# Thread: Furnace size VS. Man. J

1. Professional Member
Join Date
Jul 2007
Location
Chicagoland
Posts
483
Post Likes

## Furnace size VS. Man. J

After calculating with man. J (on paper) a house shows 70400 (71* HTD). I am looking at goodman 90+ equipment. The 70 would undersize by 4000, the 90 would over by 14. Man. J says stay tight to results. Opinions?

2. Go with the 90.

Nothing worse then finding out that the gas company is suppling gas with a 100 BTU lower content then you thought when its at design temp outside. That could put you at 10,000 BTUs light.

3. I'd go less. I did that on mine, and it keeps up just fine when it's -25˚ out.

4. Originally Posted by mayguy
I'd go less. I did that on mine, and it keeps up just fine when it's -25˚ out.
Shows that your calc was inaccurate.

Not knowing how inaccurate his calc is, and in which direction, undersizing could mean he has a 10° cooler house at design.

5. ## it is based on output

If the furnace is 70,000 x 90% = 63000 output and a 90,000 x 90% = 81,000.
I would go with 90,000 just incase R-valve was high on something. Have a small safety factor.

6. Regular Guest
Join Date
Jul 2005
Posts
717
Post Likes
Originally Posted by 21degrees
If the furnace is 70,000 x 90% = 63000 output and a 90,000 x 90% = 81,000.
I would go with 90,000 just incase R-valve was high on something. Have a small safety factor.
************************************************** ********
My thoughts exactly. Go with the larger one and get the benefit of shorter, but efficient, comfortable cycles, like my furnace with only about 2 six minute burner cycles an hour even at design temperature.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•

## Related Forums

The place where Electrical professionals meet.