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





Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old 02-21-2009, 08:59 AM  
travelover
Senior Member
 
Join Date: Aug 2007
Posts: 693
Liked 2 Times on 2 Posts

Default 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)?



__________________
travelover is offline  
 
Reply With Quote Quick reply to this message
Old 02-22-2009, 08:46 AM  
Hube
Senior Member
 
Join Date: Apr 2006
Posts: 114
Default

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



__________________
Hube is offline  
 
Reply With Quote Quick reply to this message
Old 02-22-2009, 10:15 AM  
travelover
Senior Member
 
Join Date: Aug 2007
Posts: 693
Liked 2 Times on 2 Posts

Default

Quote:
Originally Posted by Hube View Post
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?
__________________
travelover is offline  
 
Reply With Quote Quick reply to this message
Old 02-23-2009, 08:13 AM  
Hube
Senior Member
 
Join Date: Apr 2006
Posts: 114
Default

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

__________________
Hube is offline  
 
Reply With Quote Quick reply to this message
Old 02-23-2009, 04:35 PM  
travelover
Senior Member
 
Join Date: Aug 2007
Posts: 693
Liked 2 Times on 2 Posts

Default

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



__________________
travelover is offline  
 
Reply With Quote Quick reply to this message
Reply


Quick Reply
Message:
Options

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

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
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter DIY Home Repair Forum Replies Last Post
Gas Furnace Maintenance Icehouse HVAC 4 02-17-2012 12:32 PM
lux air furnace duane HVAC 3 01-04-2010 07:06 PM
Furnace Advice Needed gunit HVAC 3 01-10-2009 03:17 PM
Attic Furnace Causing Ice Dam HockeyGuy HVAC 3 01-26-2008 06:18 PM
Attic Furnace remodel101 HVAC 0 02-07-2007 11:20 PM