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Old 11-14-2009, 09:01 AM   #1
krb
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Furnace size


I am replacing my furnace and for now it will be the Lennox G61V two stage.
The problem I am running into is two contractors have two different opinions.
One wants to put in the 91,000 btu, the other 111,000 btu furnace.
The 91er used computer heat value calculations, the other used the slide ruler. The 91er used 5 degrees, the 111er used -15, (I think with windchill).
The 91er says run 5 to 7 mins. then stage two kicks in if need be and the 111 stage two will never kick in, and has enough to run more if I was to add another room.
The 111er says run 15 mins. beforestage two comes on and the 91 will run constantly in real cold wx.
Additional the 91er will add a two stage t-stat, the 111er says not needed, Use the DIP settings.
I know I just want the proper fit for my home and to run the most efficiently.

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Old 11-14-2009, 09:10 AM   #2
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Furnace size


Where are you located ?
How big is your house ?
How big is the old furnace (could be oversized so may not matter) ?
Newer windows or insulation ?
So the 91er did a Heat Loss Calc based on your houses values ?

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Old 11-14-2009, 09:24 AM   #3
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Furnace size


Scuba Dave is right. I am a Lennox tech and can tell you lots about that furnace. I need to know the name and model# and input/output of the old furnace. You should definitely use a 2 stage thermostat and that will eliminate all the nonsense they are telling you about timing. The 2 stage stat will kick into high fire if the temp drops more than 1 degF and is much better. If you can afford it the G71 is a better unit for not much more $$ ($600) and I can tell you why. Give us more info and we can help.
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Old 11-14-2009, 10:21 AM   #4
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Furnace size


Also check if it's a G61 or a G61-71. The 61-71 qualifies for the tax credit,up to $1500.
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Old 11-14-2009, 11:45 AM   #5
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Furnace size


Thanks to all that are replying and helping.
Here are more details:
2650 sq. ft. colonial in lower Michigan.
Old furnace, 20 yr. old one stage, 132,000 btu.(They all say that is too much).
Mostly newer windows, some as recent as two months ago.
Added 8" insulation in attic
I know there are many variables involved, but my main concern is what is the proper setup. Why do they conflict each other?
Juri wrote: The 2 stage stat will kick into high fire if the temp drops more than 1 degF. I'm not sure that you want second stage kicking in for one degree.
As far as the furnace, G61V is 95% and is good for the tax credit and if the G71 is the modulating furnace, the 91er who did the computer calc's, said it does not show a return on the, I believe it is called POI.
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Old 11-14-2009, 11:59 AM   #6
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Furnace size


Do the load calculation yourself:Load Calculator

There are different ways to time on a 2 stage furnace with the DIP switches but these guys are confusing you. In mild weather the G61 runs at 70% of full fire and usually that is enough. In cold weather when it cannot keep up and drops 1 deg. more than the first deg it takes to start the cycle then it goes to high. A more accurate and sensitive way to run it. Both ways work.
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Old 11-14-2009, 12:48 PM   #7
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Furnace size


I think you'll find that even in MI. A 90,000output is large for your house, with the improvements you made.

2 stage furnace should be controlled by a 2 stage thermostat.
It provides far more comfort that way.
Using the timer, defeats much of the reason to have a 2 stage. And is somewhat a waste of money.

However. If you have lots of money to waste.
Yuri and I will gladly take some of it off your hands. For beer money
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Old 11-14-2009, 12:56 PM   #8
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He needs a G71 and a zoning system. Now thats some serious coin and a couple interesting days work for me.
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Old 11-14-2009, 01:40 PM   #9
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Furnace size


Sorry about that. I missed the 91K BTU and not 90K


Last edited by Marty S.; 11-14-2009 at 01:48 PM.
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