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Old 11-21-2020, 03:21 PM   #1
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HydroHot/AquaHot Diesel Burner

I have been dealing with an intermittent problem on the HydroHot diesel burner for past couple years.
The problem is the Diesel Burner Light going out and burner shutting down indicating a problem with the Webasto Burner Control Unit. Not to be confused with the HydroHot/AquaHot main control panel.
HydroHot/AquaHot Control Panel controls the overall operation of the complete system
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The Webasto Control unit is what controls only the diesel burner. This maintains the temperature of the boiler fluid when diesel burner is on.
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The intermittent problem was not a big deal in warm temps when we just switched on diesel burner when taking showers. The burner would work fine most of the time for several hours, but would never stay on all night. With us using the coach in colder temps and not putting in storage till later I really wanted to solve the problem. Given the nature of the problem it was hard to monitor and collect good information.
I happened to have a small data collection device that I was able to connect and program to collect data to my MQTT Servers. The device was connected to the diesel burner switch light (B3) and the Fuel Solenoid Circuit (C4). This allowed me to capture status of the light and when the burner had fuel flowing.
What the data showed me was the Control Unit would fault out when the burner was on but that was about it. Confirmed the random nature of the failure. Green Lines are Fuel Solenoid, 0=Off 1=On, Yellow Lines are the Diesel Burner Light, 2=On (Ignore some of the random spikes as I was altering program.)
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With this information I looked closer at the burner schematic.
What this showed was the fault was limited to a small portion of the circuit between C4 and C6 on the Control Unit Plug. In this circuit is an Over Temp tStat(S4), Fuel Solenoid(L1) and Flame Sensor (R1).
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Flame Sensor was replaced along with Control Unit within past 18 months so ruling those out for now.
Ignition Coil was tested 100x manual firings with zero issues. Fuel delivery tested, nozzle replaced along with pressure check and burns clean, no smoke.
Over Temp seemed like a logical place to explore deeper. These are supposed to trip and require a manual reset with button on end. Some research indicated these are a known failure point.
With the above information I disconnected OT sensor and placed jumper in its place. I also connected the OT sensor to the data collection device. Disclaimer and Warning: Bypassing Over Temp sensor is for testing only and should be watched closely as this is a safety device.
HydroHot has now ran for over 24 hours without faulting! And the data is showing that the OT sensor seems to have an intermittent issue as it is tripping for brief periods but resetting.
Cyan Lines are the Over Temp Sensor Up is tripped.
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With this new found information a replacement Over Temp Thermostat (ELE-HLT-322) was ordered. Once in place I will repeat test and collect data.
I could have bypassed some of the data collection and just replaced parts till it worked but I wanted to dig deeper and actually replace parts that showed to be defective via testing.
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Old 11-21-2020, 03:34 PM   #2
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That's interesting info, Steve. We've been having persistent "FAULT" issues with our AH since we got it in 2019. Like you, during the Summer it wasn't an issue, but last week, when we stayed in the coach overnight on the night with the high winds & low 40 temps, I had to stay up ALL NIGHT, listening to the sound of the blower coming on but the burner not firing. It'd fire & within a few seconds turn off. A few seconds later it'd fire, then turn off. Sometimes on the 3rd try it'd fire, stay lit & go through it's normal cycle. Other times it'd just quit. Sometimes it'd never fire after I'd hear the blower turn on.

I had to turn it off/on 8x during the night to keep the heat on. Also like you, there are times when it works perfectly & the burner not firing was so intermittent, even our service shop couldn't recreate it. But then, they didn't test it long enough, I think.

It's in the shop for some other fixes now & I told them to keep it & use the AH on diesel until they get it to FAULT. I don't want it back until they fix it this time.

I think I'll email the shop this thread so they can read what you discovered.

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Old 11-21-2020, 03:42 PM   #3
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HydroHot/AquaHot Diesel Burner

The Webasto Burners used on HydroHot and AquaHot can be tricky to troubleshoot.
One starting point is always the Diesel Burner Light at the switch. If it goes out this indicates a problem from the Webasto Burner Control Unit.
This could be from any number of issues.
1. Clogged/Bad Nozzle
- Will not ignite or burns with excess smoke.
- Flame Sensor will not see flame and put Control Unit in Fault
2. Over Temp Tstat
- Bypass for testing
- Is same circuit with Fuel Solenoid and Flame Sensor.
3. Flame Sensor
- Check resistance with Meter by disconnecting and checking when light and dark. Less than 300 ohms when light and greater 100k ohms when dark.
4. Ignition Coil
- Manually activate 100x making sure it sparks each time.
5. Check Fuel Pressure. 145psi
6. Control Units can also fail. The older silver box units probably more susceptible to this with the many relays and analog circuit.
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Old 11-21-2020, 03:57 PM   #4
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We just had the AH serviced this Summer which included a new nozzle. Our problem didn't resolve after that so it's got to be something else.

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Old 11-21-2020, 03:59 PM   #5
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I see people talking about replacing the nozzles quite often. I would guess that is rarely the issue with the actual burner unless it has been years or dirt getting in system.
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Old 11-21-2020, 04:11 PM   #6
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It sure would have been nice if Webasto would have put some lights on their control unit indicating what the actual fault was.
Troubleshooting them for true problems is a pain.

If yours has the old silver control box on the side of burner like my picture above that would be first place to review.
If it kicks on but does not fire, tapping side of box may make it fire. This is a good sign it needs replaced.
I had same problem 18-24 months ago.
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Old 11-21-2020, 04:26 PM   #7
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Aquahot asked me to watch the indicator lights on the outside panel to see if any dim when the misfires happen. Problem is, those lights are not bright enough to see them in daylight. They can really only be seen after dark. Anyway, from what I could see, all were lit green that were supposed to be.

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Old 11-21-2020, 04:40 PM   #8
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Better to just check voltages at the correct spots on the burner when it fires. This requires opening up the unit though.

As I mentioned if a silver control box it might be a good thing to review. The other is bypassing the Over Temp and let it run an extended time. For me this was fairly easy as it is sitting in driveway, at a dealer or elsewhere might be hard to recreate and test the problem.

Assuming the control box is good and no issues with grounds or voltages. The fault light would come on from the Flame Sensor or Over Temp thermostat.

Over Temp cause would be a bad sensor or truly over temp which is not likely if burner just starts.

Flame sensor could be from a variety of places.
- Bad sensor which is easy to test with meter.
- No flame which also could be a variety of sources
- fuel supply, bad nozzle, bad fuel solenoid, bad ignition coil/wires, bad fuel pump, spark gap not set correct
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Old 11-22-2020, 08:50 AM   #9
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Well, I still have something going on.
After 42 hours without faulting and OT bypassed it shut down again. OT also tripped a couple times so it is defective. The faults are odd in that the light appears to have flickered off then on and then off again at end of burn cycle.
Restarted again to see how long it goes.
BTW it cycles about twice an hour for a 2 minute burn to maintain boiler temp in 50 temps without using hot water.
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Old 11-22-2020, 04:24 PM   #10
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Keep the tests coming, Steve. Not only did I email the shop a link to this thread, I emailed the service tech at AquaHot I've been conversing with.

With all the units out there, it just doesn't seem right both you & are the only ones having or posting about these FAULTS.

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Old 11-22-2020, 04:37 PM   #11
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I rewired my data collection device in a little more secure. In doing so I did notice one screw for the +12vdc connection to the Webasto Control Unit was not tight as it should be.
8 hours into an extended run and no faults yet.
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Old 11-23-2020, 08:34 AM   #12
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Only made it 16 hours yesterday before it faulted out.
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Burner started and 1 second later it faulted out.
Only input at this point is the flame detection and it should not fault 1 second into the burner starting.
Starting to think it is just the crappy Ukraine built Burner Control. Control unit was replaced in 2017 and these are not cheap.
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Old 11-23-2020, 11:55 AM   #13
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Found a nice bit of information to help in troubleshooting.
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This TSB from Aquahot has some good info. The more I read it seems like the Spheros black box controllers are prone to early failures. This may have prompted Aquahot to build their own version. WPE-300-901
https://www.irv2.com/forums/attachme...9&d=1580401635
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Old 11-23-2020, 02:18 PM   #14
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Thanks, Steve. I keep shoveling this info to the shop & to AquaHot. Below was the response I got today from AH. The reference Jim made to what Megan mentioned was from a prior email I sent to them. She noted checking the control pad & check a possible fuel issue, which I'm unsure of has been done on mine.

Lori-
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