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Q&A Spotlight

What’s Wrong With Our New Furnace?

Why isn’t this furnace, which was sized by a heating contractor, keeping anyone comfortable?

Brand new and not cutting it. A new high-efficiency gas furnace was supposed to make a San Francisco house more comfortable, but so far it's generating "high winds" and not much in the way of comfort.
Image Credit: John Melichar

John Melichar has upgraded the furnace in his two-level San Francisco home, one of several improvements that should have made the house more comfortable as well as more energy-efficient. The new furnace has the capacity recommended by his heating contractor, but so far the house seems less comfortable, not more comfortable.

In a post at GBA’s Q&A forum, Melichar explains his concerns:

“Our contractor told us to buy a 60K Btu/h furnace; we opted for 96% AFUE with two-stage variable blower — the Goodman GMVC960603BN.

“We get high winds from the vents,” Melichar continues. “We thought that the variable blower and two stages meant that the unit would operate on a low flame and low fan (cubic feet per minute) level but that doesn’t seem to be the case. The furnace starts out slow then ramps up to gale storm.”

These problems prompt Melichar to ask whether the furnace has been installed incorrectly. Maybe the contractor has recommended a furnace that’s too big for the house. Can some adjustments be made to improve its performance?

Further complicating Melichar’s situation is a language barrier with the contractors who installed the furnace: He’s able to speak with only one of them.

That’s the backdrop for this Q&A Spotlight.

Did the installer do his homework?

GBA senior editor Martin Holladay’s first question is whether the heating contractor has taken all the necessary steps to ensure the furnace was specified and installed properly.

That process, he writes, should begin with Manual J and Manual D calculations to make sure the furnace is matched to the heating loads in the house, and that the ductwork is designed correctly. Following installation, the system should be commissioned to make sure it operates as intended.


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  1. saltarch | | #1

    Combustion Air
    Am I seeing the image correctly that the combustion air is being pulled from the interior of the mechanical room? Is that legal? Seems like that could impact comfort in the house unless it is somehow balanced elsewhere?

  2. Expert Member
    Dana Dorsett | | #2

    Neither a comfort or legal problem @ Chris Harris
    Unless explicitly barred in the manufacturer's installation instructions it's legal to draw combustion air from the mechanical room so long as there is the code specified minimum makeup air path for the size of the burner. Codes vary on the makeup air opening requirement- not sure what it is in CA. A typical makeup air requirement for an atmospheric drafted burner is 1 square inch per 1000 BTU of make-up air opening into the mechanical room from the conditioned space is sufficient even for supplying the draft-hood dilution air of an atmospheric drafted burner. But stack dilution air isn't even used for condensing burners. It only needs enough for the combustion air.

    Then, a 60K burner doesn't draw enough combustion air to affect indoor comfort:

    Even a low-efficiency gas-burner draws no more than one cubic foot of combustion air for every 1000 BTU of source fuel, and condensing burners about 10% less. So 60,000 BTU/hr condensing burner draws about (60,.000/1000= ) 60 cubic feet per hour, or (60minutes/60 cfh = ) ONE cubic foot per minute. That's only ~1% of the air of a decent bathroom fan, and "in the statistical noise" of the amount of air leaking into & out of a typical home. A 2 mph change in wind speed would make a bigger difference in the amount of infiltration air.

  3. AlanB4 | | #3

    @ Dana
    I used to have a 75K in/60K out indoor air sourced furnace, replaced with a dual pipe unit and i noticed the indoor humidity went up 10-20% the winter after replacement. So its enough to make a difference, and one can probably consider it an exhaust only ventilation system.
    I am much happier with the dual pipe and i hope my water tank replacement can also be dual piped (then comes the dryer).
    Though i will then need an HRV once i do some air sealing :)

  4. charlie_sullivan | | #4

    Climate Consultant
    Thanks for prompting me to try Climate Consultant. What a great free tool! It seems like a good way to answer questions like whether a whole-house fan is useful in a particular climate, for example.

  5. Expert Member
    Dana Dorsett | | #5

    It's the dilution air, not the combustion air @ Alan B
    75K-in / 60K-out= 80% efficiency, and whether it was atmospheric drafted or forced draft, the largest fraction of the air needed to operate it without flue condensation is the dilution air going up the draft hood.

    The dilution air draft runs 24/365, not just when the burner is operating. The flue damper (if the old unit even had one) that opens only during burns is on the appliance side, before the dilution hood. Even if the furnace or boiler is right-sized for the load, the daily volume of passive dilution air flow is quite large.

    With condensing appliances there is NO dilution of the combustion exhaust, and the exhaust venting materials are selected to be condensate tolerant, and flue condensate management details are built into the equipment & venting design.

    If your water heater isn't a forced draft type (with plastic venting), and it vented into a masonry chimney that used to be shared with the old furnace, there is high risk of even the diluted exhaust condensing/adsorbing into the masonry, along with higher risk of backdrafting. Natural gas exhaust condensate is mildly acidic- it degrades mortar and destroys the chimney from the inside out. Look up "orphaned water heater".

  6. AlanB4 | | #6

    @ Dana
    That is very interesting about the dilution air. It was forced draft with no damper i am aware of. The orphan water tank is going to a non masonry chimney, its actually a metal chimney that runs up the center of the house. I am planning on replacing it with a power vent water tank so the chimney will be capped, and there is not much in the way of water dripping out of the chimney, i have removed the bottom cap a few times and its dry, even after rain.

  7. user882465 | | #7

    Code Requirements for Furnace Replacement
    California's Title 24, Part 6 Building Energy Standards require an ACCA Manual J, S and D for a furnace replacement. This leads me to ask whether this project was permitted. Was it? If not, this points out a benefit of getting a permit. It it was permitted, and the contractor can't show that they did these calculations, it would strengthen the homeowner's case that the contractor did not do the minimum requirements called for in this job.

  8. Expert Member
    Dana Dorsett | | #8

    Dilution air on hot air furnaces @ Alan B
    Hot air furnaces need an induced draft to overcome the impedance of the heat exchanger, but the dilution air is introduced post- heat exchanger in a draft hood (typically on one side of the box, sometimes two and is not dampered-off in many/most older 80% AFUE models. Retrofit automatic flue dampers were sometimes added post-draft hood with interlocks to inhibit ignition until the open flue was "proved" by an electromechanical switch, but it's unlikely that yours had one of those, given the over-drying symptom.

    The exhaust condensate will eat holes through B-vent over time. It won't show up as liquid water when you break it open for inspection at some random time- it DOES dry, due to the 24/365 parasitic drafting through the draft hood of the water heater. If it's a stainless liner it will tolerate it, but dual or single walled galvanized won't.

  9. Expert Member
    Dana Dorsett | | #9

    CA Title 24 @ Bill Burke
    It always amazes me, but that part of the code seems to be regularly overlooked in older homes, even though that's where it is most likely to do some good! Newer up-to-code houses are much tighter, but new construction seems to be where inspectors focus on the HVAC details.

  10. LukeInClimateZone7 | | #10

    CA Title 24
    @ Bill Burke, @ Dana
    The requirement for Manual J, (or ASHRAE HoF) calcs has been on the books for years. As someone who works on the Peninsula, I've experienced this bit of code (Section 150.0 (h) of Title 24 part 6) quite a bit since I advocate for it in my own work. My personal experience is: plancheckers and inspectors don't know about this section of code, nor have they received training as to how to review such calculations, and the vast majority of HVAC subcontractors have never heard of them either.

    On the good side-- Bill and his colleagues are actively addressing this with ongoing education at the PG&E Energy Centers in SF and Stockton. (keep up the good work!)

  11. AlanB4 | | #11

    @ Dana
    Thats very good to know and i will keep it in mind, i am already planning on replacing it, hopefully this year, and i'm not sure what kind of metal it is or how to find out. Its at least 30 years old that about all i know.

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