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How much energy is lost due to indoor sourced air for combustion in a furnace?

AlanB4 | Posted in Energy Efficiency and Durability on

I used to have a 75k input (60k out) 80% furnace that used basement air for combustion, in addition a vent was broken dumping a lot of heated air into the uninsulated crawlspace which is where a large amount of air leakage is in this house which was feeding the furnace.

I now have a 95.5% high efficiency 40k with dual pipes, but the intake pipe was full of snow so i have temporarily removed the intake pipe, so its using basement air for combustion, i also sealed the broken crawlspace pipe. I am curious how much heat the furnace is sucking out of the house to combust the gas, the basement is now about room temp (20C), the furnace uses 26k low and 40k high (Payne PG95XAT). Right now the temp is -14C and it runs at low fire for a few mins, kicks into high fire for a minute or two then shuts down, the design loss is 45k but when i hit the design temp several times last month it was still shutting down so the software or the inputs is not perfect (Hot2000).

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  1. GBA Editor
    Martin Holladay | | #1

    According to one source, a 100,000 BTU/h furnace requires about 21 cfm of outdoor air for proper combustion. If that estimate is accurate, a 40,000 BTU/h furnace would require about 8.4 cfm of outdoor air -- about 1/7 as much as a bathroom exhaust fan -- when the furnace is operating.

    Your current situation is jerry-rigged and is not recommended. As soon as possible, you should relocate the outdoor air duct for your sealed-combustion furnace to a new location that remains snow-free.

  2. charlie_sullivan | | #2

    What's your fuel, natural gas?

    I did a quick back-of-the-envelope and calculated that you need 10 CFM of air for 40k BTU/hr, and that you lose 660 BTU/hr, or 1.6% of the heat using inside air. No guarantees that I didn't miss a factor there--we'll see if others come up with the same numbers. Of course, the furnace presumably runs at a little higher efficiency when it starts with warmer air for combustion. I don't know what that works out in practice, but perhaps you lose 1% instead of 1.5% all things considered.

    1% seems small, but given that you also gain protection from backdrafting, it makes a lot of sense to do it.

  3. AlanB4 | | #3

    Thanks for your replies, do vent motors have a CFM rating? I am wondering because they would have to pull more air then stoichiometric, they have to pull the flames into the heat exchanger against gravity. When i was furnace shopping the first bid was from a guy who reminded me of a 1980s car salesman (he was in his late 50s so maybe he was), didn't want to name prices, the price would be different if i waited for more quotes (by next week), but one thing he said was i guarantee you will use 50% less gas with the new furnace, especially since it pulls in combustion air from outside instead of using heated air. I knew he was lying but that did make me want to calculate how much was being used, but the new furnace was double piped so i forgot about figuring it out till yesterday. I noticed there is no option for this in Hot2000, an oversight or an inconsequential value, who knows.
    The tech is supposed to come on Thursday and its well under the 1 year warranty so that should get it fixed properly, i'm operating it without the cover at the moment and allowing the water to drip into a bucket instead of the inside of the furnace like it was, and the tech told the secretary to tell me theres no immediate danger so i can keep using the furnace (which is natural gas)

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