Anyone know anything about a Webasto diesel furnace in a motorhome? For those who are not familiar with it, the Webasto utilizes a diesel burner to heat coolant that is pumped through heater-core style heat exchangers to heat the coach as controlled by a house-style thermostat.

We have the Webasto DBW 2010 in our coach and I am at a loss as to what to do to diagnose it. The symptoms are:

-The interior thermostat called for heat causing the Webasto to fire up.
-The Webasto burner runs for ~11 minutes from cold start at 40* ambient
-The burner cycles off and the coolant pump runs during the cool-down cycle
-The coach interior temperature has not yet reached the thermostat set temperature so the thermostat is still calling for more heat (power still at B1)
-The coolant pump cycles off and the unit will not restart until the power from the thermostat is cycled off and back on.

once the power cycles off and back on:

-The Webasto burner runs for ~6 minutes from warm start at 40* ambient
-The burner cycles off and the coolant pump runs during the cool-down cycle
-The coach interior temperature has not yet reached the thermostat set temperature so the thermostat is still calling for more heat

My understanding is that the Control Thermostat mounted on the housing of the Webasto is supposed to shut the burner off when the temperature reaches 167* and start it back up when the temperature drops below 154* and that the Temperature Limiter should not shut the unit off till the temperature reaches 203*.

The voltage coming into the Webasto at Control Unit terminal B4 is 13.5V - less than a volt different from the voltage measured at the house batteries.

On startup, the voltage at B4 drops to 12.6V and after the Webasto has run for a few minutes, it comes up to and fluctuates between 12.8-13.1V; however, it drops back to 12.6V once the burner cycles off and only the heat exchanger fans and the coolant pump are running. Once the coolant pump shuts off, the voltage is 13.3V at B4 with only the heat exchanger fans running.

The Webasto housing near the Control Thermostat and the Temperature Limiter was measured with my IR temperature thermometer at ~125* when the burner shut off and ~115* when it was restarted.

I've installed a new Electronic Control Unit (287.962), Control/Regulating Thermostat (354.902), and Temperature Limiter (608.719) within the last year using the logic that 1) the control unit was bad, 2) the Control Thermostat wasn't shutting the burner off soon enough, and 3) the Temperature Limiter had degraded to where it was cycling out sooner than the Control Thermostat. None of these parts have corrected the issue which has persisted for 2 years now.

I did jumper across the red and green wires of the Control Thermostat as soon as the burner cycled off and it did not restart the burner.

Any suggestions on what to check next would be appreciated

 

Wow, I know absolutely NOTHING about this unit, never even saw one, and I have a pretty good idea how it works. GOOD DESCRIPTION!

 

Stupid question.

The coolant circ pump is running, but is it actually moving coolant?

There is a chance that the impeller may be causing issues, and the coolant is reaching the cool-down point of the thermostatic sensor, but only at that area.

I have seen this issue with Espar brand heaters, where the impeller is simply worn out. They will run, and get up to temp, then shut down, without ever heating anything.

Just a thought.

 

No, not a stupid question at all. I can tell you that when the burner is on and the circulation pump is running, the heat exchangers do put out good heat which would lead me to believe that the fluid is circulating to some extent. I'm not sure how you are supposed to check the flow level on this unit. Other than disassembling the pump to check the impeller, any thoughts on how to check flow?

 

How about fuel issues. That thing has a visible light flame detector. I've seen commercial furnaces run fine under some conditions, flame out under others. Problem was usually bad nozzles or draft problems, coupled with a flame sensor current that was a bit low for some reason, usually missing a required lens or something.

To check out this theory, measure the flame sensor current.

Just a rambling thought at 4 am wood fire feeding time. Does the flame go out just as the pump starts?. Could be a power dip or electrical noise interfering with the flame safety stuff. Stiff bearings in the coolant pump, for instance.

Could be a simple as exhaust vent restriction by vermin of some kind. (shrew, maybe)

Fuel problems, could also cause something like this.

hope it helps.

 

Good thoughts, especially for 4 am. In our case, the burner fires up fine when the unit first turns on and burns strong (no smoke, hot exhaust) until it shuts off, presumably from reaching maximum temperature. When I refire it, it again fires up immediately and burns with no problem until shutting off again. I should have time to pull it apart tomorrow so I'll check the flame sensor then. Thanks for the idea.

The shrew-like creature that Cougar sent me was found just a few feet from the furnace compartment of the motorhome so I'm sure that the Alaskan shrews are somehow involved in this even thought the problem started 2 years ago. Thanks for tying that all together for me.

On edit, the fuel source is the chassis 100 gal. fuel tank so it's getting the same fuel as the Cummins although it has it's own separate fuel filter. I might check into changing that just for the fun of it.