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Thread: Size of furnace

  1. #1
    mathew stouffer's Avatar
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    Default Size of furnace

    Ok, this is likely beyond the scope of a home insepction but additional input is always helpful. Inspected a house today, 6189 square feet. Basement, first floor, and two bedrooms on the second floor. The basement had a theater, about 800 square feet with its own furnace. The rest of the house was heated and cooled by one 132,000 btu mid efficiency furance. So about 5100 feet heated by one furnace. Plus the basement had approximately 12 foot ceilings in the family room. Any thoughts on this configuration, other then the second floor will roast during the summer and the basement will be cold during the winter. Thermostat was on first floor, house built in 06.

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  2. #2
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    Default Re: Size of furnace

    Anything is possible but I don't see it working. With the right layout, ducts, etc. you could push a 132K conventional up to 2500-3000 sqft, maybe. Anything beyond that is highly suspect.
    Obviously if it is a super insulated house or something, you could probably get away with it. Energy cost and comfort levels are probably way out of whack. I would recommend my client ask for a copy of a gas bill from last winter and a summer electric bill.
    I'd also discuss installing two 90+

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  3. #3
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    Default Re: Size of furnace

    It may functioned by like you said, comfort levels will suck! Would you rec eval by an HVAC contractor.


  4. #4
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    Default Re: Size of furnace

    Let me try that again. It may function, but like you said, comfort levels will suck! Would you recommend evaluation by an HVAC contractor?


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    Default Re: Size of furnace

    I would definitely recommend eval by HVAC contractor. If the place doesn't get comfortable in the winter or the bills are sky high, what are they going to do? Call the HVAC guy of course because they will think the furnace is broke or something. HVAC guy comes out starts laughing and says the HI should have caught this. Please pay me 10K or so. Who do you think will get the next phone call? Either you or the lawyer.
    One of the big issues with scenarios like this is the potential for very high costs to rectify the problem. High costs like this may make a particular property no longer feasible for a client. I consider it therefore very important to let clients know about such issues. Costs like these may require them to look for another property.
    I had clients recently who told me at the beginning of the inspection that it was very important they know as much as possible about potential repair costs. The house was at the upper limit of their budget. Because of that, there wasn't going to be any money anytime soon for major repairs.

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    Default Re: Size of furnace

    Quote Originally Posted by Markus Keller View Post
    One of the big issues with scenarios like this is the potential for very high costs to rectify the problem. High costs like this may make a particular property no longer feasible for a client. I consider it therefore very important to let clients know about such issues. Costs like these may require them to look for another property.
    The house is 6000 square feet. Selling price is probably $600,000 - $1 mil+. Even at adding a new system at $10,000, that is only 1-2 percent of the selling price.

    I agree that the buyers should be made aware of the short comings but adding 10k to a million dollar price tag is not likley to make it unaffordable to the typical buyer in that price range. $6000 sq ft homes are not starter homes.

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    Default Re: Size of furnace

    Quote Originally Posted by mathew stouffer View Post
    Let me try that again. It may function, but like you said, comfort levels will suck! Would you recommend evaluation by an HVAC contractor?
    I would have to agree as well. I would have expected to see two furnaces that size in this home. Sure you didn't miss one?

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  8. #8
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    Default Re: Size of furnace

    Gunnar,
    There are two furnace, but one is dedicated to a theater room which is about 800 to a 1000 feet. That still leaves 5100 feet far a 132000 btu Mid efficieny furnace.


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    Default Re: Size of furnace

    Quote Originally Posted by mathew stouffer View Post
    Gunnar,
    There are two furnace, but one is dedicated to a theater room which is about 800 to a 1000 feet. That still leaves 5100 feet far a 132000 btu Mid efficieny furnace.
    Yeah, sorry. What I meant was I would have expected two HVA/C units for the house excluding the basement (which sounds like an addition/modification). One upstairs, one downstairs. Particularly in chilly Utah.

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  10. #10
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    Default Re: Size of furnace

    Yeah someone F'ed up.


  11. #11
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    Default Re: Size of furnace

    Quote Originally Posted by mathew stouffer View Post
    So about 5100 feet heated by one furnace. Plus the basement had approximately 12 foot ceilings in the family room. Any thoughts on this configuration, ...house built in 06.
    MS, When I took the inspection course, the instructor taught us a quick calculation: New house is 3-5 btuh/ft3, Old house is 5-8 btuh/ft3 ( 1kW=3412btuh)

    I think it depands on where you are, these formular are too much for Vancouver, BC. I have 90,000btuh furnace feeling very warm in my 2,200ft2 x 8ft high old house . Counting backward is 5 btuh/ft3. Therefore I use 3 for new and 5 for old Vancouver house. You may use couple local houses to modify the above formulars.

    In your case, new house volume = 5,100ft2 x 12ft = 61,200ft3
    Average heating input required 61,200ft3 x 4 btuh/ft3 = 244,800btuh

    It seems undersize! Anyway, this is a guideline only, please let the heating engineer to review it.


  12. #12
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    Default Re: Size of furnace

    Did it have zones with other thermostats other than the two? It might be wrong, but I would not make a big fuss saying it IS wrong. Doesn't sound right, but if this house is an Certified Energy Star home, and has dampers for other zones it might be OK. I would recommend a load calc by a qualified HVAC contractor and leave it at that.

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  13. #13
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    Default Re: Size of furnace

    Quote Originally Posted by Daniel Leung View Post

    It seems undersize! Anyway, this is a guideline only, please let the heating engineer to review it.
    Now hold on a minute. I am not sure what size the home theater furnace is but you will get some heat from that unit rising to the floors above. Admittedly by most posting here, right?

    AND, you said the furnace size is 132,000 BTUH, was this the output or input?

    When I do schematic designs I usually use 30 BTUH/SF as a rough number for estimating equipment heating capacities. Assuming the Basement furnace is 80 MBH (I know you didn't say what capacity that one was, just WAGing on my part), that still exceeds the 30 BTUH/SF number.

    In reality I am not so sure the zoning would be that bad, not perfect for sure but a lot depends on exposure and window size, etc.

    Anyway FWIW, my 2 cents.

    Oh, one more thing, if the home is fairly new the heating calcs had to be submitted with plans for permit.


  14. #14
    David Bell's Avatar
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    Default Re: Size of furnace

    I am guessing that a mid effieciency unit is a gas fired 85 to 90% unit, possibly direct vent, but probably power vented. At 132,000 in, this furnace may be able to put out 120,000 or 125,000. If by any chance the A/C coil is less than 5 ton, air restriction is probable and a decline in output btu should be expected. Even at 5 ton you can only expect 2500 cfm at the absolute top of the curve. Seems low for the square footage described.


  15. #15
    David Bell's Avatar
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    Default Re: Size of furnace

    Oh, one more thing, if the home is fairly new the heating calcs had to be submitted with plans for permit.

    More and more homeowners are pulling construction permits. Towns have very little recourse to question calcs in this case and tend to just focus on install aspects, such as insulation requirements.


  16. #16
    mathew stouffer's Avatar
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    Default Re: Size of furnace

    The Mid unit was 132,00btu input. The theater was 45,000 btu, high efficieny, however there was a suspended garage slab above the theater. The main problem I see is the height of the basement ceilings.


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