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Old 01-27-2009, 11:49 AM   #1
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For a natural gas furnace, ~170,000 BTU/H


can anyone tell me some installed prices in this BTU range?

I know how much my inefficient old furnace is wasting, so with the price of a new furnace and a couple of other factors I should be able to figure a replacement date for my old one (if ever).

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Old 01-27-2009, 12:46 PM   #2
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For a natural gas furnace, ~170,000 BTU/H


I dont know of anyone that makes a 170,000 btu furnace. You must have a gianormis house to need that much heat. WOW

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Old 01-27-2009, 12:56 PM   #3
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For a natural gas furnace, ~170,000 BTU/H


As for prices, they are highly localized due to big differences in labor cost and equipment markup. Call several different local HVAC pros to get some competing quotes.
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Old 01-27-2009, 02:08 PM   #4
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For a natural gas furnace, ~170,000 BTU/H


How do you know you need 170,000 BTUs? - Get the correct capacity determined by the heat required for the house. After that determine what kind of efficiency you want to get the input.

In my case, an 80% was more economical in the end because of the installation cost and annual demands. The cost of installing and venting a 95% efficiency was far more the money I could saved despite living in a cold climate (we also air condtion a great deal) beause we had to mate up to very new AC set-up.

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Old 01-27-2009, 02:29 PM   #5
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For a natural gas furnace, ~170,000 BTU/H


Quote:
Originally Posted by concretemasonry View Post
How do you know you need 170,000 BTUs? - Get the correct capacity determined by the heat required for the house. After that determine what kind of efficiency you want to get the input.

In my case, an 80% was more economical in the end because of the installation cost and annual demands. The cost of installing and venting a 95% efficiency was far more the money I could saved despite living in a cold climate (we also air condtion a great deal) beause we had to mate up to very new AC set-up.

Dick
That's what the factory manual showed for my furnace, and it seems to work adequately to heat my house.
Factory spec. is that 83% of the input BTU/Hr gets converted to heat.

2200 sq. ft. house, the 2000 IRC shows +20℉ for the 97.5% winter design temp., medium tightness house, for Nov/Dec I used 250 Therms.

I 'spose if I measure the surface area of my house, including the floor at 50℉, with an internal temp. of 70℉ and looking up the average temp. for my area for these particular 30 days from old newspapers I could figure total heat loss and average R value.
That's another day.

Last edited by Yoyizit; 01-27-2009 at 02:44 PM.
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Old 01-27-2009, 02:49 PM   #6
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For a natural gas furnace, ~170,000 BTU/H


+20 and 2200 sqft I dont think you need 170,000. Depending upon insulation values I would guess at 100,000. Possably 80,000 Definetly have a load calculated on the home. Are you sure you are reading the existing btu correctly. It says 170,000 on the furnace tag itself?
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Old 01-27-2009, 02:56 PM   #7
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For a natural gas furnace, ~170,000 BTU/H


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+20 and 2200 sqft I dont think you need 170,000. Depending upon insulation values I would guess at 100,000. Possably 80,000 Definetly have a load calculated on the home. Are you sure you are reading the existing btu correctly. It says 170,000 on the furnace tag itself?
I'm thinking now I needed 115K and the people before me oversized this thing. I'm glad I didn't pay for this beast.
I couldn't find a tag but I did think there was one on the furnace. The factory manual for my model and serial showed 170k in.
Oversized = lower efficiency.

One of these days I'll have to ask the t'stat what my duty cycle is.
No, never mind.
250 Therms for 30 days is 35,000 BTU/Hr input. This thing is running way below capacity for a pretty cold month for the DC region.

So I guess my question now is,
how much for an installed furnace with this capacity or higher?

Last edited by Yoyizit; 01-27-2009 at 03:02 PM.
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Old 01-27-2009, 03:09 PM   #8
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For a natural gas furnace, ~170,000 BTU/H


Doubtfull you need more then 50,000 to 70,000 otput.

Use this program, and see what you really need.
http://hvaccomputer.com/talkref.asp
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Old 01-27-2009, 03:13 PM   #9
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For a natural gas furnace, ~170,000 BTU/H


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Originally Posted by beenthere View Post
Doubtfull you need more then 50,000 to 70,000 otput.

Use this program, and see what you really need.
http://hvaccomputer.com/talkref.asp
I have requested at least one of the ACCA manuals through inter-library loan. Borrowing ASHRAE stuff this way is like pulling teeth; I hope this goes easier.
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Old 01-27-2009, 03:20 PM   #10
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For a natural gas furnace, ~170,000 BTU/H


I did the calc on my house based on the addition going in & new sunroom. With close to 3000 sq ft I will need around 108k.
My existing boiler is 131k for a 1640 sq ft space

What I didn't realize is that the size of the boiler must be adjusted based on efficiency? IE if I buy a 100k 85% efficient then the boiler puts out 85k ???
Is that correct? I thought if I bought a 100k 85% efficient that the 100k is what it puts out at 85% efficiency?

True, False?

Last edited by Scuba_Dave; 01-27-2009 at 09:03 PM. Reason: -only 131k sp
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Old 01-27-2009, 03:46 PM   #11
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For a natural gas furnace, ~170,000 BTU/H


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Originally Posted by Scuba_Dave View Post
What I didn't realize is that the size of the boiler must be adjusted based on efficiency? IE if I buy a 100k 85% efficient then the boiler puts out 85k ???
Is that correct? I thought if I bought a 100k 85% efficient that the 100k is what it puts out at 85% efficiency?

True, False?
True and True. A 100K btu 85% efficient boiler will give you 100% of it's 85% efficiency, which is 85k btu
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Old 01-27-2009, 04:01 PM   #12
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For a natural gas furnace, ~170,000 BTU/H


Quote:
Originally Posted by Scuba_Dave View Post
I did the calc on my house based on the addition going in & new sunroom. With close to 3000 sq ft I will need around 108k.
My existing boiler is 155k for a 1640 sq ft space

What I didn't realize is that the size of the boiler must be adjusted based on efficiency? IE if I buy a 100k 85% efficient then the boiler puts out 85k ???
Is that correct? I thought if I bought a 100k 85% efficient that the 100k is what it puts out at 85% efficiency?

True, False?
You buy a 100,000BTU 85% efficient, its 85,000 BTU output.

Also, with a boiler, that will have a lot of piping, or runs through unconditioned crawlspace to get to the heat emitters, use the IBR rating.

That means for a house requiring 108,000 BTU output, you would need a boiler with a gross input of around 140,000 to 150,000BTUs, at 85% efficiency.
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Old 01-27-2009, 04:44 PM   #13
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For a natural gas furnace, ~170,000 BTU/H


My concern is that for max. efficiency the duty cycle should be 100%. Practically this can't be done but mine seems to be ~100(35/140) = 24% even with a reasonably cold 30 days.
In spite of this the furnace has held up over more than 16 yrs.
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Old 01-27-2009, 08:59 PM   #14
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For a natural gas furnace, ~170,000 BTU/H


Thanks - it makes sense, just wanted to be sure
I'm actually at 97k for the Calc, so the WGO3 will work (98k)
Everything is in the basement, which is underground

I supplement with wood + we have electric radiant floor heat
Right now the boiler runs maybe 15-30 minutes every 2+ hours
Its actually a 131K *85 = 111k = maybe 40% oversized
With a fire going it may not kick on for 4-5 hours if at all
Depends upon how cold it is outside

Last edited by Scuba_Dave; 01-27-2009 at 09:04 PM.
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Old 01-28-2009, 06:54 PM   #15
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For a natural gas furnace, ~170,000 BTU/H


Do an accurate heat loss and size the boiler from the DOE output if all piping is in basement and not an unconditioned space. How many thermostats?
For more heat loss information see this link.
www.comfort-calc.net/home-page.html

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