Gas Furnace Sizing

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10-26-2005, 09:38 PM
ksspartan

I got a quote back from my contractor this morning and he has proposed a 90 MBH input, 94% Lennox variable speed two stage furnace. I called York and they told me my old furnace was a 117 MBH input furnace that had an output of 88 MBH. They also told me that the furnace was a 70% furnace which according to my math would be an output of 81.9. This furnace seems to heat the house fine. If I do the math on a 90 MBH @ 94% that puts out 84.6 MBH. I am considering telling the contractor to give me a 100 MBH furnace which would provide 94 MBH?

With a variable speed furnace and two stages of gas heat I won't have any problems being a bit oversized will I?
10-26-2005, 11:41 PM
hvac64

No that wont be a problem,heck why not put in a bigger one, why not twice as big in case it gets real cold. Dont lsten to the guy that does it for a living. You know more than him.
10-27-2005, 01:12 AM
Jultzya

Furnace Sizing Program

Helpful link
10-27-2005, 01:13 AM
beenthere

If your duct work is under sized you could have plenty of problems.

The 90,000 should do fine, most older installations are over sized.

Did the contractor do a load calc.
The purpose of 2 stage is for longer run cycles for comfort.
By over sizing you defeat the purpose.
10-27-2005, 06:43 AM
blown67

Quote:

Originally posted by hvac64
No that wont be a problem,heck why not put in a bigger one, why not twice as big in case it gets real cold. Dont lsten to the guy that does it for a living. You know more than him.

Thats what I love about this board, you say something hoping to get a little insight as to the rhyme and reason for the guys decision on maybe a better recomendation and everytime you get a smartass reply from some insecure idiot so worried someone is trying to cut them or thier knowledge out of the loop. I hope next time you ask someone for some simple info on a subject you dont know everything about they treat you as badly as you treat others.
10-27-2005, 07:10 AM
troyorr

blown 67, I don't usally reply to posters that haven't filled in their profile because I have no idea what their background and qualifications are. I will reply to your post. I say, try not to be don't be the defender of all who come here. Don't try and base your replys on the minimal, limited information usally provided by the original poster. Look at the question and then look at different situations. Is 84.6k big enough? You don't know how big the house is or any other construction features. If the house is 800 sqft, sure 80k will do it. Is the house 4000 sqft. If so, 80k probably isn't big enough. Does the house have 1 window or 200 windows. What exposure are the windows? Looking at the question at face value, the answer is no. Is 84.6k the same as 88k? No so the recommended furnace isn't as large. Simple answer.
10-27-2005, 12:38 PM
propmanage

size

You should get two or three people to size and price unless this is your everyday contractor, and in that case you should let them do the job and trust them unless you have reason not to. Do you trust then to respond late at night in the dead of winter?
10-27-2005, 05:03 PM
blown67

Quote:

Originally posted by troyorr
blown 67, I don't usally reply to posters that haven't filled in their profile because I have no idea what their background and qualifications are. I will reply to your post. I say, try not to be don't be the defender of all who come here. Don't try and base your replys on the minimal, limited information usally provided by the original poster. Look at the question and then look at different situations. Is 84.6k big enough? You don't know how big the house is or any other construction features. If the house is 800 sqft, sure 80k will do it. Is the house 4000 sqft. If so, 80k probably isn't big enough. Does the house have 1 window or 200 windows. What exposure are the windows? Looking at the question at face value, the answer is no. Is 84.6k the same as 88k? No so the recommended furnace isn't as large. Simple answer.

And I believe thats more along the lines of a responce he was looking for, no need to make fun of someone because they dont know all the pros do. I just started reading this board about a week ago, have some questions about my boiler in my house and the furnace in my garage, but dont want to ask just for the fact of the typical responces handed out by most here. I guess maybe the whole site should be for pros only and not let homeowners on to try and give them a little knowledge to help them with contracting a job.
10-27-2005, 06:18 PM
troyorr

blown67, if you want answers to questions, be polite and don't imply we are trying to rip you off. You post about converting a furnace to lp is one example. I don't think the company charging XXX was trying to rip you off. You may have purchased a conversion kit for a few dollars but is it the proper one for your furnace. Those kits are not universal. You put the wrong orifices in or set your gas valve operating pressure wrong and you could at the least, burn your furnace and garage down or at the worst, burn everything you own down.
Make sure you HIRE A QUALIFIED TECHNICIAN to perform a prestart check and setup.

http://www.my-smileys.de/smileys2/033102sor_1_prv.gif (No pricing, due to site rules )

[Edited by jultzya on 10-27-2005 at 10:17 PM]
10-27-2005, 07:27 PM
blown67

I did have a tech start it, He did remove the in and out taps to check the supply pressure and the manifold pressure with his manometer. I just supplied the parts, the part number for the conversion was on the honeywell valve, wasnt to hard to figure out. I myself am someone who works in contracting business but do not claim to know every trade, I have my oil company service, clean and test my boiler every year. They dont deal with Lennox, thats the only reason I didnt use them.
NOWHERE did I say all the pros try to rip you off, but you really mean to tell me that a job like mine that would of taken a quailified tech less than 1 hour is XXX with me suppling the parts, and yes they new I already had the parts, and agreed to install them as long as they were still in the sealed package and the right part number for my application.
This was never about me vs. the pros, and I do thank you for being honest well spoken and not insulting. All I'm saying its a matter of respect weather its mine for you as a professional who knows more about this than I do or yours for me as the person doing the hiring trying to evaluate the job and make and educated decision based on knowledge, not just: call someone, you dont know what your doing.

http://www.my-smileys.de/smileys2/033102sor_1_prv.gif (No pricing, due to site rules )

[Edited by jultzya on 10-27-2005 at 10:18 PM]