tempstar furnace smart valve cycling


Old 02-10-10, 03:37 PM
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tempstar furnace smart valve cycling

tried searching for some help on this, but for some reason the search feature is not returning anything even if i just put in "furnace".

anyway i have seen some other posts related to similar issues. not really finding a tempstar model number on my unit anywhere..? but the furnace has a Honeywell smart valve SV9501. everything generally works as it should, though the colder it gets and the more it has to work the more likely it runs into this cycling problem. the harder it works, the shorter the cycles get. at worst the burner comes on for only 5 seconds or so, then shuts off for a couple seconds, then back on.. over and over. if it gets to this point the blower might not even come on so i have to turn off the heat for 15-30 minutes. then if i turn it back on it will work well enough to heat up the house. usually it will still be cycling, but the cycles are longer and it give the blower time to come on and it remains on throughout the burner cycling.

so could it be a temp sensor that it telling it to shut off? looking for some suggestions on where to start with this.
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Old 02-10-10, 04:11 PM
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What is more unusual than most, regarding your burners going out ,is the fact the fire lights back up by itself, in seconds, without a need for the furnace to begin sequencing all over again.

Do you have a pilot ignition and the pilot is staying lit even though the main burners go out? Can you tell if there is a single-wire flame sensor at the burner that is the opposite side from where the ignition source is?

Or, didn't you word the post quite right?, and you meant to say that after the burner shuts down in 5 seconds, the entire furnace shuts down - and then the sequencing for the ignition process begins again, within seconds. If that is the case, the igniton control will make a few more tries at ignition if need be. If this is the case, it probably is a flame sensor, just the same. You can easily remove the single screw, remove the flame sensor rod, and polish it up with fine sandpaper, a scotchbrite pad, or fine steel wool - so it can sense the flame, once again. (I just did one today.)
Old 02-13-10, 08:25 AM
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Sounds like exactly what my Tempstar furnace does when the flue over-temp switch opens. Put a VOM on about a 30VAC scale across the flue over temp switch contacts, then run the furnace until the issue happens. I'll bet you'll see the voltage jump to 24VAC, indicating the switch opened (which it should not unless the air system fan quit). If this is the case, I'd guess the correct course of action is to replace the flue overtemp switch with a new one, but for years what I've been doing is just cleaning the switch contacts with 600 grit sandpaper once a year or so. THIS IS A SAFETY ISSUE; DO NOT DEFEAT (BYPASS) THE SWITCH, and my sandpaper trick should only be done at the direction of a licensed professional.

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