Furnace Sizing - HVAC - DIY Chatroom Home Improvement Forum

Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > HVAC


Thread Tools Display Modes
Old 09-17-2008, 09:30 AM   #1
Join Date: Sep 2008
Posts: 1
Rewards Points: 10

Furnace Sizing


Heat load caclulation: 59,100 BTU

Proposed Furnace #1 (two contractors):
Efficiency: up to 96.7% @ 60,000 BTU = 58020 BTU (max.)
Two stage / variable blower

Proposed furnace #2 (one contractor)
Efficiency: 95% @ 80,000 BTU = 76000 BTU
Two stage / variable blower

How close does the actual output need to be to the calculated Heat Loss? I don't want to be stuck with a furnace running full time on the second stage and yet not being able to keep up with heating the house on cold days.
On the other hand, if the larger furnace is 17,000 BTU higher than the heat load, will it short cycle on those "colder" days?


Swolef is offline   Reply With Quote
Old 09-17-2008, 09:57 PM   #2
Join Date: Mar 2008
Posts: 109
Rewards Points: 75

I would go with the second furnace despite the fact you think it is oversized. Remember that for the most part, the furnace will be able to heat your house and satisfy your thermostat while running in first stage anyhow.


statman is offline   Reply With Quote
Old 09-18-2008, 11:07 AM   #3
Join Date: Sep 2008
Location: Upstate New York
Posts: 368
Rewards Points: 250

What is the fuel: natural gas or propane?

How was the heat load calc done? If a meticulous job was done with a Manual J then #1 may be okay. If you're not sure then go over the numbers again with whomever did the calc.

Being slightly undersized won't hurt too much, but it hurts when you pay more for a furnace initially and then pay more again due to short cycling. Short cycling may also affect comfort.

Don't forget Manual J is setup to be conservative, so your actual heat loss may lower than that.

Last edited by dac122; 09-18-2008 at 11:12 AM.
dac122 is offline   Reply With Quote

Thread Tools
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 Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Heil Furnace Burner Problem timothyr HVAC 79 02-12-2014 07:01 PM
Question Re: New Furnace and Duct Sizing DSimo HVAC 3 06-17-2008 07:24 AM
Furnace sizing thumbkins HVAC 5 03-26-2008 08:05 AM
Furnace sizing opinions moneymgmt HVAC 2 05-29-2007 12:26 PM
Replacing My Furnace, few simple questions compression HVAC 10 02-03-2007 05:00 PM

Top of Page | View New Posts


Search Engine Friendly URLs by vBSEO 3.6.1