2 stage furnace sizing

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Old 02-09-08, 06:12 AM
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2 stage furnace sizing

I recently purchased a new 2 stage variable speed furnace - 75,000 btu / 92 eff rating. I was told that based on measurements that my home needed at minimum a 65,000 btu output.
If a 2 stage furnace uses the first stage for most of the time, ( I think that means it is operating at a 50,000 btu output) does that mean my furnace is too small for my house because it is mostly operating at a well below 65,000 btu that it needs?
Should I have installed a 100,0000 btu /92 eff rating instead?

I guess what I am wondering is if you should choose the size of a 2 stage furnace based on the max or min output that the funace is going to be operating at?
 

Last edited by wwp23; 02-09-08 at 06:16 AM. Reason: further explain problem
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Old 02-09-08, 07:33 AM
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How cold has it been recently over there?

On very cold days the house heat loss will be greater and the furnace (if sized properly) will operate at its higher output (2nd stage)

On mildly cold days, the house heat loss will be smaller so only the 1st stage of heat from the thermostat will be needed and it shall have the furnace run at its lower output (1st stage)

I have seen 2-stage furnaces installed but the original thermostat not upgraded. You need compatible components. Did you upgrade to a 2-stage thermostat too? If not, the furnace will start in low heat and after a few minutes goes into high heat and remains there thru the length of the call for heat, hence defeating its original purpose, which was, to be able to modulate its output and $ave you money.

If your furnace appears to only run on low heat (1st stage), it has either (a) not been too cold over there, (b) the thermostat is a single stage dude, or (c) your furnace is significantly oversized and working on low heat produces all the heat your house needs, so going into high heat is not called for by the t-stat.
 
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Old 02-09-08, 08:31 AM
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Thanks pflor for a reply!

I definately have a 2 stage furnace & a 2 stage therostat.
The outside tempatures have only ranged from 30 to 60 degrees since i put in the new furnace, and have noticed it operating only in the first stage.

My house overall just seems much colder set at the same temp. compared to my 20 yr old single stage furnace.
 
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Old 02-09-08, 09:57 AM
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Originally Posted by wwp23 View Post
I recently purchased a new 2 stage variable speed furnace - 75,000 btu / 92 eff rating. I was told that based on measurements that my home needed at minimum a 65,000 btu output.
If a 2 stage furnace uses the first stage for most of the time, ( I think that means it is operating at a 50,000 btu output) does that mean my furnace is too small for my house because it is mostly operating at a well below 65,000 btu that it needs?
No, your furnace is NOT to small. The ideal behind two stage is long run time... long run time means comfort.

I don't know where you are out of, but with your temps that mild, You may of been able to get buy with a 60,000 furnace.

Should I have installed a 100,0000 btu /92 eff rating instead?
God no, You would NEVER see 2nd stage when it really gets cold out. Be an expensive furnace that you will never see 2nd stage!

I guess what I am wondering is if you should choose the size of a 2 stage furnace based on the max or min output that the funace is going to be operating at?
You want to size it to the max out out...


I have a two stage on a two stage stat, I have long run times and in the day time it always stays in 1st stage to maint temp in the home..

When it starts getting down to -5˚ outside, then my first stage runs non stop, and around -10 to -15˚ 2nd stage starts cycling on and off.

Only time I see 2nd stage when I am coming out of set back of 60˚ night/away time, and when it's -15˚ outside.

Much colder??? Was the old furnace oil? How old was the old one?
 
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Old 02-09-08, 07:57 PM
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hey Jay11J thank you for the reply.

Your reply makes me feel better that i choose the right size furnace.
I live in the Cincinnati area & it can get quite chilly there too.

It was just the fact that i used to run the 20 year old one stage forced air gas 100,000 btu furnace @ around 68* & now i'm running the new one @ about 72* to feel just as warm so I thought maybe the unit was too small.

I just always thought that if I had a more energy eff furnace, that I would not have to have the therm set higher in order to keep as warm. I guess the fact that it is a 2 stage as opposed to a single stage takes some getting used to.
 
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Old 02-09-08, 08:53 PM
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You shouldn't have to rise the temp higher to feel warmer... I don't, and we keep ours at 68˚ and feel plenty warm.

What brand is your furnace?

Was a temp rise taken? The blower may be to high, and/or the house is lack of humidity.
 
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