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View Full Version : Understanding the sizing of a replacement gas furnace

gary_g
11-30-2007, 11:05 AM
I am just a lowly homeowner with a heat pump so I don't comprehend gas furnace sizing that well. Below is a post from another site. Why would a high-efficiency replacement gas furnace have to be less btus than the original less efficient furnace? Doesn't a 95% efficient 100,000 btu furnace put out the same 100,000 btus as an 80% furnace, only using less gas? Please educate me. Thanx.

"An easy way to determine the size of a new furnace is to multiply the input of the old unit by 60% then divide the efficiency of the new furnace into the result. For instance, if your old atmospheric furnace was 100,000 Btu times 60% = 60,000 Btu output. If your new furnace will be 94% AFUE divide 60,000 Btu by 94% =64,000 Btu. That is the input you need to match your old furnace. In this instance, if you match size for size the system will be 36% over sized."

m kilgore
11-30-2007, 12:05 PM
Gary

The whole problem with that post is that a very high percentage of furnaces are oversized from the beginning. The best way to see what size you need is to have a manual J load calc. run on your house. This will give the closest idea as to what size you truly need. You don't ever want to size off of what is in the house now.

As far as furnace selection goes a furnace with a 100,000 btu input will not get that as the output also. If you look on your furnace nameplate it will give you the input and output for that model. Yes a 90% furnace will have a better output that an 80% because of the efficiency. You want to select a furnace off of the output matched as close as you can to your structure load.

gary_g
11-30-2007, 12:14 PM
OK - I think I'm starting to understand.

100,000 btu in x 80% efficient = 80,000 btu out.
100,000 btu in x 95% efficient = 95,000 btu out.

So, a higher efficiency gas furnace not only saves operating \$\$ but puts out more btu's versus a lower efficiency furnace.

Yes/no?

Thanx.

t527ed
11-30-2007, 12:19 PM
yes

gary_g
11-30-2007, 02:39 PM
Thank you for the education.

Take care.