Originally posted by bydabeach
Let's say the ductwork is alread in place, and you are replacing an existing, old furnace on the fire floor. Is there any harm (other than overpaying for capacity you don't need) to the variable speed/modulating approach, meaning that you are counting on the furnace to operate on the lower firing cycle(s)?
Since the old furnace was most likely over sized, a 45,000 90% should be ok, just heat the place alittle faster then a 45,000 80%.
Re: 2 stage furnace - way to go !
I'm curious how you determined that your Trane furnace is derated 12%.
Originally posted by xv80satisfiedcust
My 2 stage furnace (derated for high altitude) gives 42240 in high mode. The low stage only gives 27456 (high altitude). High altitude reduces the output capacity by 12 percent for the Denver altitude. So, at sea Level (NJ) it would do 31,200 btu (low mode) like the Trane literature says.
Xcel does not deliver "natural gas" in the Denver area; they deliver "mixed gas" that has a heat content of only about 830 Btus per cu ft, depending on the altitude of delivery (due to Boyle's law).
The Tranes and American Standards I have seen delivered in the Denver area are not derated at all. So the combined deration would be about 17%.
One problem not mentioned so far: every time a furnace cycles on/off the heat echanger and flue go thru a temp zone called the "WET" zone. during these periods metal in the unit is below the dewpoint of the flue gasses. If a furnace cycles too rapidly you can have early heatexchanger failure due to corrosion. Over sized is NOT good on a furnace either.
If You don't have time to do it right, when will you have the time to do it over?
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We had the same problem and solved it by using a "water coil" type heater. It uses hot water from a gas water heater.