ICP Short Cycle and intermittent no light.

[BigWave]

19 yr old ICP GUI100 (RPJ II), WR 50A55 Control (replaced June 2006).

I am in need of some troubleshooting direction please. When the furnace lights, it will run for approx 5 minutes with a nice even blue flame, then the flame stops. Both fans run for a short time and the furnace restarts. This will repeat for an undetermined amount of time. Some times the furnace will not light and locks out (1 flash). After shutting off power for 30 seconds the furnace will attempt to restart - the HSI will glow, the small solenoid on the gas valves clicks (I do not hear nor smell gas during this part of the cycle), the HSI goes out, and this process will repeat approx 3x's until lock-out. I have not been home during the day to observe how many lock-out cycles the furnace goes through before eventually starting again with the dreaded short cycle syndrome. Unfortunately my wife has been home during the day, and she has reported (read "not happy" ) temps in the low 60's...

So far I have cleaned the flame sensor, checked the pressure switch - 0 ohms when fan starts (with the leads jumped to check) - and 24 volts. 24 Volts to gas valve. Rollout switch closed. I did notice an interruption in the madness when I wiggled the wires to the control valve when I was checking the voltage - (it's getting late so I have not investigated this further - and the furnace is at least providing heat at this time). I have not done anything with the limit switch. As I finish this post the furnace has completed the heat cycle and is currently just waiting for another call for precious heat from the t-stat (Honeywell digital - new in 2010 and has performed great). We are looking at a forecast of 5-degrees F on Friday, so any help would be most appreciated. Thank You!!! -John

[nomadpeo]

it really sounds lilke a flaky connection. has it failed again ?

[BigWave]

Yes. This morning it was in lock-out (1 flash), and 66-degrees-F in the house. I turned off the power at the furnace for a few seconds and then turned it back on and the furnace started up normally. went into the short cycle sequence and did not restart -it was back in lock-out within approx 10 minutes. I turned the power off momentarily, then back on and the furnace restarted normally. I taught my wife how to reset with the power switch (so the heat will sort of work today) !

After another reset the furnace started right up again. I expect the same cycle to repeat...

[BigWave]

I checked all of the connections to the gas valve and the control board - everything is tight - no loose connections. (There is no ground wire to the gas valve, and the ground to the control panel is good.) I wiggled the wires at the gas valve during operation and the furnace continued to run properly. The furnace seemed to start up okay all day today (no resets required by my wife ); however, the short cycle syndrome continues ... Any other troubleshooting ideas for the short cycle? Even though I cleaned the flame sensor, and it appears okay (no cracks) - could this still be the culprit? Is it logical that a faulty flame sensor would work for 5 minutes then cut out? Can the limit switch cause short cycles, and if so, what is the proper way to verify the switch is functioning properly? Thank You!

[nomadpeo]

hate to say it, but that old technology was pretty flaky. you may not even have a fuse on the circuit board. i'm sure you don't have any flashing lights telling you what circuit opened. if it utilizes a 'smart valve', there are even more variables.

as dilkey said, look at it from the viewpoint of performance. check your temp rise rating on the nameplate and measure (using the same thermometer for both) the return air (entering) and the supply air (leaving - after the first turn) to see if you are moving enough air across the heat exchanger. if it is too high, you may have a restriction, typically a restricted evaporator coil or dirty filter, possibly too many supply grills closed. if the rise is ok, see if you can catch it in the act. monitor 'w' to 'c' when it short cycles. if the signal remains, check the gas valve terminals for 24 volts. if no signal, you will need to continue ruling out components with a meter. a flame sensor is relatively cheap by the way.

given the age of this unit, i would be equally concerned about the condition of the heat exchanger and consider plans for replacement before spending too much money. be sure to do a thorough inspection.

[BigWave]

Thank you both for your help!

Here's where I am now. Note - I have limited (ie none) HVAC tools - just my trusty volt meter and typcical array of hand tools .

The AC coils were cleaned over the summer and I did replace the flame sensor this weekend because I was able to get a new one for only a few bucks and new is better, right? I do not think the flame sensor was really the issue because I did remember recently closing a few registers to try to force more heat into another bedroom so I opened these and the problem has gone away. In retrospect, I think the unit was working aok, doing exactly what it was supposed to do and the limit switch sensed the high temp in the plenum...

Although the unit is old (and I expect to replace soon), the WR control board does have a fuse and the diagnostic light. I did not see a code blinking for the limit switch; but, I may not have looked at the right time and if I am not at the exact correct angle to view the light it is easy to miss. At this point I will continue to monitor but all appears to be working as designed. I'm sure my wife will let me know if it isn't!

Thanks again - great job!!!

[austen]

Thanks for the information