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Old 01-20-2009, 01:16 AM   #1
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furnace size?


I just moved into a brand new home and i'm a little concerned. Here are my concerns:

1. the heat comes on about every 5 to 10 mins
2. I lose about 5 degress in those 5 to 10 mins
3. a gas guy come out to my house for unrelated issuses and I asked him the same Questions and he said my furnace is to small

The furnace I have is 66,000 btu's. I have about 2700 sq. ft. of house with 9 ft ceilings and alot of windows I looked in the attic and in the crawl space as well, and insulation is not the issuse ( there's alot ). Is the gas guy right?

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Old 01-20-2009, 07:21 AM   #2
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furnace size?


If the furnace is shutting off, then it can't be too small.

If it was too small, it would run 24/7, and your house would not be warm.

If your house loses 5° in those 5 to 10 minute off times, it may be that your stat is not calling for heat again as quick as it should.

What is the outdoor temp, when it is losing those 5° in 5 to 10 minutes.

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Old 01-20-2009, 08:39 AM   #3
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been is correct. What was your thermostat temp set at (set point), what was the approximate outdoor temp when this was happening?

A unit properly sized for the dwelling will meet its heating needs virtually all of the time, except on those days when temps are far beyond the norm. With the arctic blast many locales received in the last week, many folks furnaces were working overtime.
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Old 01-20-2009, 08:54 AM   #4
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furnace size?


If it is short cycling it could be a few issues...

The hi/low/limit (fan switch) settings could be wrong. If there is a narrow gap between the hi and low settings it would make it run in short bursts, but more often... at least that was the case with my furnace on the wood side. The hi setting was 125, low was 100 and the motor for the fan (belt drive) was set for the fastest speed so it was cooling the heat exchanger too fast.

If you are loosing heat that fast, then I could see it running alot but in short cycles if the furnace is throwing alot of heat. How fast does it heat up when it does run?

When we went through that cold snap last week, there was one morning where I went down to get the fire going after running the oil all night. When I went down the temp in the house was 64. 20 min later it was 57. The temp outside was -30.
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Old 01-20-2009, 09:32 AM   #5
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Standing on the curb with my thumb out in front of me and looking at your house with one eye closed, 66,000 btu seems weak for 2,700 sq with 9ft ceilings. But (and ain't there always a but), we don't know where you live, so a house in Miami will have much different requirements then a house in Fargo.

That said, at 40 btu/sq, you are looking at 100,000 btu+. Even if you have all the best materials, your still probably looking at 25 btu/sq and that puts you at 67,500 btu and that is output. If your 66,000 btu furnace is an 80%, you are only getting 52,800 btu output. 93% and you get 61,000 btu, still short of your probable need.

Now, the short cycle does not compute for a need for btu. Listen to the above posters. Your problem beyond size is more likely in settings and adjustments, or bad design. Again, don't know where you live, but that thing should be running long just based on size.

Good Luck
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Old 01-20-2009, 09:41 AM   #6
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I have to let the techs talk adjustments, but maybe you should look at and report your T-Stat location. Is it located too close to a hot spot? If your T-Stat is located where the warmest air is generated in your system, it will satisfy the stat. Then, check to see if there is cold air leaking into the back of the stat from the attached wall. This could cause the stat to require the need again. Rare to have both of these issues, but there is no substitute for bad design.
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Old 01-20-2009, 10:02 AM   #7
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I don't know where you live, but unless it is somewhere warm I'm going to make a generalization that a 66,000 btu/h furnace is much too small for a 2700sqft home. There are of course a number of variables, but for comparison my 2000sqft home has a 120,000btu/h 93% unit.
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Old 01-20-2009, 10:47 AM   #8
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I live just north of seattle,wa. The average temp outside is 45 day and 30 at night.
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Old 01-20-2009, 10:52 AM   #9
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were would I find the fan switch settings?
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Old 01-20-2009, 11:01 AM   #10
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Quote:
Originally Posted by dr? View Post
I live just north of seattle,wa. The average temp outside is 45 day and 30 at night.

Even the most basic HVAC calc shows 108,000 - 118,000 btu needed for your selected area. Now understand that many many factors can alter this calculation, but the fact remains that you live in pretty consistantly chilly area (lots of humidity) and you did say you had 9ft ceilings and lots of glass. Both require extra heat.

Mouse over your climate #, and then enter your sq ft and then hit calculate.

http://www.hvacopcost.com/equipsize.html

This by no means is accurate, but it will somewhat close and may help answer your size question.

Good Luck
Jay
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Old 01-20-2009, 12:10 PM   #11
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furnace size?


HomeAir Direct: that's an interesting link...just so as I get the definitions straight, square footage means "of living space" ie areas of each room - or outside dimensions (length times width)?

Thx
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Old 01-20-2009, 12:28 PM   #12
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furnace size?


on the approx sizing of heating & ac. wondering how basement area would figure in. many variables there. i have the foam block walls, and 2 " of the 250 foam under floor. should i double the sq ft?
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Old 01-20-2009, 12:43 PM   #13
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Originally Posted by gene_champ View Post
on the approx sizing of heating & ac. wondering how basement area would figure in. many variables there. i have the foam block walls, and 2 " of the 250 foam under floor. should i double the sq ft?

As I said, that calculator is just a simple tool and it's outcome is flawed by lack of real data. To get the true measurement of "Load" a Manual J calculation needs to me performed.

To answer the OP, the Sq Ft needs to be the considered actual living spaces. If in fact, that space is the whole of your above ground structure, then LxW will work, but often that is not the case.

As for the basement portion of the structure, that one can be a little tricky. Variable can be % of exposed sub-level walls, walk-out basements, and in your case, insulation (and it sounds like a lot of it).

If your basement is part of your living space, you can consider that space at 50% and still come out with a close calculation. (***Disclaimer - For reliable load data, do the Manual J)
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Old 01-20-2009, 02:06 PM   #14
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have a tech do a full manual J and a manual D this must be done correctly as one or two miss inputs can drastically skew the results.

66,000 btu 93% is not too far off for Seattle if the wall insulation and ceiling insulation R values are way up there and the house is tight.

That said furnace cycling is an indication that the furnace can reach set point and is not too small.

Check your air filter

And tell us about your duct work (the manual d can be done easily in short form) this is more likely the source of your trouble
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Old 01-20-2009, 02:12 PM   #15
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Quote:
Originally Posted by thekctermite View Post
I don't know where you live, but unless it is somewhere warm I'm going to make a generalization that a 66,000 btu/h furnace is much too small for a 2700sqft home. There are of course a number of variables, but for comparison my 2000sqft home has a 120,000btu/h 93% unit.
Dang. You live in the artic or something?

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