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Old 12-20-2009, 11:51 PM   #1
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How to determine best size and type for replacement furnace


We have a gas furnace that is more than 20 yrs old. We're thinking of replacing it and need to figure out the optimal size and type for a new one. The current one (Kenmore 867.762032) is 100,000BTU. Single-stage, I was told, whatever this means. Since the units made now are more efficient, is it possible that a smaller unit would suffice? Also, what types of furnaces are available and what's the main differences between the cheaper ones and more expensive ones?
Thanks much!

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Old 12-21-2009, 12:14 AM   #2
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How to determine best size and type for replacement furnace


Best to have a load calc done to determine what size furnace you really need. Most likely your current one is oversized.

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Old 12-21-2009, 12:24 AM   #3
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How to determine best size and type for replacement furnace


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Originally Posted by beenthere View Post
Best to have a load calc done to determine what size furnace you really need. Most likely your current one is oversized.
Thanks. Is this something I can do or do I have to call a pro? Of course, sooner or later I will have to call one if I decide to get a new furnace. But for now I am just trying to determine an approximate price range to figure out whether we can afford it.
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Old 12-21-2009, 12:46 AM   #4
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How to determine best size and type for replacement furnace


You can do your own.

here is one HVAC Calc it has a 49 dollar fee. But is well worth the money. And has tech support.
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Old 12-21-2009, 03:41 PM   #5
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How to determine best size and type for replacement furnace


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Originally Posted by ilyaz View Post
We're thinking of replacing it and need to figure out the optimal size and type for a new one. The current one (Kenmore 867.762032) is 100,000BTU.
I'd like to try out a spreadsheet to size your new furnace based on what your old 100K input BTU/hr furnace is consuming.

Can I get the Heating Degree Days for a winter month for your location using
http://www.degreedays.net/
and your
Outside Design Temperature from
http://www.hrt.msu.edu/energy/Notebo...by_AJ_Both.pdf
and your BTU/therms/ccfs used for that same month (it's on your gas bill)
?

Thanks.

A lot of people replace HVAC stuff at 20 yrs, but it has an even chance of making it to 35 yrs., give or take. Replacement age is usually less than the age where everything starts to fail and the average is somewhere between 22 yrs and 28 yrs.

Last edited by Yoyizit; 12-21-2009 at 03:55 PM.
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Old 12-21-2009, 04:06 PM   #6
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How to determine best size and type for replacement furnace


Quote:
Originally Posted by Yoyizit View Post
I'd like to try out a spreadsheet to size your new furnace based on what your old 100K input BTU/hr furnace is consuming.

Can I get the Heating Degree Days for a winter month for your location using
http://www.degreedays.net/
and your
Outside Design Temperature from
http://www.hrt.msu.edu/energy/Notebo...by_AJ_Both.pdf
and your BTU/therms/ccfs used for that same month (it's on your gas bill)
?

Thanks.

A lot of people replace HVAC stuff at 20 yrs, but it has an even chance of making it to 35 yrs., give or take. Replacement age is usually less than the age where everything starts to fail and the average is somewhere between 22 yrs and 28 yrs.
HDD:
Month starting HDD 12/1/2008 820 1/1/2009 1096 2/1/2009 743 3/1/2009 640 4/1/2009 324 5/1/2009 116 6/1/2009 22 7/1/2009 6 8/1/2009 2 9/1/2009 52 10/1/2009 297 11/1/2009 438
Outside Design Temperature

From table 2 -- 14

Thanks!!
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Old 12-22-2009, 06:51 AM   #7
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How to determine best size and type for replacement furnace


Use the program I linked to. Instead of someone's guess work(I know, his way is free.
Its also unproved to work.

You don't want to be a statistic on how often guessing is accurate.
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Old 12-22-2009, 11:55 AM   #8
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How to determine best size and type for replacement furnace


Quote:
Originally Posted by ilyaz View Post
HDD:
Month starting HDD 12/1/2008 820 1/1/2009 1096 2/1/2009 743 3/1/2009 640 4/1/2009 324 5/1/2009 116 6/1/2009 22 7/1/2009 6 8/1/2009 2 9/1/2009 52 10/1/2009 297 11/1/2009 438
Outside Design Temperature

From table 2 -- 14

Thanks!!
So what was your gas usage for that HDD= 1096 month?

Give Been There's program more credibility. It has to be way more sophisticated than a single spreadsheet page.
I think now my spreadsheets give a furnace size a few percent lower than what you need. A more useful answer would be the max size needed, since there seems to be an industry bias in the direction of oversizing.

One problem is that every time the furnace turns on, it has to reheat the air & everything else, in addition to overcoming heat loss.
My program doesn't take this extra BTU requirement into account. Heating the 1800# of air in your house 1F might take several thousand BTU, and this could happen up to 6 times/hr, depending on your duty cycle.

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Last edited by Yoyizit; 12-22-2009 at 12:02 PM.
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