controling a 2 stage furnace; length of each stage


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Old 11-26-19, 08:07 AM
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controling a 2 stage furnace; length of each stage

Lennox signature 2 stage furnace, 9 years old.
Had a new tstat installed last season and tech had to modify the wiring. IIRC he tested operation of both low and high (second) stage heating.

Got my tune up for this season and tech said there was no wiring for the second stage. He added a jumper and told me that the furnace is more efficient running in second stage. So now it starts in first stage and very quickly (a few seconds) goes to second stage. Assured me that Lennox heat exchangers, in general, do not crack and my delta T was well within spec at second stage. And that a single stage furnace normally runs at the higher gas pressure (3.5) anyway.

I really don't see how it is more efficient. Yes, set temp is achieved quicker and the blower runs for a shorter period of time, but higher speed. I have actually heard that you want a system to run almost constantly (or is that a/c?). With a 2 story house it seems like running the blower as much as possible would help equalize the temp.

I know people have their own ways of doing things and opinions on what is best, this just seems odd to me and after running it one way for 9 years now I've got a big change. Previously I was under the impression that it would only go to second stage as needed, and that would be rare. What is the point my having a 2 stage furnace if it runs nearly all the time at 2nd stage?

Is running it this way indeed better?
As a comfort issue, I find it annoying that I hear more airflow at the vent (noise). Because heat rises and this is a 2 story house we did close down the second story vents, I wonder if adjusting the dampers a bit would help?
 
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Old 11-26-19, 04:47 PM
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First stage is marginally less efficient than second when the unit runs continuosly due to a worse fuel to air mix and shorter flame.

the difference may be 5% or less.

however, when the unit is cycled, first stage lengthens on cycles and reduces total number of cycles. cycling wastes energy - it takes 5-10 minutes for the furnace to reach full capacity and efficiency.

so it's kind of a wash.

2-stage is for comfort mainly, the entire idea is to get more consistent heat. the unit is supposed to run to maintain, not satisfy/overshoot the stat quick and shut down.

at a minimum, the jumper should be removed and the board set up to run on low for the first 10 minutes (varies by brand) or so of each cycle.

ideally, w2 terminal should be hooked to the stat so second stage is only turned on when the house needs it and it can drop from second back to first stage.
 
 

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