Atwood Cycle On/Off

[InGodWeTrust]

Just curious what could be causing Atwood to cycle on and off every few minutes I do get heat but is not continuous ? I have Hydro Flame 8900-III not sure where to start thanks for any insights. 

[Jim Byrd]

Sounds like furnace is short cycling.  The most likely cause for this condition is an over temperature condition that is caused by restriction in the air flow,  You need to check the intake and discharge air duct work for any restrictions.  A faulty over temperature protection device that is operating at a lower temperature.than designed could also cause your problem.

When I purchased my 04 Holiday Rambler I had the same problem, After much frustration finally determined that it was caused by insufficient air flow in the duct work, the air temperature was found to be greater than setting of the over temperature device.  Checked back pressure with a water manometer and found back pressure was greater the manufactures specification. Added an additional discharge vent, problem solved.

Jim

 

 

 

 

[InGodWeTrust]

Jim,

Thanks for your input. I did find a vent hose under fridge that seemed to be blocked up I will run furnace tonight I bet that was it I appreciate your input. 

Will

[Harvey Babb]

Other possibilities are hot air from a vent blowing directly on the thermostat or a defective thermostat.

 

[InGodWeTrust]

Update..ran furnace last night and worked as it should it was a blocked vent hose thanks for all your help and suggestions. 

[Jim McGarvie]

This discussion raises the question, what is normal cycling?

We have an Atwood 8535-IV-DCLP furnace, 19 years old. Our burner is normally on for 60% of the time the blower is running. Should it be greater than that?

I was concerned we were wasting battery power (we most often dry camp) with the blower running but no burner, until I checked the air temperature at one of our ducts. When the burner is running it is approx. 160 degrees, but when the burner shuts off and the blower keeps running it only drops to about 140, so I guess we aren't losing too much.

If 60% burner-on time is about normal I won't worry about blockages; if it isn't I will start looking.

We have another issue which may or may not be related. Intermittently (yeah, I know) the furnace will shut down prior to reaching the thermostat setting temperature. It will run normally for three or so burner on/off cycles, then after an off cycle the burner comes on with a "wumph" sound like it is trying to light but doesn't stay lit. After the cool-down cycle the blower shuts down. I can repeat the cycle by turning the thermostat off and waiting a couple of minutes, then turning it back on. If I don't manually recycle it, the furnace will reset itself after an hour or so and try again. I am guessing this issue might be caused by the flame sensor.

Any thoughts would be very welcome!

[Harvey Babb]

In normal operation the burner should never cut off until the thermostat tells it to.

It sounds like your burner is blowing out, which will cause the control board to shut off the gas, wait for purge time, and re-light it. If so, definitely not normal!

 

[Jim McGarvie]

5 hours ago, Harvey Babb said:

In normal operation the burner should never cut off until the thermostat tells it to.

It sounds like your burner is blowing out, which will cause the control board to shut off the gas, wait for purge time, and re-light it. If so, definitely not normal!

 

Wow! Thanks for the information, Harvey. What would cause the burner to "blow out?" You mean by wind? This occurs all the time, not just when it is windy.

I wish we could remember whether this has always been the case. If so, maybe we have inadequate ducting. If not, perhaps something is blocking a duct or ducts.

It would be great if I could get it to operate as you describe.

[Jim Byrd]

8 hours ago, Harvey Babb said:

In normal operation the burner should never cut off until the thermostat tells it to.

It sounds like your burner is blowing out, which will cause the control board to shut off the gas, wait for purge time, and re-light it. If so, definitely not normal!

 

A faulty HI TEMPERATURE cut-out switch will shut the burner down.  Once the unit cools down the HI  TEMPERATURE will automatically reset and if the thermostat is still calling for heat the burner will relight.

 

[Jim McGarvie]

1 hour ago, Jim Byrd said:

A faulty HI TEMPERATURE cut-out switch will shut the burner down.  Once the unit cools down the HI  TEMPERATURE will automatically reset and if the thermostat is still calling for heat the burner will relight.

 

Thanks for that info. I have a "tune-up kit" coming which includes the limit switch as well as a burner assembly, ignitor/flame sensor and sail switch.

[Harvey Babb]

There are several things that can cause a flame to be lost including (as Jim B notes above) a high temp cutout switch, an intermittent sail switch, incorrect fuel/air ratio, burned/corroded burner, damaged firebox  and probably others I don't know about.

If the on/off cycling is regular (always about the same amount of time between cycles) that would point to the high temp shutoff. If the timing is erratic (sometimes a short run, other times a long run) that would point more toward a fuel/air problem or another air or fuel issue causing the flame to blow out.

Update: Reading your tune up kit contents reminded me of another fairly common issue. Another thing that will cause erratic cycling is the flame sensor losing contact with the flame. This causes the controller to THINK that the flame is out and turn off the gas valve in response. If this is happening you will hear the gas valve click closed before the flame goes out. If the flame goes out first, followed by the gas valve click then the flame sensor is OK.

At this point an inspection while running is going to be necessary.

Edited January 21, 2021 by Harvey Babb
Other things remembered....