Armstrong furnace problem

[akntrotter]

i have an armstrong furnace about 5 years old that as of a week ago has started to shut off in the middle of the night. I can turn off the power switch and turn back on and it will come back on. It will sometimes fix it for a little while and sometimes itself on and off several times before finally running. It usually runs about 10-30 seconds and will shut off and sometimes start right back up and sometimes starting up 5-10 min later. Any help or suggestions would be greatly appreciated.
Thank You

[mdshunk]

You forgot to mention if your unit was gas or oil fired. If gas, it would be helpful to know if it is electronic (spark) or hot surface (glow plug) ignition. Both styles, if it is only 5 years old are self diagnostic. Model number helpful. Please advise for further help.

Common problems for oil:

Nozzle and or filter needs changed
Electrodes dirty
Cad cell flame sensor sooted up.
thermostat damaged

Common problems for newer gas furnaces:

Goofy gas pressures (in town) during peak demand times
Flame sensor needs cleaned
Intake or Exhaust PVC pipe blocked (buried in snow, bird/bee nest, etc.)
Draft inducer or purge motor bad/intermittant
Combustion chamber draft pressure switch bad
Thermostat damaged

[akntrotter]

Thank you for replying. The furnace is natural gas,not sure of the ignition type. The model is an Ultra SX 80. Is there a place for a layman like myself to find out the ignition type. Also, there seems to be no specific time at which it shuts down, can be just kicking in, running for a few minutes or running for period of time. Sometimes, before the flame ignites it shuts off if that helps at all. I also tried to run just the fan without the heat and it did the same thing. Thank you again for a response.

[mdshunk]

You just said something very interesting in your last post that may just put me on the trail of the problem. You said you tried to run the fan and the furnace shut down. Do you mean that you set the thermostat's "FAN/OFF/AUTO" switch to "FAN" and the thing only ran for a short while? If that's the case then the problem is either in the thermostat OR the 24 volt circuit in the furance. There's really no safety's involved when you're just running the fan. WAIT.... the door on your furnace has in interlock switch that kills power to the whole thing. Are the front covers both on tightly? Let me know what you did to run the fan and exactly what happened. We can solve your problem in time.

With regard to ignition type, I looked it up and your system has hot surface ignition.

[akntrotter]

Thank you again very much. I set the fan on the tstat to on instead of auto and it seemed to act the same way, turning on, then off again. I set the heat/ cool to off. Problem seems to be getting worse over the last week. Initially flipping the switch would get furnace to run for a 10 hours? Now seems to go on then off in about 5 seconds, never kicking on the heat. Greatly appreciated, thank you.

[mdshunk]

Welp, I'm going to give you the sequence of operations for your furnace, and we'll see if we can pin down the stopping place. Take off the top front cover of the furnace and observe a few things. If there's a switch that pops out when you remove the cover, you may have to wedge the switch in to do your troubleshooting, to simulate the door being in place. When the thermostat is turned up (or furnace turn on in your case), a small motor should kick on on in the front of the furnace. After a short delay to prove that this small motor is running, you should see a glow in the firebox as the hot surface ignitor is heating. After another short delay, the gas valve opens and the burner lights off. After another short delay, the main blower (for the ductwork) kicks on and sends heat out through the house. Where in this sequence does the system shut down?? Does it shut down in this same place in the sequence every time you have trouble? Have you checked the ends of the PVC pipe(s) coming out ouf the furnace firebox to make sure that they aren't stopped up outside?

[akntrotter]

You explained the sequence perfectly I believe. I checked outside and I believe I checked the correct things. There were two slanted covers on the side of my house. I put my hand in and felt a grated cover inside that did not feel blocked or any foreign objects detected. As far as where in the sequence it shuts off, I do not believe it stops at a specific spot. I took the covers off and had it stop 3 times in the 1st sequence, 1-3 seconds into it. One thing I hear when it does fully kick on sometimes is a miss. The best I can describe it is like an engine running on 3 of 4 cylinders. Usually when I hear this it kicks of within a few seconds. Sometimes though it will run for 30 minutes with no problems. Usually only once a day. Will heat from 60 to 68 degrees. I believe that it could be random as to when it shuts off. Thank you so much for your input.

[mdshunk]

I'm going to sleep now... 5 am EST comes early. I think more and more like a low voltage problem. Try this and see what happens. Unclip the thermostat and jumper the "R" and "W" terminals (normally a RED and WHITE wire), and see if the furnace starts working reliably. This basically just overrides the thermostat and causes the furnace to run. Of course, the furnace wil run and run and run until you take the jumper out. This test is to include or eliminate the thermostat as the culprit. (if you don't have small wire, an old wire bread or baggie twist tie will do, just skin the ends). I like to eliminate things that "people touch" first.

[akntrotter]

Thank You again. I tried shorting the two but it did not turn on. Last night I opened up the bottom door. In there is a circuit board? I pulled out the top larger plug (looked like 5 conductor white plastic connector) . When I closed the panels back up it came on and worked fine until about 3am. It has run longer when I do this but still shuts off. Thank You. Hope you got some sleep last night.

[mdshunk]

If I were at your house, I could have this thing troubleshot in 5 or 10 minutes. It's hard to do with posts. Your Armstrong furnace is not the simplest one on the block. It has the Honeywell Smart Valve, a propriatory control board, and numerous safetys. It may be time to call a serviceman, since your problem is so intermittant. If your furnace was flat broke, it would be easier to troubleshoot via email. By the way, jumper the stat R and W connections again. There's likely a time delay built on the board. You might have to wait a minute after you jumper it. You could also jumper R and W at the furnace, where the thermostat wire connectes on the control terminal board. Either end will do the same thing. Jumpering at the furnace end would eliminate the the thermostat wire itself and the thermostat as the possible cause of the problem.

[akntrotter]

Thank You so much for your time and expertise. I will give that jumper a try again. I have a technician coming out tomorrow. I will post what they find just in case you hear it again. Once again, Thank You.

[akntrotter]

Thanks Again, found problem to be control board. Is that the same as Honeywell Smartvalve? It was under warranty and only cost 243 dollars to get swapped out.

[mdshunk]

The control board (module, as it is sometimes called) is the "brains" of the whole furnace in your case. The Honeywell Smart Valve is the actual gas valve that opens and closes to let gas into the burner. Thank your lucky start that it was under warranty. That board retails out between 175 - 225 bucks, depending on your market area and how nicely your house is furnished. ;) Control modules don't go out all that often, so I'm always a little suspicious when guys start replacing the most complicated part that they don't know how to test, but it has obviously solved your problem. Great.....thanks for the update.