Undersized Furnace


Old 02-18-04, 06:11 PM
Visiting Guest
Posts: n/a
Undersized Furnace

11 month old house; Bryant high efficiency gas furnace; Honeywell programmable thermostat. I keep the house fan on for even temperatures between floors. During construction, an independant company came and inspected for energy efficiency (insullation, house leaks, etc.)

On cold day (-3F), furnace reached only 63F when demand was 68F I checked & replaced filter and thought I was OK - the furnace worked all day and finally got to 68F.. 5 days later, it got cold again (-5F) and again the house was only 63F. Havent had the problem on warmer days.

Attempted resolution:
Installer came and checked system on a warmer day -- "OK, no problems - call when it happens again."

I am concerned that the system is undersized and I will have to fight this problem every winter on the coldest days.

How can I tell if the system is undersized (I dont have confidance in the installer because they stand to lose $ if they have to install a bigger unit).

Thanks in advance!
Sponsored Links
Old 02-19-04, 04:59 AM
mattison's Avatar
Join Date: Mar 2003
Location: Cinti, OH
Posts: 5,549
Received 0 Votes on 0 Posts
Try this link and you should be able to do a heat loss calculation on your home to determine the size of furnace.

Old 02-19-04, 09:04 AM
Ed Imeduc's Avatar
Join Date: Aug 2002
Location: Mountain Williams Missouri
Posts: 18,389
Received 0 Votes on 0 Posts
Not a year old . Call the builder and the furnace company NOW
You dont say where you are but when they ran the heat loss on this home for where it is. This should have all been taken in.That you should have 70o no matter what is out side. For where you are. So call them now and fix it are you will call your lawyer. Do it now before the year is up ED
Old 05-05-04, 11:37 PM
54regcab's Avatar
Join Date: Jul 2002
Location: Oklahoma City
Posts: 661
Received 0 Votes on 0 Posts

Wow this the first time I've ever heard of this, almost all of them are grossly oversized around here.
DO make sure the issue is with the furnace itself and not a ductwork/insulation issue.
-3 is pretty cold, where do you live ?
Old 05-06-04, 10:42 AM
hvac4u's Avatar
Join Date: Aug 2001
Location: NW atlanta
Posts: 3,147
Received 0 Votes on 0 Posts
this is a common problem here, however the a/c is usually undersized, as 54 mentioned.

first do a load calc, you can get it at www.hvac-talk.com
this will give you the numbers to compare to your unit to determine if you have a gripe on sizing or not.

you have NO idea how often the installers grab the wrong unit, install the upstairs down and vice-versa, supply house sends wrong one etc.

last week i found 2 furnaces side by side that were swapped.

if sizing is correct, i would look at the duct. it may have come apart at a seam somewhere (have seen this too on ductboard trunklines improperly supported).

i would do the load calc first, then call the installer again
ask if he did a load calc and ask to see it

good luck
Old 05-09-04, 06:36 AM
GregH's Avatar
Super Moderator
Join Date: Jan 2001
Location: Manitoba
Posts: 10,139
Received 37 Votes on 35 Posts
Just wondering how you are making out on this?


I agree that it should be the contractors problem not yours.
Make sure you carefully document dates, who you talked to and get everything in writing.

Where do you live and what size is the furnace?

It sounds to me as well that the contractor could have goofed up in the sizing or something is not working right.
Your furnace should maintain about 72 degF on the coldest day of the year running nearly continuously.
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 Off
Refbacks are Off

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: