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I just moved into my house and I am wondering if this is normal for a furnace.

So I have my furnace set to 68 degrees. The temperature on the thermostat says 66 degrees. The furnace is on, but does not appear to be blowing warm air (I guess I would describe the air as not warm, but not cold).

So then I turn the furnace up to 71, and the thermostat still at 66 degrees. The air coming out of the vents is warm.

I have not tried this long enough to see what happens as the thermostat approaches 71 degrees.

Is this normal? As the temperature on the thermostat approaches its setting does the temperature of the air coming out of the vents decrease?

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Does your thermostat have a switch for the fan, and is it set to "on" or "auto"? –  BMitch Nov 18 '11 at 3:44
    
It is set to auto. –  Josh Pennington Nov 18 '11 at 9:34
    
Is your furnace a two-stage furnace? –  The Evil Greebo Nov 18 '11 at 10:38
    
And in addition to Greebo's question, how long has the furnace been on when it's running cool and set to 68? In other words, is it just after the furnace cuts on, or are you giving it 2-3 minutes to see if it will get warm without adjusting the thermostat. –  BMitch Nov 18 '11 at 11:23
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I meant, is there flame when the furnace is running? You may not have a pilot, it might be lit only when heat is called for. So you'll have to turn it on, then open the panel to make sure the burners are lighting. –  Tester101 Nov 18 '11 at 13:07
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2 Answers

up vote 5 down vote accepted

It sounds to me like you've got a two stage furnace.

Stage 1, low heat, just enough to maintain temperatures. Won't produce warm air, just warm enough air - much more efficient than running full burn.

Stage 2, high heat, for when it's actually cold. Produces WARM air intended to raise temps. Less efficient than stage 1, but gets the house warm. Stage 1 follows.

The alternative is a 1 stage furnace that's full on, full off, and thus must cycle on and off over and over again.

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The other possibilities are that stage one isn't working, or that it's a single stage furnace wired to the second stage. –  BMitch Nov 18 '11 at 13:36
    
I'm not familiar with the furnace but I would place a reasonable amount on a bet that says stage 2 can't ignite if stage 1 can't ignite, since it only makes sense that stage 2 would be more gas/burners in use than stage 1. –  The Evil Greebo Nov 18 '11 at 15:52
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I was thinking more of a mis-wired thermostat rather than a mechanical failure. That said, the most likely answer is yours, a properly functioning 2 stage. –  BMitch Nov 18 '11 at 17:15
    
I had someone out to look at the furnace. It turns out it the flame sensor was bad. –  Josh Pennington Jan 23 '13 at 14:42
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Try taking the thermostat off the wall and checking the temperature in the hole the thermostat wire is coming through. Sometimes cold air will pull up the wall cavity and being that digital thermostats are 99.9 % accurate the thermostat will get confused thinking it's colder in the house than it actually is. Stuff cotton or a piece of sponge in the hole.

Also check and make sure your return is fairly close to the thermostat. If it's far away from the thermostat your differential will be off. Again that will confuse the thermostat.

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See the accepted answer and attached comments where the OP has already resolved this issue. –  BMitch Jan 8 at 16:11
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