DIY Home Improvement, Remodeling & Repair Forum > HVAC > Sizing a new gas furnace

Help Support House Repair Talk by donating using the link above.
 Thread Tools Search this Thread
02-21-2009, 09:59 AM
travelover
Senior Member
Join Date: Aug 2007
Posts: 693
Liked 2 Times on 2 Posts

Sizing a new gas furnace

I recognize the importance of correctly sizing a gas furnace. I used a spreadsheet program to do my own Manual J calculation and came up with 31,400 btu / hour heat loss at 5 degrees, for SE Michigan for my 2100 sq ft. Colonial house. To cross check this I looked at my gas consumption for 2008 and compared to the heating degree days for my area in the same time period. This calculation gave me a heat loss of 32,400 btu / hour.

My question is this : What kind of a safety factor should I use to size the furnace (beyond the efficiency rating correction)?

02-22-2009, 09:46 AM
Hube
Senior Member
Join Date: Apr 2006
Posts: 114

A 40,000 input furnace @ 90 % efficiency will give an output of 36000.

02-22-2009, 11:15 AM
travelover
Senior Member
Join Date: Aug 2007
Posts: 693
Liked 2 Times on 2 Posts

Quote:
 Originally Posted by Hube A 40,000 input furnace @ 90 % efficiency will give an output of 36000.
Thanks, that part I understand. But a furnace is typically slightly over sized to account for worst case conditions - very low temperatures for a few days. My question is how much should a furnace be over sized?

02-23-2009, 09:13 AM
Hube
Senior Member
Join Date: Apr 2006
Posts: 114

If I know the heat loss has been calculated correctly by using up to date heat loss/gain methods and also having the proper design temperature for the area, I like to have approx 10 % added.
In your case,I suggested 36000 total output, which would be approx 10% more than your calculation.
One of the main reasons to go a bit over the actual heat loss figure by 10% is to compensate for any "short-lived" temperature drops in your local area weather.
Note; oversizing a furnace by too much could create what is known as "short cycling" and hence cause temperature "swings " within the home

02-23-2009, 05:35 PM
travelover
Senior Member
Join Date: Aug 2007
Posts: 693
Liked 2 Times on 2 Posts

Thanks - short cycling is exactly what I'm trying to avoid.

 Thread Tools Search this Thread Search this Thread: Advanced Search

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules

 Similar Threads Thread Thread Starter DIY Home Repair Forum Replies Last Post Moose71275 Framing and Foundation 38 09-21-2016 10:38 AM llatour77 Framing and Foundation 10 04-30-2015 11:36 AM tractng General Home Improvement Discussion 11 01-05-2015 02:52 PM Mvlawn HVAC 7 07-15-2014 10:34 AM CallMeVilla Plumbing Forum 2 08-06-2012 01:09 PM