# How many BTU furnace do I need?

Hello guys,
I'm planning on rerunning the duct work in my house and replacing the furnace. My question for you is: how many BTU furnace should I buy?
I live in Iowa where it can get VERY cold, and for days on end. We've just reinsulated the exterior walls with R13 fiberglass insulation (best you can get for 4" studs). Also, replaced all sheathing, wrapped the house, and installed vinyl siding. Livable above ground area is 1,000 sq. ft. The house is also two story. Basement area is ~450 sq. ft. We haven't installed the insulation in the attic yet, but it will most likely be in that 40-50 R value range.
Thanks!
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can
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Do a heat load on the house. That's the only way to tell what you need. DON'T look at a similar house and say "Since that's the same size as mine, I'll put in the same size furnace." It doesn't work that way.
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sx460 wrote:

There is only one way of determining what you need. That means having someone come in who can take the measurements and do the calculations using the right formula. They also will need to determine if the duct work you have is proper, another calculation using a standard formula. The only way to get around this is to find a home "exactly" like yours that someone already did the calculation for.
In short call in the pro and make sure they are doing "Manual" calculations.
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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6
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Depends. On lots of factors we can only guess at. You really want to get some recommendations on hvac contractors in your area, telling them your story. Pick the one you want to do the whole job, starting with the appropriate load calculations. Then you can talk about hardware. And get the best package for you.
In Iowa, you might want to look into corn stove for backup/supplemental heat, too.
HTH, John
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You'll need to do a heat load analysis for your house if you want to know the actual answer. Otherwise, you'll need a competent HVAC guy who's got lots of experience with installations in your area who can just tell you what will work and what won't. So, basically, you can spend a lot of time figuring out what your local HVAC retailer can probably already tell you.
If you want, you can do a rudimentary heat load calc by accumulating all your U-factors and R values for windows, door, ceilings and floor, walls and air infiltration estimates and using this data to figure out your BTU loss per hour. Basically, the BTU loss per hour for a window equals its U-factor multiplied by it's area in sq. ft. This product, when multiplied by the degree difference b/t the inside temp and outside temp in F will give you the BTU loss per hour of the calc'd area. This process is repeated for every door, window, ceiling, floor, wall & etc. The other big factor is air infiltration and exfiltration. I use a U of .02, multiply this by the total cubic volume of the structure and that product by the number of air changes per hour and that product by temp difference to generate the BTU loss.
Obviously, this can get very complicated since the heat loss is, among other things, compartmentalized - heat source and loss load is not uniform throughout the stucture - but you should be able to get a ballpark estimate of your load by accumlating loss data for your exterior surfaces.
For example:
A 3020 window will lose about 67 BTU's per hour on a day when the inside temp is 68 and the outside temp is 35: U-factor of .34 X 6 X 33 = 67. Similarly, 180 square feet of wall insulated with R-19 fiber, sheated, sided and drywalled for a total R of 21 will lose 283 BTU per hour: (1/21) X 180 X 33 = 283. And so on.
You can find tables of R-values on the web that will tell you what the R-value of 1/2" drywall is, for example, & etc. R-values are the inverse of the U-factor - an R-21 = U-.05.

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Thanks for all your respones guys. Great info!

just
the
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