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Community and Q&A

Make-up air for gas furnace?

John_H12 | Posted in Mechanicals on

Our builder is finishing our new 2,000 sq ft house with a basement in the St. Louis Missouri area.  Due to some crazy change order pricing I could not afford both air sealing and sealed combustion furnace and water heater.  I chose air sealing the house in accordance with Energy Star 2018 guidelines.  The insulator estimates we are at ACH3 @ 50 pascals, but no blower door tests have been performed.  The gas furnace 80,000 Btu (80% efficient) and water heater are open combustion appliances. 

The home has a 400-cfm range hood, a 125-cfm electric dryer, 120 cfm central vac, and a 100 cfm and 50 cfm bath fans. I’m not sure what the Btuh of the water heater is but my math for 80,000 Btuh gas furnace at 50 cu ft per 1,000 Btu is 67 cfm. Total cfm is roughly under 1,000 cfm., but that seems like a lot of leakage.

I’ve estimated the house volume at 35,000 cu ft.  At ACH3 @ 50 that’s roughly 1,750 cfm.  But does gas furnace back draft start at a lower differential, 30 Pa?  

Would It be prudent to plan for a makeup air system?  If so, my concept is for the make up air system to provide unconditioned filtered air to the basement buffer area. 

I’m a mechanical engineer who has designed HVAC systems in the past and have no problem designing and testing a make-up air system.  

John H

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  1. [email protected] | | #1

    This situation is hard to understand. What prices are you being quoted? The incremental cost to go from a worst in class 80% efficient furnace to a condensing sealed combustion 93% unit with an ecm blower ought to be negligible in the scheme of things--maybe $500. And by the time you factor in a chimney I'd think a heat pump water heater might be cheaper as well, especially if you are looking at an on demand gas unit.

    3ACH50 should be pretty easy to hit if sheathing and windows and doors are taped properly. The incremental cost of doing that right should also be pretty negligible I think.

    But to your original q... I think i'd wait until it's built and then have an auditor test to see if you got your money's worth on air sealing and then see if it meets bpi standards for caz. If it back drafts, then install make-up air.

  2. John_H12 | | #2

    To change to a 90+ furnace in a 2,000 sq ft house the adder was $3,800. I was a bit incredulous at that price so I asked if that included a 14+ SEER A/C and the answer was no. For that price I could purchase a new furnace and throw away the old one.

    The change order pricing reflects a volume builder who does not want to make changes and a adder to cover the cost when they mess up the change oder. I'm generally happy with the builder except for the change order cost. I've tried to get the items I can't change later.

  3. Yupster | | #3

    In Ontario, Canada we have a -5 Pa depressurization limit for natural or induced draft furnaces.
    I've attached a form for a depressurization calculation sometimes used here, distributed by our provincial HVAC association. It's obviously a rough estimate but should get you in your ballpark.

  4. Expert Member
    Dana Dorsett | | #4

    >" 2,000 sq ft house..."
    >"...gas furnace 80,000 Btu (80% efficient)..."
    >"I’m a mechanical engineer who has designed HVAC systems..."

    Have you calculated heat loads? Do you realize how RIDICULOUSLY oversized that furnace is for a 2000' code-min tight NEW house (or even a leaky but insulated 2x4 framed house that size)?

    The odds are pretty good that your design heat load at St. Louis's 99% outside design temperature of +8F (yes, I know it gets colder than that) is under 25,000 BTU/hr, and that furnace puts out nearly 3x that much, as if you were expecting a Polar Vortex event to deliver sustained periods when the outdoor temps were in the -50s F or something!

    AHRAE recommends an oversize factor of 1.4x the heat load at the 99th percentile temperature bin. If my WAG is right that you're looking at 20-25,000 BTU/hr the furnace should have an output of no more than 1.4 x 25,000 = 35,000 BTU/hr, which would still have you fully covered WELL into negative double-digits. An 82% efficient (minimum legal efficiency) 40,000 BTU/hr furnace puts out ~33 KBTU/hr which is good enough.

    I'm not just out to bust your chops here- "bigger" is the opposite of "better". At that level of oversizing will lead to significantly lower comfort levels, despite the tightness of the house. A right sized furnace will have a high enough duty cycle so that all rooms get heated when it's actually cold out. With an 80K furnace you're going to get the noisy hot-flash followed by the chill in some rooms, and other rooms at the far end of the duct runs or those with somewhat different heat loss characteristics (such as a bonus room over a garage) won't get enough heat when it needs it.

    Nate Adams has a pretty good primer on the negative effects of gross oversizing on his "Nate the House Whisperer" blogs:

    Even the short videos do a pretty good job of describing the problem, and the solution:

    If it's not too late, scratch that 80K furnace, run a reasonably aggressive Manual-J or similar, and spec something more appropriate.

  5. John_H12 | | #5

    It's to late. I'm not offended by your comments as I did not specify the equipment. It is frustrating to buy a new house and not be able to specify the construction techniques and systems. My focus has been on the items I can't change later like air sealing and insulation.

    So yes a smaller more efficient heater and A/C are in order but that's not economically justified today. After we close I'll talk with the HVAC installer about a swap or credit for the old equipment. There's not much of a market for newer but used residential HVAC equipment.

    Back to the original topic, make-up air now or wait...going through Yupster's pdf later this evening...

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