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ALWAYS make a heat estimate!

Posted by Harold Kestenholz - on April 14th, 2002 02:57 AM
In reply to Maybe not correct information by R.C. on April 14th, 2002 12:27 AM [Go to top of thread]

It was understood that the old furnace was a single-stage, there were no 2-stage furnaces 23 years ago.

You want to install a 90-plus 2-stage furnace now. The answer, as written is the same, do a heat loss estimate first. A heat loss estimate takes the worst case scenario of a house heat requirement during the coldest nights of the year with no people inside and therefore, not making heat from their bodies nor from electric appliances. A heat loss always is at least 10 percent over what you need in the worst night. Then you will not have to guess the size furnace you need, simply buy a furnace that has the output to match at least the heat estimate. No guessing, no mistake, and no oversizing. I live in a 1200 square foot home on the east coast with similar insulation and my heat loss is 32000 btuh, your region is not twice as cold nor does it reach minus 70F frequently. Your 92,000 btuh boiler is closer to three times as large as you need for 1400 square feet, than anywhere near what you need. If you mean that your home is 1400 square feet upstairs and 1400 square feet downstairs, making it a 2800 square foot home, then you are closer to your goal. Telling an estimator that your home is half the square feet it is can make for strange calculations. For most manufacturers a 90,000 btuh furnace is the largest they make in 2-stage furnaces and they are not meant for little homes.

Oversizing is not good for a two-stage furnace. The furnace comes on at high fire to establish a good draft, then after five minutes, if the controls sense that high fire is not needed, it will go to low fire to extend the heating time for the purpose of comfort. If the unit is oversized, most of the season it will come on at high fire and shut off in a few minutes. When the weather nears freezing then the unit will go to low fire for a while. Understand that a unit that is twice as large as necessary only has to run half the time in the coldest night of the year. The oversized furnace will run less than an accumulated 30 minutes of any hour of the year on any day warmer than the coldest day of the year. How will you feel as the house cools down while the furnace is off most of the time and the unit goes on and off rapidly, sometimes going from high to low fire repeatedly?

Your furnace will never peel the paint off the walls (It is a low-temp unit), but it can make you wonder why it goes on and off and changes sounds so frequently, while it never seems to make you warm at 72F. Many people take the Tim Conway approach to heating and want to put a 250 horsepower engine on their leaf blower. Comfort comes from matching a furnace to the heat load estimate; ask any manufacturer, utility, and the Department of Energy - the meaning of the AFUE rating. The first step is doing a heat loss estimate and having it in your hands before selection.

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'R.C.' on 4/14/2002 3:50 AM
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