furnace problem

[nwclassic]

Our year old AS is in the shop to fix a furnace short cycling problem. So far swapping components has had no effect. Can anyone shed some light on this condition and where to look for its cause? Many thanks in advance.
Brian

[ke4pk]

Think about the thermostat location.

Mine is in the bedroom and is above the shelf where the TV in the bedroom is and if it is on.....guess what? HEAT!

[smily]

What do you mean by short cycling?

[nickcrowhurst]

Brian, the simplest reason for short cycling is the internal adjustment of the thermostsat. Most modern thermostats can adjust what is known as the "switching differential". This is the difference between the temperature at which the thermostat switches on, and the temperature at which it switches off. For fine control of industrial processes, this can be as little as one tenth of a degree. If you used such a thermostat in an airstream, the furnace would constantly switch on and off. A switching differential of about 2 degrees is a reasonable compromise between comfort and switching frequency. On mechanical thermostats there is an adjustable slider on a wire under the cover. On digital thermostats this is effected by pressing buttons. This is a basic matter which your dealer should know all about, and has probably eliminated as a cause. However, just in case, I offer these details. Beyond this, an over-sensitive heat limit safety switch is a possibility. This switches off the furnace if the temperature close to the furnace becomes excessive. This could be tested by temporarily shorting out the switch to see if the problem continues, or replacing the limit switch with a new one. A low voltage from the battery is another possibility. If the voltage drops too far, the fan will not blow hard enough on the sail switch to close the contacts, and the furnace will stop. If the voltage comes back, the furnace will start. If you check out the switching differential, check out the battery voltage, and short out the limit switch, you have checked three possible causes, without entailing any expense. Beyond that, if the circuit board is found to be faulty, I suggest you get a Dinosaur Universal Igniter Board as a replacement. (NOT the original type) This is soldered on both sides for resistance to vibration, has fuse protection, and has 3 tries at ignition before locking out. About $115. Good luck. Nick.

[nwclassic]

Thanks for everyones help on this... to flesh out the problem a little further -- Atwood furnace, Dometic ac/heat pump and comfort control center. Furnace runs until selected temp is reached. From that point on it cycles (off 30-90sec, on 2-5min). The auto sequence engages the fan, then lights the burner. At the end of that cycle the burner is turned off while the fan continues to run until the box cools. The fan stays off only a short time before repeating the sequence. An independent thermometer agrees with the control setting. The controller is attached to to an interior wall housing the refer. There are 'dip switches' which enable various components/functions that the controller responds to. One of them labeled 'differential' only reduces the nominal spread (3deg) by half. My initial thought was that it had been switched on (no), then that it was wired backward (Dometic expert says no). A remote sensor was attempted - same result. Voltage is good, heat limit switch might be a problem but the temp sensor should be determining the restart, not the burner box temp. One last thing -- out dry camping in cold weather we needed the furnace but could not tolerate the near constant 7A draw. At one point I put a towel over the controller and the cycling stopped. I mentioned this to the Dometic guy. That's when the component swapping started. After that, he got on top and reprogrammed the unit... all w/o effect. He's still working on it. I expect an update call from him after he has a summit mtg w/Dometic Central. Thanks for your ideas and any to follow. I'll post any updates.
Brian

[66Overlander]

This past week while at the Tall Stacks Rally, our furnace started doing the same thing (never done it before). When first turned on, it would run until the set point temperature was reached, but from that point forward, the cycles would become shorter and more frequent until the were mostly "cool down time" with just a few seconds "off" before the cycle began again. The furnace would heat, as I could just begin to feel heat before the flame turned off and the cool down cycle began. This lead the the furnace blower running almost non-stop one night, but not with much effective heating going on. If I turned the furance off for a while and let the trailer cool down a bit, the whole cycle would begin again with one normal cycle, followed by increasingly short and more frequent cycles. My first thought was that this was a thermostat issue, but this thread raises some other possible causes, too. I guess it'll be time to pull out the Service Manual and get checking the next time I pull the trailer out of storage.

[Kevbo10]

i have a problem with my furnace. atwood furnance and thermostat. it's a 2005 AS. i bought the unit brand new. only used the unit perhaps 5 times in the last yearand it worked perfect. now it won't turn on. i checked under the cover outside and the exterior is clean as a whistle.

if i "jump" the two wires leading to the theromstat (only thing i could think of) for perhaps 5 secs, the blower comes on, then the ignitor goes, the heat comes on and it will continue to run, until the selected temp on the thermostat is reached. it will however not restart.

any thoughts on the fix, would be a blessing.

i'm packed for a rally, supposed to leave in the morning, and it's going to be a cold rally. 22 at nite.

thanks,

kevin

[steelbird312]

Kevin, We won't hijack this thread, but sounds as if ou have your own answer. You took the thermostat out of the loop and it worked. try a new thermostat.

[doorgunner]

Kevin, I had a like problem, the furnace would not kick on until I tapped the thermostat, I took the cover off and gave it a shot of contact cleaner, No problemo. Enjoy 29, tilt one back for me!!

[Kevbo10]

thank you, and thank you. i didn't realize i jumped in the middle of a different problem (i used search, and this thread appeared, won't happen again). thanks, kevin

[vikx]

There is a possibility that the ducts are restricted, which can cause short cycling. Sometimes they get crunched or sometimes not installed correctly. Check and see that air is able to freely flow thru the ducts and that the proper number for your furnace are installed. Just a thought....VK