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  1. #1
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    Default General rule for sizing gased forced air units

    Is there a general rule for sizing gas furnaces per square foot of house? I understand taking the windows, insulation etc... into consideration. Just looking for a general rule of thumb.
    I heard 30-35 BTU Per SQ Ft in CA. But if I'm checking a unit in the high mountains, where the temp was 30 degrees today + snow, wouldnt 40-50 BTU be more appropriate?
    What say you?

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    Last edited by Marc M; 12-09-2009 at 09:02 PM.
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    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Marc M View Post
    Is there a general rule for sizing gas furnaces per square foot of house? I understand taking the windows, insulation etc... into consideration. Just looking for a general rule of thumb.
    Inspectors do not really need to worry about that unless you see some gross difference in size.
    That I leave to the experts.

    Normal would be I think somewhere around 75,000 - 100,000 for average size living unit or home.

    Still it is up to the HVAC guy to do the clac on that stuff and past SOP of any Association I know of.


  3. #3
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    Default Re: General rule for sizing gased forced air units

    The house i was in was 1100 sq and the unit was a small 32K unit. I was there for two hours and the unit never exceeded 50 degrees starting from 35 degrees. I didnt test the rise because I wasnt inspecting the house, looking to buy it.


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    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Marc M View Post
    Is there a general rule for sizing gas furnaces per square foot of house? I understand taking the windows, insulation etc... into consideration. Just looking for a general rule of thumb. I heard 30-35 BTU Per SQ Ft in CA. But if I'm checking a unit in the high mountains, where the temp was 30 degrees today + snow, wouldnt 40-50 BTU be more appropriate? What say you?
    Marc,

    In general, it would be helpful if we had more specific information as to where you are located. All your info says is CA, which has a lot of different climates. San Diego barely needs heating and Tahoe needs lots. Different needs for different areas. Insulation and dual glazed windows will be a huge factor. Also, high efficiency compared to a 30 year old furnace would be much different, probably about half the BTUs. In addition, if there is A/C, a higher CFM blower is necessary and the BTU input with high CFM blowers for some reason tends to be higher as well. Clearly, sizing a furnace is beyond the scope of a home inspection and even if you have a rule of thumb, it may not work in practice. I inspected a home a few years ago that had an appropriately sized furnace that was on for 2 hours straight and did not push the heat up more than 6 or 7 degrees. Clearly something was wrong there. Just today I inspected a 2000 sq.ft. home that had a mid-efficiency 60,000 BTU furnace. I felt that it was undersized and reported it as such.

    My area is pretty temperate, but can drop below freezing on occasion. What I see is generally about:

    Old: 60,000 to 80,000 BTU per 1000 sq.ft.
    New, Mid-Efficiency: 50,000 to 60,000 BTU per 1000 sq.ft.
    New, High-Efficiency: 35,000 BTU per 1000 sq.ft.

    Since you are referring to a mountainous area that will get pretty cold, I would probably up my numbers by at least 25%, probably more.

    Once again, your mileage will vary depending on where you are located and my numbers may not be valid for your needs. Maybe you could contact a heating contractor for some rough numbers. Maybe someone from a similar climate will chime in with their observations.

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    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Marc M View Post
    The house i was in was 1100 sq and the unit was a small 32K unit. I was there for two hours and the unit never exceeded 50 degrees starting from 35 degrees. I didnt test the rise because I wasnt inspecting the house, looking to buy it.
    Marc,

    I was typing when you posted this response. I would expect about twice the size of the one you saw. Clearly, by the operational test that you performed, it is nowhere near adequate.

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  6. #6
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    Default Re: General rule for sizing gased forced air units

    do a Manual J workup you will get a the exact values you are looking for.
    forget general rules.


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    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Richard Pultar View Post
    do a Manual J workup you will get a the exact values you are looking for.
    forget general rules.
    I must have missed that section in class.


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    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Bob Elliott View Post
    I must have missed that section in class.
    Not going to happen in a home inspection. Not a full heating load calculation, at least not around here. Too much work, liability and no real reason.

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    Default Re: General rule for sizing gased forced air units

    I see many houses here in MN where the furnace and a/c were sized for the finished square feet of the home. Then the homeowner finishes the basement, basically doubling their finished square footage without upgrading the furnace or a/c. I'll generally suggest a load calculation test be done.

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    Default Re: General rule for sizing gased forced air units

    Hey Gunnar,
    The place is in Frazier Park, CA. top of the grape vine between Santa Clarita and Bakersfield.
    The place is invester owned and it looks like a typical flip hack job as far as the install goes.


  11. #11
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    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Marc M View Post
    Hey Gunnar,
    The place is in Frazier Park, CA. top of the grape vine between Santa Clarita and Bakersfield. The place is invester owned and it looks like a typical flip hack job as far as the install goes.
    Marc,

    Was this a wall furnace? I thought Henry the homeowner could not purchase a forced-air unit. I always thought it was funny that homeowners can buy wall furnaces, even though they are arguably the most dangerous if installed incorrectly. If it is a wall furnace, was it even installed correctly? If done by a homeowner, probably not.

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    Default Re: General rule for sizing gased forced air units

    I'm totally dropped the ball on the info huh?

    The house was actually 2200 sq ft. Actually, 1100 up and down. A FAU in the attic and one in the garage. Both were the same at 32K BTU. I have never seen this type of furnace before. So small, it looks like an RV furnace.
    Yea, it looked like a total DIY job. Hey, just wondering...what is this business about taking into consideration the altitude (+8000 feet)? Just wondering how that effects the units performance- negitive or positive. I will never see that situation but good to know anyhow.


  13. #13
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    Default Re: General rule for sizing gased forced air units

    Boy, that really was incomplete information!

    I still say 70,000 BTU is way too small for a home that size. Don't know about the high altitude stuff since I don't need it. Maybe someone from Colorado will chime in.

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  14. #14
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    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Gunnar Alquist View Post
    Boy, that really was incomplete information!

    I still say 70,000 BTU is way too small for a home that size. Don't know about the high altitude stuff since I don't need it. Maybe someone from Colorado will chime in.
    Yea...I agree. It's pretty dang cold there. IMO, anything below 65 is dang cold.
    As far as the high altitude stuff goes; I thought per every 1000 feet above 8K, you had to deduct something. Or add something. I dont know what the hell I'm talking about right now.


  15. #15
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    Default Re: General rule for sizing gased forced air units

    the btu's number has almost no relevance to what is appropriate unless you consider the load demand.
    It all depends on what you want to do with what you have to work with.
    Doing a Manual J review is the Industry standard. Not doing one is lazy.


  16. #16
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    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Richard Pultar View Post
    the btu's number has almost no relevance to what is appropriate unless you consider the load demand.
    It all depends on what you want to do with what you have to work with.
    Doing a Manual J review is the Industry standard. Not doing one is lazy.
    When do you start classes?


  17. #17
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    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Richard Pultar View Post
    the btu's number has almost no relevance to what is appropriate unless you consider the load demand.
    It all depends on what you want to do with what you have to work with.
    Doing a Manual J review is the Industry standard. Not doing one is lazy.
    I'm all in...


  18. #18
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    Default Re: General rule for sizing gased forced air units

    search Manual J , pull up a free trial and fill it out takes about 10 minutes. The results are required for rebates from the utilities in NJ . The furnace and /or air conditioner can not exceed 15? per cent ?? of the required load demand for the degree days zone the house is in.


  19. #19
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    Default Re: General rule for sizing gased forced air units

    Good morning:The only true way to size gas equipment is by doing a heat loss calculation on the building.Using square footage of building(or square ft of radiators for boilers) is guess work.As very few contractors take the time, effort, or even know how to do it heat loss calculation is rarely done(in province of Ontario I've met installers who size by from 10,000 Btuh per room to 30,000 Btuh per room).

    As to altitude issues at higher elevations the actual input rate may be lower than the value listed on the rating label/plate.You may require a smaller orifice(s) if you exceed max elevation manufacturer allows.

    hey I used "enter" bar.am I meeting the requirements of the bosses?


  20. #20
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    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Phillip Bates View Post
    hey I used "enter" bar.am I meeting the requirements of the bosses?
    Cool! Capital and lowercase letters, punctuation, carriage returns... Excellent! passing grade.

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  21. #21
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    Default Re: General rule for sizing gased forced air units

    I attached a PDF file that I found somewhere a year or two ago that may be of some help as a general guide.

    Attached Files Attached Files
    The above statements are expressed solely as my opinion and in all probability will conflict with someone else's.
    Stu, Fredericksburg VA

  22. #22

    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Marc M View Post
    Yea...I agree. It's pretty dang cold there. IMO, anything below 65 is dang cold.
    As far as the high altitude stuff goes; I thought per every 1000 feet above 8K, you had to deduct something. Or add something. I dont know what the hell I'm talking about right now.
    CLOTHING!

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  23. #23
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    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Dana Bostick View Post
    CLOTHING!
    So. California minimum clothing standards: Shorts, tank top, flip flops.
    Hazardous conditions require "steel toed" flip flops.

    Last edited by Marc M; 12-11-2009 at 10:30 AM.

  24. #24
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    Default Re: General rule for sizing gased forced air units

    I live in Colorado at 7300 ft elevation, and my house has 43 BTU/sq ft. with a mid efficiency furnace. I think the elevation skews this result a little high, because the furnace has to be re-calibrated (leaner jetting) to compensate for the thinner air, and loses some capacity in the process, but I don't know how much. It was -15F earlier this week with no problems for the furnace (I do have R38 in the attic).


  25. #25
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    Default Re: General rule for sizing gased forced air units

    Your better off under sized then over sized. If under sized you can add some need caulking, insulations or windows which will make it more comfortable on an older home. You don't want the heater to take a "cold" house from 35 to 75 in 2 hours. Once the sheetrock gets that cold it can take 1-3 days to warm the house up so your not getting the radiant cold of the walls. In 2 hours the air should have felt warm, but the house cold. Sure hope all the plumbing is drained being that cold.

    I can attach a rule of thumb chart. It has 3 pictures of a house that you cut out and hold at arms length. Using the different sizes of paper cuts will give you a rule of thumb of heater needed. (it's a joke)
    There is some good software out there that allow you to have 30 days of use for mimumun fee.


  26. #26
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    Default Re: General rule for sizing gased forced air units

    Quote Originally Posted by Gunnar Alquist View Post
    I still say 70,000 BTU is way too small for a home that size.
    You want to be careful to not oversize the furnace because that'll lead to short-cycling and result in the furnace condensating and possible corroding at the heat exchanger. I think the requirements up here are somewhere between 25 to 40 btu's per sq.ft.

    I often see oversized appliances, I guess it's that Tim-the-tool-man mentality... the bigger-the better-the more macho, ourgh, ourgh, ourgh!

    Joe Klampfer RHI
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