# furnace size?



## dr? (Jan 20, 2009)

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:huh:

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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## beenthere (Oct 11, 2008)

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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## dac122 (Sep 5, 2008)

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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## CowboyAndy (Feb 19, 2008)

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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## Home Air Direct (Jan 6, 2009)

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
Jay


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## Home Air Direct (Jan 6, 2009)

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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## Termite (Apr 13, 2008)

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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## dr? (Jan 20, 2009)

I live just north of seattle,wa. The average temp outside is 45 day and 30 at night.


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## dr? (Jan 20, 2009)

were would I find the fan switch settings?


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## Home Air Direct (Jan 6, 2009)

dr? said:


> 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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## ccarlisle (Jul 2, 2008)

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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## gene_champ (Jan 4, 2009)

*interesting link -*

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? :huh:


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## Home Air Direct (Jan 6, 2009)

gene_champ said:


> 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? :huh:



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:thumbsup.

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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## Dr Heat (Jan 14, 2009)

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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## hvaclover (Oct 2, 2008)

thekctermite said:


> 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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## Home Air Direct (Jan 6, 2009)

thekctermite said:


> 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.


Not that uncommon. Wrong mind you, but not uncommon. I think that happens when the homeowner has too much input in the choice. I am a firm believer that the HO is the one holding the checkbook and should be "part" of the decision, but they often have a "bigger is better" view.

Back in the day when I was VPin the Goodman distribution, more times then not, the burnouts that required replacement (Hvaclover/Drheat: you remember those, I am sure) were also related to oversizing. That is not to say that Goodman didn't have a HX problem, but it was not uncommon to see a 100,000 btu on 1200 sq.


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## Scuba_Dave (Jan 16, 2009)

Our current oil fired hot water boiler is sized at 115k BTU for a 1640 sq ft house

I did the Manual J & included the additions I am (or have) built, sq footage will increase close to 3000 sq ft. Addition is all R19 walls & R38 ceilings (old was R7). As well as an insulated unheated attic & garage. Some of the old walls are still R7, some R13, some R15

It came out around 108k BTU for the new size of the house


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## hvaclover (Oct 2, 2008)

Home Air Direct said:


> Not that uncommon. Wrong mind you, but not uncommon. I think that happens when the homeowner has too much input in the choice. I am a firm believer that the HO is the one holding the checkbook and should be "part" of the decision, but they often have a "bigger is better" view.
> 
> Back in the day when I was VPin the Goodman distribution, more times then not, the burnouts that required replacement (Hvaclover/Drheat: you remember those, I am sure) were also related to oversizing. That is not to say that Goodman didn't have a HX problem, but it was not uncommon to see a 100,000 btu on 1200 sq.



i was lucky. i dumped the line before I got burned. But I never had an install that was over sized either. Nor would I let a customer dictate what size he wanted.

I take that back...had one on a Consolidated 90%er. 1000 sq ft home customer wanted 100kbtu. I argued with him and until he said he was going to use the gas company warranty plan. 

I don't know how many years later I got a call from the customers home but a gas co tech was calling ME (yeah, the fur flew). He wanted me to back up the labor on a bad heat exchanger. I told him since the HO had the monthly warranty plan with the utility how come they wouldn't do it.

Gas guy says they don't cover Ht ex. He started yelling at me saying we weren't honoring or contract with the HO.

Told gas company guy labor warranty was voided as soon as they signed with gas company.





Gas co guy did it on the side. I called the city and busted him. 
No license. Boy did that feel good:thumbup:


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## JohnH1 (Jan 9, 2009)

The stat may be short cycling on the heat anticapator or cycle rate adjustment. You may get better performance setting for a *slightly* longer run time. Dont go to far or you will overshoot the desired temp and have large temp swings. What type of T-stat do you have?


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## key1cc (Nov 21, 2008)

Home Air Direct said:


> 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.
> 
> ...


I live in NJ in a 2800 sq. ft, 30 year old home with 3 massive 5-pane each bow windows on the the front (north side). I have a 100 btu 95% 2stage furnace locked into 1st stage (due to high static pressure) which means I have available theorhetically 61750 BTU output. It actually may be a little less because my manifold pressures were not adjusted yet. Bottom line is it hit 8 and 9 degrees "F" last week and my home temp held the 72F temp setting. It even cycled (occaisionally went off for a few minutes).

Key1


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## beenthere (Oct 11, 2008)

There's a lot of 3000 sq ft homes that could be heated with 80,000BTU 90% furnace, that have 120,000 BTU 90s in them.
Because someone just thought it had to have a large furnace.


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## hvaclover (Oct 2, 2008)

key1cc said:


> I live in NJ in a 2800 sq. ft, 30 year old home with 3 massive 5-pane each bow windows on the the front (north side). I have a 100 btu 95% 2stage furnace locked into 1st stage (due to high static pressure) which means I have available theorhetically 61750 BTU output. It actually may be a little less because my manifold pressures were not adjusted yet. Bottom line is it hit 8 and 9 degrees "F" last week and my home temp held the 72F temp setting. It even cycled (occaisionally went off for a few minutes).
> 
> Key1[/qu
> 
> I know you had a man J done (if memory serves me) but I put 80kbtu in homes like yours.100k Seems a little big from my experience.


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## key1cc (Nov 21, 2008)

hvaclover said:


> I know you had a man J done (if memory serves me) but I put 80kbtu in homes like yours.100k Seems a little big from my experience.


No Man J, no static test, no combustion analysis, no pressure adjustment, no temp rise, no airflow balancing, no anything the  hacks.....that's why I'm on sites like this......but I digress....

Actually I thank them now for contributing to me becoming more and more a HVAC :nerd: everday....

Key1


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## hvaclover (Oct 2, 2008)

key1cc said:


> No Man J, no static test, no combustion analysis, no pressure adjustment, no temp rise, no airflow balancing, no anything the  hacks.....that's why I'm on sites like this......but I digress....
> 
> Actually I thank them now for contributing to me becoming more and more a HVAC :nerd: everday....
> 
> Key1


They did tell you that you have survive an a close range acetylene tank explosion to join the HVAC club didn't they?:huh:


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## key1cc (Nov 21, 2008)

hvaclover said:


> They did tell you that you have survive an a close range acetylene tank explosion to join the HVAC club didn't they?:huh:


I have not even started on the AC system yet.....waiting for the cooling season...then I'll start my flurry of inquireries on super heating, super cooling, superman, etc...

Key1


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## dr? (Jan 20, 2009)

Dr Heat, what would you like to know about my duct work. Also, I put a new filter in 2 days ago.


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## dr? (Jan 20, 2009)

my t stat is a honeywell (th2110D1009) or pro 2000 series


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## Dr Heat (Jan 14, 2009)

hvaclover said:


> Dang. You live in the artic or something?


Now I am scared We agree :laughing:


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## Dr Heat (Jan 14, 2009)

hvaclover said:


> They did tell you that you have survive an a close range acetylene tank explosion to join the HVAC club didn't they?:huh:


hey I've done that:furious: burned my hands and face three years ago


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## Dr Heat (Jan 14, 2009)

dr? said:


> Dr Heat, what would you like to know about my duct work. Also, I put a new filter in 2 days ago.


 as much as you can tell us 

A manual d is a program for laying out duct to a specific velocity or static pressure 

start at the furnace and give the trunk size and number of take offs per duct as you go try to scan in a diagram if you are able.


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## hvaclover (Oct 2, 2008)

Dr Heat said:


> hey I've done that:furious: burned my hands and face three years ago


Well, that is the official initiation to get in. You just had to pay your dues like the rest of us:laughing:


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## JohnH1 (Jan 9, 2009)

dr? said:


> my t stat is a honeywell (th2110D1009) or pro 2000 series


Factory setting for that stat is 5. If you furnace is a 90% eff it should be at 3. If it is a 80% try it at 4. Here is a link to the instruction manual go to page 7. http://customer.honeywell.com/techlit/pdf/69-0000s/69-1968EFS.pdf


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## Dr Heat (Jan 14, 2009)

hvaclover said:


> Well, that is the official initiation to get in. You just had to pay your dues like the rest of us:laughing:


Finally i fit in now if only I could be a real boy. Wait sorry that Pinocchio not the wizard of oz.


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