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Old 09-30-2011, 07:23 PM   #1
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Help sizing new furnace..please


ok our furnace was installed in 1987. We are planning to replace before winter strikes, so very soon! Our problem is we had 4 heating/cooling companies come to give estimates and no a single one did a heat/loss calculation.
The current furnace is 58,000 BTU. They all planned their estimates around that. some bigger, some smaller.

can someone on here help me with what size would be optimal?

House is a single level on slab foundation. 2x4 walls with r-11 insulation.

front of the house faces West it is 45' long it has 1 bay window approx 5x7' . 2 single windows aprrox 3x4' each. so total glass opening of the front is 59' of glass

side of house is 25' with 2 small windows 2x3' so 12' of glass on that wall

back of house again 45' 2 small windows 2x3' and one patio door so again about 59' of glass.

remaining side is 25' with no windows.

8' ceilings with cellulose insulation in the attic. probably about r-19. I know I need more!

My question can someone help me to make a educated guestimate on the size furnace would be optimal.

any info I am missing?

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Old 09-30-2011, 07:33 PM   #2
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Help sizing new furnace..please


For 50 bucks you can do your own load calc, and see what size you really need. http://hvaccomputer.com/talkref.asp

Unless your current furnace wasn't able to maintain temp, you don't need one larger.

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Old 09-30-2011, 08:48 PM   #3
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Help sizing new furnace..please


Are you replacing your furnace because of age, not working properly, or never been enough for your home? The existing furnace is what most companies use as a guideline. There are several sizing guides available online, some free, some with a nominal charge for the software. Talk to your neighbors, see if they have any company they deal with and do your own research as well. It's not rocket science, it's BTU science.
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Old 09-30-2011, 08:59 PM   #4
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Are you replacing your furnace because of age, not working properly, or never been enough for your home? The existing furnace is what most companies use as a guideline. There are several sizing guides available online, some free, some with a nominal charge for the software. Talk to your neighbors, see if they have any company they deal with and do your own research as well. It's not rocket science, it's BTU science.
replacing due to age 1987 and mainly because of a cracked heat exchanger. parts no longer available and it's only 80% efficient. existing is 58,000 btu and to my knowledge no issues with heating. We have only owned the home a year.

Estimates came in with a 40,000 2 stage furnace on one extreme to a 70,000 on the other end of the spectrum. Just wondering what is optimal.
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Old 09-30-2011, 09:04 PM   #5
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Help sizing new furnace..please


Some of it depends on your location. A two stage, larger unit won't be a bad choice, since it can work on reduced fire unless the second stage is needed.....so it acts like a smaller furnace under most conditions. A reputable company can steer you in the right direction.....but do as much research as you can.
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Old 09-30-2011, 10:35 PM   #6
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The only way to know for sure is to do a manual j load calculation. Most furnaces are oversized and it's not a good idea to go by that size if you want optimal performance.
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Old 10-01-2011, 05:29 AM   #7
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The only way to know for sure is to do a manual j load calculation. Most furnaces are oversized and it's not a good idea to go by that size if you want optimal performance.
That's what I am hoping to get help with. I have little over 1000 sq ft, small house no basement.

I know the program mentioned above or $50 would help the sizing and manual J load calculation, but I was hoping to find someone on here who already has the program and would possbly help me out. $50 isnt a lot but I hate using my cc # online, and hope someone on here much more knowledgable than I could punch in my dimensions and get me in the right ballpark

Thanks everyone for your input so far
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Old 10-01-2011, 07:50 AM   #8
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Help sizing new furnace..please


The 50 buck is a limited time license. So anyone that has it, only has it for 60 days.

A 1,000 sq ft house, probably really doesn't need more then 30,000 BTUs of heat. So an over sized 70,000 BTU furnace would be a waste of money, since it would virtually never never need to go to second stage.
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Old 10-01-2011, 08:12 AM   #9
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Help sizing new furnace..please


Additional to what beenthere is talking about, the efficiency rating of a staged furnace is at its high output, so running an oversized unit in the first stage would be counter productive as you will be paying for a high efficiency unit and running it in a low efficiency range.

Spend the $50 and do a load calc, not only will that give you piece of mind, it will also enlighten you so you are better prepared to talk to a perspective contractor. You could also insist that any contractor quoting an installation to you must do a load calc, but you will undoubtedly be paying more than $50 to have someone do it for you.
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Old 10-01-2011, 08:36 AM   #10
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A 1,000 sq ft house, probably really doesn't need more then 30,000 BTUs of heat. So an over sized 70,000 BTU furnace would be a waste of money, since it would virtually never never need to go to second stage.
That's the case with my 1000 square foot home. Even when the low did hit -20F one morning the thermostat hovered one degree below set point so second stage never came on. That's a 45,000 BTU 2 stage furnace and was before we added another R25 in the attic.
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Old 10-02-2011, 10:25 AM   #11
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That's the case with my 1000 square foot home. Even when the low did hit -20F one morning the thermostat hovered one degree below set point so second stage never came on. That's a 45,000 BTU 2 stage furnace and was before we added another R25 in the attic.

more research and talking with the estimators, I was told the 40,000 95% 2 stage furnace would be what to go with.

Now I am researching what thermostat to upgrade to, to make sure I utilize the multi-stage furnace
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Old 10-02-2011, 04:32 PM   #12
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Help sizing new furnace..please


I prefer the Honeywell line. they provide comfort, by having a very tight temp tolerance.

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