how to determine furnace size needed?

Old 12-04-02, 10:59 AM
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Join Date: Dec 2002
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Question how to determine furnace size needed?

A contractor estimated I needed a Weil-Mclain CGa-5 (140,000 BTU) unit.
I found out online the many variables in making the determination - almost none of which he could have considered, or had access to. The house has 2 zones for 1,208sq ft. total. The unit is $399 from the local gas supplier. The CGa-4 (105,000 BTU) is free.
He estimated the job (including 2 new pumps and a blower to assist the venting (although none is present now) at $4,300. I thought that seemed excessive.
Any help would be appreciated, thanks in advance.
Old 12-04-02, 05:53 PM
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Join Date: Jul 2002
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Lightbulb 140,000 for 1200 sq ft

DO you live in the top part of Canada in an old uninsulated house ??
Sounds like overkill to me, but it depends where you live and how well insulated your house is.
Typically a HVAC contrator guesses the size and you normally end up with more than twice what you need.
Your heating load is probably closer to 50,000 - 70,000 BTU.
There are several online heat load cacluators that will tell you actual heat load.
Don't be shocked by the seemingly low #'s
Old 12-05-02, 06:05 AM
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A great Heat Loss Calculator

I found a GREAT program with everything you need to accurately figure heat loss (and more). You can download a demo for free and you can purchase a limited license ($39.00) that will allow you to do the calculations for a single house. This is the same program used by contractors. I have added their web link and phone number below.

Questions? For More Info, Call Toll Free! 1 888 736-1101
8:00 a.m. to 8:00 p.m 7 days/week

HVAC Computer Systems Ltd.
613 233-1101
888 736-1101

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