Furnace Questions

[TOMLESCOEQUIP]

I have a tempstar 80% furnace which has been acting up lately. it will cycle on , run the burner about 30 seconds or so then shut off............About a minute or 2 later is starts another sequence all over again but won't keep running. It's about 10 years old but hasn't given any problems till now and with Christmas just barely past, I'm not ready for a replacement expense just now ! Things I've checked already are: Flue vent not obstructed, forced power vent fan is working, sensor in plenum seems to turn fan on when unplugged and cuts flame off when I unplug it , sensor at front(toward me) on burner seems to work too............things shut down when I unhook it also. I have tried a jumper accross both sensors with no effect............It still shuts down after about 30 seconds no matter what. Also a small vaccumn diaphram off the power vent fan cage (connected by a tube) is working or seems to be. The brain module says honeywell continuous retry and that's what it's doing..........It acts like a flame sensor is not registering ignition but the main burner lights just fine............Any ideas would be really appreciated..........heating the house 30 seconds at a time is going to get old real fast............THANKS..............Tom

 

[EdKing]

I would try replacing the sensors, we had a simular problem with the oil furnace in our garage and replacing the sensors fixed it. It could be one is bad and your tests aren't making it work.

 

[Mark_in_NH]

I would quickly guess that some sensor is defective.
Do yourself a Big favor and call your oil supply co. Have them come out and service it. Save yourself alot of hassle and be done with it.

This is one of those times when (at least for me) that a person needs to know when they are out of the knowledge territory and to call in the people who do have the knowledge.

 

[rozett]

It sounds like the flame sensor to me. Try replacing that one first.

 

[mainiak1]

I don't know if what you have is an oil fired furnace. I had one that acted like yours and turned out to be the electric
eye under the transformer. It unplugs easily and only costs around five bucks. If the eye doesn't see the flame after so many seconds it will shut the burner down.

mainiak1

 

[Egon]

As the others have said, call in the man with the instuments.

The 30 second run time on startup is consitant with a sensor failure. There may be times when one sensor at a time may be bypassed to determine the faulty one but this require bypass knowledge for the instumentation on the system.

The first step is to jiggle and tap anthing that looks like a connection or electrical module. If this fails the second step is the telephone.

Egon

 

[PineRidge]

Many times over a flame sensor will build up a thin layer of oxidation that can be easily cleaned off using some fine steel wool.

You did not state what type of thermostat that your system uses but if it’s a type with an adjustable heat anticipator you can also check the amp draw on the gas valve to be sure the 2 are in sync. A heat anticipator that is incorrectly set will rapid cycle the burner.

 

[TOMLESCOEQUIP]

The furnace is nat. gas........I forgot to add that last night. Where is the adj. on thermostat supposed to be set ? It's a honeywell...........The std round model with heat and a-c functions on the same unit........ We're Currently not freezing by putting the fan on manual and using the 30 second runs to stay warm..........Tom

 

[shoppintractors]

If the furnace has a condensate drain line, as mine does, every so often it gets plugged up with crud, and then acts as you describe. There is a small settling tank inside the furnace that I remove and flush out with water and then reinstall.

Don

 

[Inspector507]

The flame sensor on mine requires a good rub with steel wool every once in a while. Usually once a season. Fixes the same problem you're describing.

 

[PineRidge]

If you grasp the round painted ring on the round T87F Honeywell stat and gently pull it towards you it will pop off. The heat anticipator is the sliding scale on the bottom half of the stat. Look on the gas valve on the furnace and write down the amp draw to match it to the thermostat. The settings should be the same. If not gently realign the pointer within the T87F thermostat. The numbers are small and normally marked .4, .6, .8, etc. those are tenths not whole numbers

If the amp draw is already set correctly then you need to look elsewhere for the problem. When this furnace short cycles place your hand on the plenum or furnace bonnet (careful it may be real hot) and let me know if it is lukewarm or too hot to touch when the furnace is acting up..

 

[TOMLESCOEQUIP]

Well.........It's fixed now........In our large christmas gathering it appears that the kids knocked the cover of the t-stat ( throwing something) and trying to put the cover back on must have disturbed the setting on the cycling whachamacallit and got it screwed up. Thru trial & error ( mostly error) i now have a furnace that is more responsive than ever. I had it converted from LP to nat gas when they ran a line down our road 2 years ago, and when they (NOT ME ) changed the furnace over I don't think they ever readjusted the t-stat after changing the gas valve. THANK YOU !! for the wealth of information guy's............Saved me some $$$$ on this one.................TOM

 

[PineRidge]

Glad to help you Tom. That's what this site is all about. /forums/images/graemlins/grin.gif

 

[TOMLESCOEQUIP]

If you ever need to set the floats on a holley carb on your tri-power vette or rebuild the tilt cylinders on your forklift.......... call me !! I don't claim to know everything..........BUT I'll tell what I do know..........Just ask my wife .............Tom

 

[boustany]

I had exactly this problem with a high efficiency furnace.

I fought it for a while - it would work perfect if it hadn't run for a while (days), but once it was needed - on for a couple minutes, off for a while, etc., etc..

Then I though I heard funny gurgling sound one day ........

I noticed a black plastic sort of water hose about 1/2" diameter going into the fan housing. So I removed it an a quart of water poured out. It seems that the high efficiency furnaces condense water out of the output gasses. If the pipe is blocked they have a sensor which shuts off the burner.

So, the burner would run for a few minutes until the water got to the sensor. Then it would shut off. Then the water would slowly drip out and it could start again.

So I did the logical thing. I removed the hose and sucked/blew until the obstruction came out. Ever wonder what a bunch of dead bugs taste like? /forums/images/graemlins/tongue.gif

Anyway - problem solved. Hope it works for you!

Let us know in any event.

Oooooops. Guess I should have read all the posts. /forums/images/graemlins/blush.gif

Never mind!

 

[TOMLESCOEQUIP]

Hey......I'm just glad for all the help........my sensor didn't have any bugs but it was a little strange running the furnace while testing the vacumn sensor........use your imagination on that one !!...........TOM