# Is there a reason I shouldn't disable second stage heat?



## zootjeff (Jul 11, 2007)

Ok,

So I have a 1800 sq foot house and I have a two stage 92% Trane 80,000 BTU gas furnace.

Portland's all-time record low was set on February 2, 1950 at 3 below zero.

My House load is such that even at -10 degrees F the first stage would be able to keep up and keep the house warm without going 100%

My question is if I should just disable the second stage all together if I'm not worried about the speed at which it heats the house. The justification would be energy savings.. I waste 6400 BTU per hour running the second stage, and only waste 4160 BTU per hour running the first stage. Now the first stage will run longer and so in theory the same amount of energy is wasted per cycle.. But Isn't the 92% number a combo of a more efficient first stage and less efficient second stage?

I'm thinking of putting a toggle switch on the W1 W2 jumper (single stage thermostat) and if I get super cold or want the furnace to turn on really fast, I'll through the toggle. 


Thoughts?


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## yuri (Nov 29, 2008)

You need to check the temp rise thru the furnace after 10 then 15 then 20 then 25 minutes. It may start to ride/cycle on the limit control. Your bonnet/furnace/ductwork can get progressively hotter after a 10 minute cycle and then the supply temp increases to close to the limit setting if you have marginal sized ducts or minimum airflow. See it happen a lot. Otherwise it should be okay. Will wear out the bearings in your ventor fan a LOT sooner as those suckers run at 3000 or more rpm. Robbing Peter to pay Paul IMO.


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## Marty S. (Oct 31, 2009)

Just put a thermostat on that works on temp differential. Second stage will only come on when first stage can not keep the house to temp or for quick warm ups if you use set back. 

The AFUE rating is for high fire on all furnaces.


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## zootjeff (Jul 11, 2007)

yuri said:


> You need to check the temp rise thru the furnace after 10 then 15 then 20 then 25 minutes. It may start to ride/cycle on the limit control. Your bonnet/furnace/ductwork can get progressively hotter after a 10 minute cycle and then the supply temp increases to close to the limit setting if you have marginal sized ducts or minimum airflow. See it happen a lot. Otherwise it should be okay. Will wear out the bearings in your ventor fan a LOT sooner as those suckers run at 3000 or more rpm. Robbing Peter to pay Paul IMO.


Are you sure we are talking about the same thing? I'm talking about disabling High heat, not low heat.. I want to run low heat all the time for a longer cycle..

That means less RPM and less heat rise and less heat over longer periods..


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## zootjeff (Jul 11, 2007)

Marty S. said:


> Just put a thermostat on that works on temp differential. Second stage will only come on when first stage can not keep the house to temp or for quick warm ups if you use set back.
> 
> The AFUE rating is for high fire on all furnaces.


The problem is I just bought a VisionPro8000. And it is only my backup heat.. Right now it's high 20s out so the backup heat is only getting used. The Vision pro has really tight control on the temperature so it cycles a lot..


Is the AFEU rating any better for low heat? How much?


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## beenthere (Oct 11, 2008)

First stage is generally less efficient then second stage.
2 stage is not for saving on fuel, energy cost. Its for comfort.


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## yuri (Nov 29, 2008)

On low fire your fan runs at a lower speed so your heat rise may be the same or higher or lower depending on the rpm of the motor, needs checking anyway. The faster you can get the job done (on high fire) the less wear and tear on the ventor and main fan motor. Kinda splitting hairs but some guys like to do this theory stuff. :whistling2:Then again we can argue the energy wasted by the pre and post purge cycle of the burner when it stops and starts. Pull out the Tylenol.:yes:


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## zootjeff (Jul 11, 2007)

yuri said:


> On low fire your fan runs at a lower speed so your heat rise may be the same or higher or lower depending on the rpm of the motor, needs checking anyway. The faster you can get the job done (on high fire) the less wear and tear on the ventor and main fan motor. Kinda splitting hairs but some guys like to do this theory stuff. :whistling2:Then again we can argue the energy wasted by the pre and post purge cycle of the burner when it stops and starts. Pull out the Tylenol.:yes:



After 30 minutes it is a 45 degree rise on high heat.


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## zootjeff (Jul 11, 2007)

beenthere said:


> First stage is generally less efficient then second stage.
> 2 stage is not for saving on fuel, energy cost. Its for comfort.


If that is the case, why would anyone make a two stage furnace?


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## yuri (Nov 29, 2008)

Comfort. Lennox and Carrier and others have 3 stage and modulating furnaces. Mods can simulate that nice warm comfy feeling you get from perimeter or hot water heating or infloor heating. The Lennox SLP is 98.2 % efficient and the York is close. You would have to start talking to their design engineers to see the efficiency ratings at different stages but who cares anyway? Go to a movie with the wife and dinner and you blew the savings anyway. Nobody is going to live on bread and water and have rabbbit ear antennas on tube TVs are they?:laughing::yes:


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## kb3ca (Aug 22, 2008)

I don't believe there is any significant savings by running in first stage only. It takes the same amount of BTU's to satisfy your heat requirement whether you're running in first stage only or both stages. The amount of energy required is the same. However, using first stage exclusively may provide more comfort since the furnace will run longer. As far as the furnace lasting longer, it's a wash I believe.


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## zootjeff (Jul 11, 2007)

yuri said:


> Comfort. Lennox and Carrier and others have 3 stage and modulating furnaces. Mods can simulate that nice warm comfy feeling you get from perimeter or hot water heating or infloor heating. The Lennox SLP is 98.2 % efficient and the York is close. You would have to start talking to their design engineers to see the efficiency ratings at different stages but who cares anyway? Go to a movie with the wife and dinner and you blew the savings anyway. Nobody is going to live on bread and water and have rabbbit ear antennas on tube TVs are they?:laughing::yes:


Yaa I know, it's peanuts.. The problem is that I'm dorking out with it for fun and want to know the nitty gritty. I'v actually put wireless temperature sensors on one of my vents, near my thermostat, and in my office, and I'm graphing it and seeing the results..

In the mild weather, you get a longer run time, and in the extreme cold, you get a bump in heat for extra comfort. 

When you jumper W1 to W2 on my Trane, you get 10 minute first stage and then second stage there after...

My run times are pretty consistent around 10 minutes with the Second stage kicking on only after 10 minutes when the house has reached steady state, so only when the furnace comes on in the morning and then again in the evening does it ever hit second stage.... At steady state would you expect a cycle time to be more than 10 minutes? Or is that good?

In my case with an over-sized furnace, I think I'd fall in to the case where I want to avoid short cycle.. What is considered a "short cycle" anyway?

I'm currently cycling about 2.5 times per hour with it 32 degrees outside..

Hmm.. If you are sure that it is less efficient to run on low stage I'm not sure what I should do because that's my goal..


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## kb3ca (Aug 22, 2008)

My Trane furnace is 95% efficient in each stage according to the furnace specifications. You can check the paperwork on your furnace or go on-line to find out what yours is. I have my thermostat (VisionPro IAQ) set to 3 CPH and that seems to work pretty well. My furnace only goes into second stage when it's recovering from setback. Runs in first stage most of the time.
From what you've described I would leave yours pretty much like you have it and get a programmable thermostat if you don't already have one. That may be your best option for increasing overall efficiency.


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## beenthere (Oct 11, 2008)

zootjeff said:


> If that is the case, why would anyone make a two stage furnace?


For comfort.

Thats why they are for comfort.


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## beenthere (Oct 11, 2008)

kb3ca said:


> My Trane furnace is 95% efficient in each stage according to the furnace specifications.



Check it with a CA, and you'll find out different.


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## kb3ca (Aug 22, 2008)

Trane documentation states that both stages are 95% efficient. That, of course, is the design specifications under ideal circumstances. When other variables are introduced in the field then you will get different results. If you use a combustion analyzer to determine the actual efficiency of each stage I would think that they would be close to one another even if not at the manufacturers specification. If they are drastically different then there's something wrong with the furnace.


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## beenthere (Oct 11, 2008)

kb3ca said:


> Trane documentation states that both stages are 95% efficient. That, of course, is the design specifications under ideal circumstances. When other variables are introduced in the field then you will get different results. If you use a combustion analyzer to determine the actual efficiency of each stage I would think that they would be close to one another even if not at the manufacturers specification. If they are drastically different then there's something wrong with the furnace.


Its the furnace design. And part of the install. If it isn't set up with a CA. then first stage is a fair amount lower then second stage out of the box.


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## kb3ca (Aug 22, 2008)

How much is a "fair amount lower"? What's the worse you've seen out of the box? Good information, by the way.


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## beenthere (Oct 11, 2008)

kb3ca said:


> How much is a "fair amount lower"? What's the worse you've seen out of the box? Good information, by the way.


Out of the box, as low as 78%.

Not too many that low though, often mid 80s.


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