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
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.
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"
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.
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
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.
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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