Derating a Furnace

[lksmith]

Homeowner question here...

Should a furnace be "derated" when installed at a high elevation, say 5,200 feet?

And then the really dumb question, what does derating mean (to me as the user?)

Larry Smith

[mckcd]

Everything you need to know should be in the installation manual.

[lksmith]

Well now, I suppose if I were an installer, and had an installer manual, I'd know the answer. But since I'm NOT an installer, and therefore DON'T have an installer manual, I don't know anything about the issue.

My request would be that if you don't want to give me a reasonable answer to what I thought was the right forum for this sort of question, then just don't answer at all.

Larry Smith

[ARPA]

Before this thread goes off on some tangent, and it gets closed, please read the site rules about how we on this site treat each other.....

[4l530]

Originally posted by lksmith
Homeowner question here...

Should a furnace be "derated" when installed at a high elevation, say 5,200 feet?

And then the really dumb question, what does derating mean (to me as the user?)

Larry Smith
Typically at a higher elevation they install smaller spuds henceforth known as 'derating'. I'm not sure that all makes and models require this but I suspect they would. This is where the installation manual comes in handy. They are wanting to maintain the combustible mixture ratio in the thinner air to not overfire the furnace.
To the homeowner it means nothing except possibly consuming slightly less gas.

[lksmith]

Thankyou for your response. That's what I wanted to know.

Last question and I'll move on. Does derating effect furnace sizing? In other words, if load calcs called for a 100K furnace, yet you knew it would be derated due to the elevation, would you need to consider a 110K or 120K furnace?

Thanks in advance.

Larry Smith

[mckcd]

I was trying to help you learn, not just tell you. It's the same thing I would tell an employee. "Check the manual first then call me if it doesn't answer your question."

When was the furnace installed?
What brand and model?

Did the contractor leave the manual to the furnace?

Yes derating affects furnace sizing. Also, your load calc specifies the output BTUH not the input.

100,000 BTU 80%AFUE furnace = 80,000 output
100,000 BTU 90%AFUE furnace = 90,000 output

etc.

[Edited by mckcd on 10-14-2006 at 10:04 AM]

[lksmith]

mckcd

Didn't mean to snap at you last night, no excuse, but it was just one of those days.

The furnace that is currently installed is a LP fired Ruud Achiever 90 Plus 120K BTU. It's about 10 years old. It works like a champ. I had recently done some load calcs and discovered it to be a bit oversized (82,500 BTU by HVAC Calc). I had a service check done on the unit this summer and the tech said it had not been derated (elev. 5,200 ft.). Since it was working fine he suggested I just leave it alone. Which sounds fine with me. I was just gathering information for the next time around (hopefully many years from now) trying to figure out if that's something I should consider or at least bring to the attention of the HVAC contractor and then if it is derated, does that mean I may need a slightly larger furnace to offset the derate? When you derate, does that mean you reduce the BTU input and thus also reduce the BTU output?

It's all really hypothetical since nothing's about to happen anytime soon.

Anyway, sorry to have flamed you last night. It's amazing what a bit of sleep will do for you.

Larry Smith

[mckcd]

I know that some Rheem furnaces (I assume this to be true with RUUD also) do not require deration up to a certain elevation.

Which model it is, I'm not sure. I'm not familiar with their equipment. You might check their website to see if you can download a manual.

I think the general rule for a 90% AFUE furnace is 2% per 1,000 feet. 10.4% @ 5200 ft. Of course manufacturer instructions supercede this rule.

120,000 BTUH input, 90% efficient = 108,000 output.
108,000 @ 10.4% deration = 96,768 output

Your furnace is probably sized right. The next smallest size may have been too small (100,000 furnace would be 80,640).

Even mechanical engineers include a fudge factor to cover their butts. Most people won't complain that their furnace is slightly over sized. If it's too small that's when you really have problems.


[Edited by mckcd on 10-14-2006 at 03:07 PM]

[lksmith]

Thanks for the info. That's exactly what I wanted to know. I appreciate your help.

Larry Smith

[lksmith]

Oops, one last question...When you talk about a 100K BTUH furnace are you always talking about the INPUT? Then, when you do load calcs you come up with OUTPUT required? So, in general terms, if you have a furnace rated at, say, 120K BTU 80% AFUE, then your heat load ought to be less than 96K BTU? Is that correct? Thanks in advance.

Larry Smith

[comfortdoc]

Originally posted by lksmith
Oops, one last question...When you talk about a 100K BTUH furnace are you always talking about the INPUT? Then, when you do load calcs you come up with OUTPUT required? So, in general terms, if you have a furnace rated at, say, 120K BTU 80% AFUE, then your heat load ought to be less than 96K BTU? Is that correct? Thanks in advance.

Larry Smith

Correct

[lksmith]

Thanks!