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NickPDX 04-18-2012 10:10 AM

Is this furnace big enough?
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Hello everybody, this is my first post on the DIY forums. Sorry if it is a long one. My sister and I have been working on updating and remodeling our fathers house. The house is 768 sq ft on the main floor and is in Portland, OR. It is a single level house with a basement. It has a forced air oil burning furnace which has always worked well in the past. The original duct work is still in tact on the main floor. In the basement it was never framed properly but there were a couple rooms "divided" out with some non anchored walls. Those two rooms each had one duct in the past coming straight off the furnace.

We have properly framed in the basement at this point with insulated exterior walls. We now need to get some proper duct work into the rooms. There are existing ducts going through the rooms that we planned on tapping off of to get vents opened up in the rooms. Before I do anything though I was curious about how many sq ft the furnace should be able to heat. If it heated the space before without insulation I would assume that it would work just fine now too. If anybody out there knows a bit about HVAC I would appreciate some input. I have taken a picture of the model info from the furnace ... I hope it helps.

Thanks for any information or opinion you can provide. :-)

Doc Holliday 04-18-2012 11:02 AM

A Manual J which is a heat load calulation performed on a structure is how a system is sized to a home via how many btu's a structure loses or gains per hour.

Without one I'd have to suspect that your existing system will be okay if not too big now that the envelope (insulation on exterior walls) has been tightened.

You no longer will be losing as much heat as it was when the system was first installed in the home.

REP 04-18-2012 12:36 PM

The furnace you have is twice if not three times too big for the small sq footage you have.
A maual "J" and a manual"D" is the only way to know what size you really need.
Because the furnace is so big it will continuely short cycle and cost way more for fuel than it should.

beenthere 04-18-2012 04:54 PM

Yep, that furnace was over sized before you insulated in the basement. Of course, back when that furnace was made, it was hard to get a smaller oil fired furnace.

NickPDX 04-19-2012 04:22 PM

I had a good feeling that the furnace would be more than large enough to heat the house. Even though I understand the only way to know for sure would be doing the proper tests ... thank you all for confirming it based on your knowledge and gut feelings! :-)

Now that it looks like we are ready to get the duct work installed I do have some questions for that process. Would it be best to create a new topic post or just continue the conversation in here? My questions are mostly around heat vent locations and cold air returns in the basement.

Thanks again for the help!

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