. It puts out 38.4K at high fire, 26.9K a low fire. There are others. Within the same vendor, the
    Comfortmaker G9MVE0401712A -stager would also be about right, 39K out at high fire, 25K out at low fire:

    With a 2-stager like those right-sized for the HVAC contractors' Manual-J at least the oversize factor for the actual load would not be ridiculous, and the comfort levels would be high, even if it literally never had to kick into high fire.

    A 1.4x oversize factor on the cooling specified isn't terrible, but it wouldn't hurt to go with a lower oversize factor than that. Most codes don't have a requirement for cooling capacity, whereas the boilerplate IRC code for heating is for all habitable spaces to be capable of hitting 68F at the 99% outside design temperature with a thermostatically controlled heating system (not space heaters).

    Whether or not the county code people would accept an ASHRAE calculation (even from a professional engineer) is a question for your code officials.

    With heat loads that low you might be better off with a hydronic air handler running off a condensing water heater rather than gas furnace. eg: Firsco 18HBXB-HW with a 1.5 ton cooling coil (or worst case, the 24HBXB-HW with a 2 ton coil, if the cooling loads demand it) would get you there at domestic hot water entering water temps. The blower speed, pumping speed, and water heater temp could all be tweaked to dial in the right amount of heating output:

  11. Expert Member
    Dana Dorsett | | #11

    BTW: A heating load of 28KBTU/hr for 3900' of conditioned space is ~7 BTU/hr per square foot of conditioned space. That's an unusually low ratio for a code-min house even if some of that is below-grade basement. Are you sure the contractor plugged in only code-min numbers?

  12. Expert Member
    RICHARD EVANS | | #12

    Based upon the information you gave us: I just did a quick heat loss calculation and came up with around 34,000 btu's/hr. (assumed slab is equal area as roof and a delta T of 62 degrees for above grade walls and 20 degrees for below grade.) This excludes heat loss from infiltration/exfiltration.

    My guess is that the Manual J is fairly accurate. It seems about right anyway, given the size of home in zone 6.

    Obviously, Like Dana said- the equipment, as usual, is still too big. I've seen far worse however. My co-worker in Vermont had her 23,000 btu/hr home fitted with a 120,000 btu/hr oil furnace last year. Total cost was $11,000. But the installers were courteous and fastidious so she gave the contractor a 5 star review on Angie's List.

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