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What size furnace to buy
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Bob    Posted 12-10-2003 at 22:19:54       [Reply]  [Send Email]
I need to replace my oil fired furnace. It is a 85,000 BTU output unit. All the calculators for furnace size recommend about 45,000 to 50,000 Btu for my size home and location. I can get a 55,000 unit but the 85,000 unit has a better warranty on heat exchanger. Also better burner and is air cond. ready. What problems could I run into by over sizing? This will be second furnace replacement in 25 yrs. Just wondering if being oversized would have anything to do with cracking the heat exchangers.

buck    Posted 12-11-2003 at 09:28:06       [Reply]  [No Email]

Just a little info on your heat exchanger cracks.When a hot spot develops in the exchanger then the cracks are soon to follow. Happened to be home when the man cleaning my furnace was doing the job. Said that my furnace had never been cleaned and I told him that I got it cleaned before each season. He said you need an exchanger and it has never been cleaned before and showed me that the exchanger has to be removed to be cleaned properly so take the time to look at your old unit and see if this is the case.

deadcarp    Posted 12-11-2003 at 09:22:44       [Reply]  [No Email]
I might mention the age & condition of the house is a far more significant factor than anything else. years ago with sawdust insulation and wood heat, we used to gather a big enough woodpile to cover the size of the building by 4 feet deep. well for our 28x40 farm-house, that was about 35 cords/winter(cookstove and neater). nowdays with glass fiber matts that would be 2-3 years of overkill. in fact, few years ago i built a (20-foot with a 10 foot bedroom) dome with good 6-inch insulation, and the sun darn near heats that thing by itself. (at least it keeps itself above freezing).

and expectations are different - then we laughed if the dipper froze-in or the drinking bucket got slushy or we had to thaw the pot out before dumping it into the outhouse - nowdays everybody expects 70 degrees, no matter what. :)

Ron/PA    Posted 12-11-2003 at 08:42:02       [Reply]  [No Email]
Bob, you don't say what area you are from. Most furnace Mfg. reccomend anywhere from 30 btu's to 60 btu's per sqare foot.
Cracking a hot air heat exchanger is not about furnace size, but your burner set up, or normal wear.
UNLESS,, you are so oversized that you are not heating up the exchanger enough to achieve a good temp. If you are that big, you constantly heat up the exchanger a little bit then it cools, and it never really gets hot enough to do it's job. In order to work correctly a heat exchanger has to get hot enough to heat the air after the burner shuts off. The convection action is what really heats the air.
good luck

VADAVE    Posted 12-11-2003 at 04:05:47       [Reply]  [Send Email]
The larger furnace has nothing to do with cracking the heat exchanger. What a larger furnace does to is heat the house up faster and shut off so that it seems to be cycling faster. The bigger blower is better particularly when you put in AC. The "A" coil will put a restriction in the air flow.

Jimmy    Posted 12-10-2003 at 22:25:49       [Reply]  [No Email]
I see nothing wrong with a larger furnance. Some say slightly less efficiency can result in to large a furnance, however the sizes you stated are not that much different. The larger furnance should also have a larger cfm rated blower. Good for AC if added.

Note: LPG is selling for a huge $1.31 a gallon in MI. A few years ago it was $0.88. We use wood for the most part here in MI.

Okie-Dokie    Posted 12-11-2003 at 04:04:53       [Reply]  [No Email]
Looks like you might be better off over-sizing. That would be cheaper in the long run than getting one that would barely be up to the job, because it wouldn't be running all the time. Would you save the differance in initial costs by the time the usefull life span of the heater is reached? Bad time to be having to deal with it any how. Them salesmen know you are over a barrel, with winter here already.

Bob    Posted 12-11-2003 at 08:58:26       [Reply]  [Send Email]
Actually, there is very little cost difference between the two. I was just concerned that being over sized might be what was causing the heat exchanger cracks. Apparantly not, maybe it's the brand, both were Heil furnaces. Thinking about going with a York this time. Thanks for the replies. Bob

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