Thanks for all your respones guys. Great info!
"N7RX" wrote in message
...
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.
"sx460" wrote in message
...
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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