I live in MD and have a 30+ year old gas furnace that soon needs
replacement. BGE is the local company and their man recommended a two-stage
Carrier unit exactly matching my old one's BTU rating. In the last 2 years
we have added insulation, replaced all doors, (particularly a sliding door
that might as well have well been a hole in the wall!), and put in really
With these upgrades, do we have to match BTU for BTU with the old and new
units? From some other posts here, I wonder if it's necessary. Also, can
anyone give me some feedback on BGEs estimates and savings with upgrades?
Im sure a much smaller unit will work , you didnt even refer to Output
btu, output btu will be much more with a 90%+ Carrier unit. Your sales
guy is taking the typical lazy ass way out. First you may be oversized
now, but with more insulation and more efficient units there are 3
reasons to demand to see a written load calc. Never replace till you get
the load calculation.
I thought furnace BTU ratings were output BTU or Bonnet BTU....
BTU per Hour actually.
And to the OP... look at the duty cycle (% of on time to off time) of
your present furnace on the coldest days. If it is say 50% and you get
a new furncace that is 50% BTU rating then it will run 100% of the time
i.e. continuously on the coldest day...
Using the operation of your present furnace is probably the most
accurate way to judge your BTU requirements.
That's correct. And you don't need to wait for the coldest day
to make measurements. Pick a day when it's overcast, so there
is little heat gain from the sun. Let system stabilize for
say, 4 hours. Measure duty cycle. Now, if the outdoor temp
has been steady 35F and indoor is 70F, the diff is 35 degrees.
Take the furnace "output" BTU/Hr rating and multiply by
the %hour running time. That gives you BTU/Hr.
Divide that by 35 to give BTU/Hr/Degree diff.
If the design temp (minimum) you want is, say, 10 below zero
multiply by 85 (70 to -10) to give the BTU/Hr required.
Add another 10% overcapacity if desired.
"Why should a theoretical estimate be better than a direct measurement?
Cheers, Wayne "
That's exactly what I was thinking. Any inaccuracies in back
calculating it from the duty cycle of
his existing furnace is likely to be less than a theoretical
Because a manual J load calculation done competently is not theory. It
is based on actual numbers, data and fact. Its also standard of the
Your way is kind of like lighting match sticks until you have enough
of them lit to warm the house sufficiently.
On Wed, 1 Feb 2006 13:12:21 -0500, "Gerry Gardiner"
He's done it all wrong. He needs to give you TWO furnaces that
size...................NOT just one. It will heat your home much
faster so you wont be burning that expensive natural gas long periods
of time. This will relate to a huge savings.
Bigger really is better, you know?
I'll be over your way next week. I have a slightly used bridge that I
need to sell and Im sure I can get you to buy it.
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