BTU and quote question for New Trane XV95 + AC XB-13

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Old 04-20-09, 11:59 AM
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BTU and quote question for New Trane XV95 + AC XB-13

I live in Western part of PA in 1200SF Ranch house with 1 car garage + 850SF finish basement. I got quote for Trane XV95 -60K BTU + XB 13 -2.5Ton AC for 5800$ including trane rebate and tax. With federal tax credit for XV95 it will come down even more. Plus 10yrs parts and labor warranty from installer.
Question:
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1. Is this price reasonable? Installer will use existing duct work and no need to do chimney liner. I also opt out of mechanical/electronic filter.
2. Is 60K BTU enough for this house? House is 52years old with wall insulation + good attic insulation + blown in insulation in garage ceiling. Roof is simple asphalt. I have around 10 double pan windows about 10yrs old and new FG insulated entry door. I plan to heat main floor (1200SF) and only half of the basement (400SF). 50% of the Basement walls are insulated. I got quotes for 3 installers and BTU they suggested varies from 60K to 80K.
My existing 20 years old furnace is still working fine and it is 100K input with 80% efficiency output is 80K BTU. Installer says with this old furnace it may be only 70% efficient and it was common practice to oversize in old days.
He is giving heating guarantee of 72F for 0 F outdoor temp.
He does not mind putting 80K BTU furnance it cost only 100$ more but I want to get the correct size and he thinks 60K BTU is correct size for this house.

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Old 04-20-09, 12:57 PM
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The proper way to size a furnace or air conditioner is by doing a heat loss / heat gain calculation. This takes into account the prevailing temperatures during the coldest and hottest days of the year over a long number of years. This is called the design outside temperature. The calculation also takes into account the desired indoor temperatures for both cooling and heating seasons. In addition, it takes into consideration the construction of the house, including number and type of windows, doors, insulation, weatherstripping and many other things. Your contractor should perform this calculation as part of his bid.

A really quick and dirty method is to think back to the coldest outside temperatures you have known while living in this house. How much did the furnace run? If it was anything less than 80% (or more) of the time the furnace is oversized. At design conditions the furnace should run almost constantly. The advent of two-stage furnaces has made this distinction a bit fuzzy as with cold, but not necessarily coldest, outside temperatures the furnace will likely run continuously on the first heating stage and only cycle on the second stage as necessary to maintain the design indoors temperature.
 
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