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Old 06-08-2010, 03:13 PM   #1
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How to size a replacement gas furnace


I am thinking I should replace my 24 year old Lennox G12Q5-165 furnace before the heat exchanger goes bad. This is the orginal furnace and I am not sure if the builder spent a lot of time sizing it correctly for my 3 story home with two stories above grade = 2,400 sq ft and 1,400 sq ft in an unfinished basement. Should I look for a 165,000 btuh unit or has technology improved so that a smaller unit would do the job?
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Old 06-08-2010, 05:08 PM   #2
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How to size a replacement gas furnace


Unless you live in an area with -30F winter temps. That thing was grossly over sized.

Are you looking to upgrade to a 90%plus efficient furnace, or stay with an 80% furnace.

Might want to spend the money and do your own load calc to find out what size you really need.

http://hvaccomputer.com/talkref.asp

Its 49 bucks. And it can save you that much on the lower cost of the smaller right sized furnace alone.
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Old 06-08-2010, 06:58 PM   #3
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How to size a replacement gas furnace


You ain't seen nothin until you have seen one of our ICG 275,000 BTU lowboys, now that was a big furnace.
Beenthere is right and a multistage furnace will be more comfortable in your house. For that size of house a continuous running fan with energy saving ECM motor is recommended to even out the hot/cold spots. Check out the Lennox G71 on their website.
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Old 06-08-2010, 07:31 PM   #4
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How to size a replacement gas furnace


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Originally Posted by B4Dark View Post
my 3 story home with two stories above grade = 2,400 sq ft and 1,400 sq ft in an unfinished basement. Should I look for a 165,000 btuh unit or has technology improved so that a smaller unit would do the job?
With the BTUs used from last winter, which will be on your gas bills
and your Heating Degree Days and Outside Design Temperature for your location from the Web
you could get some idea of how oversized your furnace is.
Mine was only on 1/3rd of the time for a reasonably cold winter, for a house and furnace roughly the same as yours.

I haven't heard from anyone who noticed a reduction in their heating bill after buying a more efficient furnace so the overall difference from old to new must be only a few percent.

Last edited by Yoyizit; 06-08-2010 at 07:37 PM.
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Old 06-08-2010, 08:49 PM   #5
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How to size a replacement gas furnace


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Originally Posted by Yoyizit View Post

I haven't heard from anyone who noticed a reduction in their heating bill after buying a more efficient furnace so the overall difference from old to new must be only a few percent.

Lots of people notice a big difference in their heating bills.
Have heard back from many customers about it.
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Old 06-08-2010, 09:46 PM   #6
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How to size a replacement gas furnace


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Originally Posted by beenthere View Post
Lots of people notice a big difference in their heating bills.
Have heard back from many customers about it.
I hear it all the time how they love how much money that new furnace saved them. Oversized and low AFUE furnaces eat up a lot of gas.
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Old 06-09-2010, 10:10 PM   #7
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How to size a replacement gas furnace


Thanks for the good advise on checking the BTUs used and calculating the needed size. I was suspect that the builder had a truck load of these furnaces and used them up in my development with the "Big is Better" concept. I am in Colorado at 7,500 feet.
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Old 06-09-2010, 10:26 PM   #8
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How to size a replacement gas furnace


Do a load calc. Then add 20% to that for the derate for your high altitude.
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Old 06-10-2010, 08:47 PM   #9
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How to size a replacement gas furnace


Got it, add 20% for altitude. God, I hope I don't have to get an engineering degree to replace this old furnace............my math scores are not that good! Thanks
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Old 06-10-2010, 09:40 PM   #10
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How to size a replacement gas furnace


I must have only heard from the people who didn't notice a difference in their bills; the other ones are happy so they didn't post.
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Old 06-10-2010, 09:55 PM   #11
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How to size a replacement gas furnace


Vast majority of people. Won't post on a web site just to say. they are happy with their new A/C,HP, or furnace. They don't think about it much if its working good. And not costing them much to use.

Most people only post when they have a problem. or are just unhappy with their system. Or their bills.
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