Dometic heater fan cycling off/on every 2 seconds (fan pulsing?)

[Stu]

Hi Everyone.

Looking for some help from the community.  New to RVing and woke up cold this morning.   :(     We're FT so difficult to get unit into Service even though it's under warranty.

The Dometic DF-series heater running on LP is making a pulsing noise, like the fan is starting then immediately shutting off, then starting...

The thermostat is calling for heat.  Initially thought we ran out of propane, so switched closed off LP tank valve and opened the other one.

Stove/oven work fine.

[Kirk W]

Welcome to the Escapee forums. Do you have a connection to 120V power from the RV park or are you operating on the batteries? If the battery voltage falls below 11V the furnace will stop working and that might be what you have happening. 

Is this cycling off and on happening with no action from you? Your furnace is a newer model that I have worked on, but most RV furnaces go into lockout with low voltage.

[Stu]

Thank you for the quick response Kirk!

We're plugged into 120vac power - that all seems good.  Battery is good per the 'panel' but can throw a multimeter on it to verify.  (all other 12v accessories appear to be working properly too).

Correct - fan pulsing/cycling is automatic (no inputs from me) about every second or two.  Almost seems like the air switch is seeing air flow, the flag/sail switches contact(s), and the fan cuts out until the flag/sail comes back to normal position then repeats?  ...though looking at the schematic, it appears only the NO contacts are being used.  Can't tell from the print what controls the fan as it's fed by the circuit board.

[RandyA]

Being new a new heater this may be off base.  Guess it depends on how much the furnace has been used?  I  just when through similar problems with our older model Dometic furnace.  Like you, at first I thought it was the propane supply (Nope), next item was sail switch (Nope again), wondered about igniter board and swapped out one I had and knew was good (Nope once more).  Finally I pulled the furnace out to be sure the igniter and propane nozzle was OK.  But what I found was the internal burner head (shaped like the front of a shoe with lots of 1/8" holes on the bottom side - propane is fed into it .) was corroded, warped and had a big hole burnt in it so propane entering the fire box was not dispersed as designed. As such it would light then cut off seconds later only to keep repeating the cycle over and over.  I replaced the burner (about $30us), gaped the igniter and no more problems.

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[Kirk W]

The only schematic that I can find for your furnace is the one in the instalation manual, so I am a little bit limited. I don't find a copy of that service manual. It appears that the sail switch is wired in differently than those in earlier versions but I would bet that it still works in the same way. The purpose of that switch is to insure that there is enough air movement through the furnace for it to operate safely since the motor that provides air movement has a blower on each end, one providing warm air distribution through the RV and the other one provides combustion air through the firebox. If it does not come fully up to speed the sail switch does not open and gas valve won't open. I have never seen one open and shut rapidly as you describe, but suppose that could be possible. In normal operation, the blower starts up and runs for 15 to 30 seconds after the sail switch has closed before the propane valve is opened and the ignitor fires. That is done to insure that all combustion gasses are removed from the firebox and fresh air is provided. The blower should start almost immediately after the thermostat calls for heat via the blue wire. 

[Stu]

Thank you Randy and Kirk...all good info.

Randy, I don't think our unit is making that far into the process where it would call for gas yet.  The fan pulses are maybe every second (continuously).  I haven't pulled the heater out - have only had the cover off to check wire connections.

Kirk, battery voltage is 13.25 so should be good there.  The battery fitting on the battery case cover was a little loose to the cables but the cables were tight to each other so don't think the 12v source is an issue.

Thanks again for taking the time to respond.

I've attached a pic of the unit and the schematic from the book.  I don't see a '2nd motor' listed.  Any chance their using one motor to drive two (both) fan air circuits?  Heater looks fairly simple - not many components.  The main unknown is what they're doing on the circuit board.

[Wrknrvr]

   I would suggest looking at The igniter board. There is many problems with moisture on those boards. Look ok the back and see if there is corrosion. There may be some on the front also.

  I seen many problems with that model a Domestic furnace that I will not work on them anymore. Yes I am a Domestic dealer.

 

   Vern in a T-shirt 

[Kirk W]

3 hours ago, Stu said:

I don't see a '2nd motor' listed. 

Only 1 motor but that motor has a blower on each end, one for heat distribution and the other for combustion air. 

[Stu]

Wrknrvr - thanks for the info.  The board 'looks' good - clean/dry/corrosion free but who knows if any of the components on the board are bad.

Kirk - got it - thank you.

Was able to click the reset and get it temporarily working but it's intermittent.  I may need to limp it through until I can get it to a dealer.

[Stu]

Just received an email from Dometic - am guessing I won't hear back w/ an answer though until Monday or Tuesday.

The support tech did however describe the problem to a 'T' so am assuming they've seen it prior.

Will post their response when I find out.

Thanks again everyone for the help!

[Robbie R]

Stu, what did you ever figure out what the problem was? We are having exactly same issues. Please let me know what resolved the problem. Thank you 

[Kirk W]

Robbie, unfortunately Stu doesn't seem to have been back since Dec. 27, 2020. 

[Robbie R]

Kirk thanks for the response. Any chance you can help me out? We have a brand new 2022 keystone Montana 351bh. Just tested out the furnace to find out she’s not working. Fan comes on, igniter clicking, fires and goes out. It will do that till we shut it off. Any thoughts? I’ve already replace sail switch. Also just seen from somewhere else they were changing the  bracket on sail switch as it was able to open all the way. Fix is to shave off some plastic on bracket. Haven’t done that yet. But anything thoughts on what else to check? Mother board not in water as it’s stands up side. We’re supposed to go on trip week after Christmas. Let me know what else I would need to check or test. No shops here in Virginia can see me till after new yr. 

[Kirk W]

It should be under warranty if you are anywhere close to the dealer. Both Keystone and Dometic should be willing to cover it. Have you ever had the furnace running in the past as it can take 5 or 6 tries to get propane back there to ignite if the lines have filled with air. I always light the stove top first to make sure that as much air as possible is out before trying the furnace. 

If the sail switch doesn't close the ignitor never sparks and the propane valve does not open so if what you heard was the ignitor it is safe to say that the problem is not the sail switch. Can you tell if there is propane igniting by looking into the unit or by holding your hand in the exhaust coming out? If you are just starting the furnace the exhaust should not be hot but you should feel some warmth very quickly if propane does ignite.  The only Dometic recall that I am aware of is for the wires to the high limit switch and the fix is a wire tie. Dometic hasn't yet published a service manual for the furnaces with their name but they use the Atwood service manual from just before the purchase.

Edited December 20, 2022 by Kirk W

[Wrknrvr]

    Robbie R

    The best thing I would if I was there on a service call.  

 

  You need two people to really do this test. One person needs to be inside and one outside at the furnace exhaust pipe.

   So I would start with one person outside to determine if the furnace actually does ignite. That person should be next to the furnace exhaust so as to tell if it is actually igniting. So we need to determine if it is igniting or not. The outside person should hear the gas valve click when the ignitions sequence happens. This will happen after the blower runs for a short time. Then the outside person should be able to feel the exhaust if there is heat coming out.  Do not hold you hand next to the exhaust port, but away from it several inches.

    So to actually do this Simple test is to have a range top burner on while testing the furnace operation. The inside person can watch the flame on the range top while the furnace is trying to ignite.

    So the flame should not reduce its flame in size when the furnace gas valve opens. That is a crude test for gas pressure.

    So then try the furnace for operation. Does it ignite.

    Does it stay burning. If the flame on the range gets low there is a gas volume problem. But that normally will have the furnace flame go out slowly. Not all at once.

 

   Now you need to do this with the outside cover on. If the outside cover is off, it will do what you are describing.    Yep that is a hard lesson to learn. Back when this model furnace came out it was extremely frustrating to try and work on these. Logic was not on your side.

   The first thing was happening was the ignition board was getting wet, and failing. Back then the igniter board was laying flat.

  On the old style Atwood furnaces you could work on them with the exhaust vent off. But I was so frustrated at Atwood I called and vented a little frustration. So someone there with more knowledge than I had said the new Dometic made Atwood furnace will not keep burning with the exhaust pipe off the system.

  What do you mean it will not work that way. Person on the other end of the conversation said the newly designed furnace needs resistance on the exhaust to keep burning. As in back pressure. O yea, look in the exhaust vent on the cover.

 

   There should be a kink in the inside of the exhaust pipe.

 

   So if the unit is igniting, but going out and the flame on the range is not getting smaller. And the outside cover is on.

  Then the igniter board is not sensing flame. If you look at the diagram above it shows the igniter probe. That probe actually has a igniter probe and a sense probe. Look at the diagram. That probe has ground probe also. It has to have a good ground.

   Then the igniter board has to have a good ground also.

  When there is ignition and a flame, the ground probe on the igniter provides a few millivolts of electricity, through the flame to the igniter probe and the printed circuit board determines there is a flame, so the gas valve stays open to continue burning until thermostat is satisfied.

 

   So there needs to be two good grounds. One on the board. You could try to unplug the wire connection on the igniter board. Look for corrosion. That is rare, but remove and push on that connection a few times. That may fix it.

   Now if it still will not stay burning, then there is a ground problem between the igniter probe and the board.

  If this was a Suburban furnace, 70 percent of the time it would be the screws holding the burner assembly not providing ground.

 

   But since it is a Atwood, then it could be more frustrating to solve. There should be a few screws holding the burner assembly on to the burn chamber. Maybe loosen those and tighten them again.

  Repeat ignition test.

  Keep us informed, if you still have a problem, post it so we can keep helping.  

[Wrknrvr]

   I use to be a Dometic dealer. I still am registered with them. But retirement has been talking to me.

   The last one I did about two years ago, tested everything saying the board was bad. It was not registering flame, as in it would ignite and shut the flame off. Then ignite and shut the flame off.

   Called Dometic and talked to warranty person, discussed everything I tried. We decided the board was bad. They sent me a new igniter board. So when the board arrived, I went to install it. Darn multi wiring connector plug was not pushed on the igniter board properly. Pushed the plug on and it worked. Never seen that on a Dometic product before.

  Installed the new board and it also worked.    Sometimes simple things can be wrong that does not make sense 

[Customer1]

On 12/19/2022 at 5:46 PM, Robbie R said:

...new 2022 keystone Montana 351bh. Just tested out the furnace to find out she’s not working. Fan comes on, igniter clicking, fires and goes out. It will do that till we shut it off. Any thoughts? I’ve already replace sail switch...

In the power center fuse panel you probably have a black 15a auto reset circuit breaker. Pull that circuit breaker and replace with a 15a fuse, now try it.