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I,m building a new 2800 sf colonial home, one contractor says 100000 btu furnace and another says 120000 btu furnace. Why the differance, both are same brand and 92%. The one with the 100000 btu says I can save as much as $50 per month on the gas bill….
AL
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Homy,
It you need a 100,000 btu furnace in a 2800 sf house you should look into the insulation and infiltration details. I have never needed 100,000 btu even in a 6000 sf house in Upper MI. The monthly energy savings will be more than the added monthly amount to your mortgage for better insulation and air sealing. You will be ahead from day one. I pay less than $50 a year to heat my house at 5000HDD (typical upper mid west states).
Ron
*Ron:Maybe Homey lives in Alaska?Homy:Ask prospective HVAC contractors (or whatever they call 'em in Alaska) to see their heat loss/gain calculations sheets for your house. If he can't produce, keep looking.
*Hi Homy,Like Matt suggested, look at his calculations and see who did their homework. Aside from any initial cost, there is little difference between the operating cost of one or the other.Take Ron's advise with a grain of salt. A 6000 sq. ft. house with less than 100,000 btu had better be in Arizona, without windows and not here.Gabe
*Matt: Here in Alaska, we call them wood cutters - the guy who you buy cord wood from each fall.Homy: 100,000 vs. 120,000 isn't a big difference. Although they may have both started with the same specifications, slightly different models or assumptions could easily yield a 20% difference. The guy claiming $50/month saving for going 100k vs 120k is snowing you. Dump him. Other things being equal, a smaller, more appropriately sized unit will save a bit ($1 to 3 dollars a month). Even a spiffy 93% efficient unit won't save you that much in a month, in a new house, and any HVAC person ought to know that.I agree with Ron, that for new construction, 100,000 BTU/hr seems high. If you have moderate insulation and typical (shoddy) workmanship, my rough calculations are that 100,000 BTU/hr would be good to -50F. Got to -41F at my place last year, but probably not at yours. Build really tight, well insulated, and with good labor and you'd only need 30,000 BTU/hr for the same house.My best guess: They are both spec'ing oversized units. They never get call-backs for oversized units. Most people don't know that a well-sized heater doesn't come on for only 9 minutes each hour and don't complain that they were just sold too much furnace, ductwork, etc. People only complain when heaters are under-sized. So HVAC people spec bigger units than are needed. CYA for them. Matt suggests a good way to call their bluff. -David, Kenai, Alaska
*Get a good accurate heat loss calculation for the type construction you have then add 15% for duct and flue losses. Select a furnace with a output BTUH rating equal to this amount and you cant go wrong. In most locations the maximum heat loss occurs only a few days of the year so if you oversize, the furnace will cycle on and off too often and a lot of heat will go up the flue.Also for a great heating system you should use 2X12 floor diffusers located to cover the large glass areas of each room. This may take a few more outlets but it has advantages in that it directs the air away from people and stops the cold air at its source. When using this system you should use 5 or 6 inch pipe to each outlet and you should set the fan switch to (ON) and let the blower run continuosly day and night. Beleive me you will have a more comfortable house. You will not hear the burners come on or the fan reving up, the heat exchanger will never get real hot so you will not lose as much heat up the flue and the air in the house will stay mixed for more even floor to ceiling temperatures.Also motors are designed to run run run and they use less power and take less punishment if they do not have to start and stop all the time. I designed systems for Lennox for about eleven years and we used to call this the C.A.C. system for Continuous Air Circulation, since then people have opted for less outlets abd bigger pipes to save money and have less comfort as the result.
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You make some good points. Since you are apparently experienced with forced air furnace systems, I have a question for you regarding the life expectancy of furnace blower motors. Nobody seems to oil these motors anymore even if they have oil ports on the ends of the shaft and I see these motors being replaced in newer furnaces everywhere I work because the bearings are burning up. I just looked at one that was being replaced and it had a decal on the motor stating " oiling of the motor is unnecessary, but oiling will extend the life of your motor considerably" Now what does that tell you?? I have asked many a furnace installer about this and they all have the same story.. (No need to oil these new motors, they'll run for a long time) I have two neighbors with 4 and 5 year old furnaces and burned out bearings on the motors. How long did Lennox expect their motors to last running continuously??
*Can't understand why anyone would go with scorched air systems, understand it with A/C, but don't like it. Hydronics is so nice bit costs more. 100mbh is a fair amount for your system, insure that the ductwork is sized correctly, supplies should equal returns and that the temp rise across the heat exchanger meets the manufactor's spec's, usually 40-70 degree max. !! I replace quite a few cracked heat exchangers every year for just that problem and make good money redoing the entire ductwork system so that the problem will not reoccur.
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I,m building a new 2800 sf colonial home, one contractor says 100000 btu furnace and another says 120000 btu furnace. Why the differance, both are same brand and 92%. The one with the 100000 btu says I can save as much as $50 per month on the gas bill....
AL