Gas furnace restarts before reaching temperature


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Old 12-10-17, 02:43 PM
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Gas furnace restarts before reaching temperature

We have lived in our current home for almost 30 years. For the first 22 years or so, we made do with the 1970's vintage NG furnace. It had the habit of regularly having to restart before the house was fully heated in the morning. By restart, I mean the programmable thermostat would call for heat, the unit would turn on, but before reaching the set point it would cycle off, then immediately restart.

I thought that was odd, and tried a couple of different thermostats over the years. The last being a Nest, which detected this condition and admonished me for being a bad furnace owner and neglecting it in some way. We've always been regular with the filter changes, and that older furnace had little to nothing in the way of pressure sensors... why should it? It was old school.

Fast forward to 2012, when we pulled the trigger on a new Day and Night N9MSE. Imagine my amazement when it immediately continued with the series of restarts (usually 3 starts were needed to heat the house in the morning). After a while I hit up the troubleshooting chart, and lo and behold it led me to the condensate trap, which was full of yucky mold. This seemed promising, at least there could now be a reason it was unhappy. So I cleaned it, and if I recall, to the best of my ability to notice, the issue went away.

We've now had this unit about 5 years, and it does continue the restart pattern, albeit irregularly. I just pulled the trap out and although it's a little fuzzy in there, it wasn't blocked.

Despite all the angst, it never throws a code on the blinking LED.

I'm starting to wonder if there is some way our house and ducts are configured that cause cold air to somehow sit someplace until the unit goes off, then rush out and chill the thermostat. This hardly makes any sense, but I'm grasping. When the D&N was installed, we had the ducts tested for leaks and they passed, and no one has said there is anything odd about the way they're run. We have a very ordinary single level ranch house with one long supply run and two returns.

Maybe it's poltergeists?
 
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Old 12-10-17, 04:39 PM
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It's hard to believe that the new furnace doesn't display a fault code when it shuts down.

You should check or have checked the heat rise thru the furnace. That requires checking the return temperature as close as possible to the furnace and the same for the supply temperature. The difference is the heat rise and the acceptable range is posted on the ID tag inside the furnace near the burner.
 
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Old 12-10-17, 05:42 PM
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F,
There is nothing unusual about the way your furnace is running for the age and most likely not having all the blower adjusts of the new furnaces.

I believe the stat really has nothing to do with it. Back in the day this was commonplace.

What seams to be happening is the furnace may be a little oversize and is making heat faster than the blower is delivering it to the house and the furnace is reaching it's high limit setting. When that happens the burner shuts down but if the stat is still calling, the blower continues to run and eventually will reach it's low setting which will turn the burner back on. This cycle will continue until the stat gets satisfied.

Back in the day people used to call it free heat because they were getting heat without using fuel.

Your furnace being older you most likely had a mechanical fan/limit switch which could be manually adjusted. Today they are on timers and not adjustable.

If you have one of the Honeywell fan/limits if you pull the cover you will see a wheel with set points that you can adjust farther apart.

Hope this helps a little.
 
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Old 12-10-17, 05:44 PM
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spott..... this is a 2012 Day and Night furnace.
 
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Old 12-10-17, 06:11 PM
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Pete, completely missed that. Saw the stat replacements and didn't pick up on the furnace.

Completely different than the older ones. Just hope they sized it right.

Thanks. Pete. I guess he can disregard pretty much everything since it doesn't apply in this case.
 
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Old 12-11-17, 07:39 AM
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Thanks guys. After cleaning out the trap yesterday I waited with bated breath to see what would happen this morning. When the unit first came on, it stayed on for a good long time and I thought, By Golly, it was a clogged trap after all. But then, it had to cycle once, briefly, to finish coming to temperature. As soon as I heard it restart, I ran in to check for a code, but the green light was blinking normally.

I hate feeling like this pattern being common to the old and new units is an apples/oranges red herring. But so similar.

My other question in all this is... how common is it for condensate traps to clog with mold? If Iím having this problem, Iím sure thousands of others are too, but obviously these traps are not designed to be a routine maintenance item. On my trap, there is a small port with a hose attached, at the top, which I could use to squirt in some bleach. Might give that a try.
 
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Old 12-11-17, 07:43 AM
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PJ-thanks for the temp rise comment. I have a probe I think will work, will check that. Is there a tricky way to gain access to the supply plenum, which I assume is where the high side is checked?
 
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Old 12-11-17, 06:30 PM
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Depending on the size of your temperature probe you can drill a small hole in the duct work.
 
 

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