Webasto Air Top 5000 troubleshooting

My new-to-me boat came with a Webasto Air Top 5000 heater that seemed to be in good condition. I struggled with ducting for the first month, which had only been put together with tape, not clamps, but the heater itself ran for many hours keeping me warm.

I decided to replace the existing thermostat as it was mechanical, not digital, and boxed into a nice little teak box, blocking most of the air going to it. As a result, we would have 15-20 degree swings between heat cycles - not ideal.

Sure Marine sold me a SC1600B digital thermostat which I installed yesterday, and the heater no longer works. All that happens now is when I power things up, and then turn the thermostat to heat, the LED briefly flashes and goes out. Nothing else happens, no fans, no pump noises, etc.

Putting the old thermostat back results in the same problem, the heater no longer turns on, but the light flashes briefly when told to heat.

There are three wires going to the thermostat - red, black and white. The red and black are 12V +/- respectively, and the white is the thermostat "on" wire. They go back through the wall to a big connector that looks like it was meant to plug into the big digital thermostat with timer that was available for this model when it was new.

The black and red wires go to respective black and red wires on the connector. The white goes to white, but from that white wire there is what appears to be a 2.2k resistor that jumps across to a blue wire.

There are no LEDs I can find that flash error codes or lights, as some others have suggested, unless I don't know where it is. I have the older thermostat (picture attached) which doesn't have a dial with an LED in the middle.

I've completely removed power from the unit both by the circuit breaker, and by unplugging the connector on the top.

I've tested the voltage in all of the places I could think, and it all looks within the expected norms - ~13v at the mains coming into the terminal strip, tested it along the way past the pop up 20 amp fuse on the terminal strip, at the connector, etc.

I've already sent a note to Sure Marine asking for their opinion and hope to hear from them soon, but looking for other opinions as well.

I'm worried that somehow the ECU or other circuitry inside the heater somehow got damaged. It is not going to be easy to remove it if it has to be repaired, and given the pains I've had with the ducting and installation quality, I will probably just replace it with another brand (Wallas) that I've used before.

However, that is an expensive solution, when what I was trying to do was get a better temperature situation by replacing the thermostat!

Try calling sure Marine. They have always been very responsive to me when I have called.

wwestman said:

Try calling sure Marine. They have always been very responsive to me when I have called.

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Yup I already mentioned that above, have called them.

They did just call me back, but their only suggestion was that perhaps the wiring is wrong and that the black wire is actually the one that should go to the white terminal, and that I need a separate ground.

I don't think that's true, but a simple way of testing the heater is to cross those two wires to see if it starts, which I did not do yet. I will try later today.

I still think the wiring is correct (Sure did the original install years ago) and that something is hosed with the heater.

If you have removed all power from the unit, you can use an Ohm meter against a known good ground to check which wire is actually grounded. Also, check for in-line fuses in the power supply or thermostat.

Possibly you bypassed the internal sensor with the new thermostat installation, and need to install an external sensor for it to work properly. The basic heater measures it's heat in the heater, and I installed an external sensor so it measured the room temperature instead of the temperature at the heater so the room would be the temperature I set the thermostat for.

It's worth looking into for you. The optional external sensor comes with a new cover for the heater and plugs into a port in the heater. My original wire harness also has a connection at the thermostat for an external sensor but I could only locate the sensor that plugged into the heater itself, not one that plugged into the thermostat unit.

gsholz said:

If you have removed all power from the unit, you can use an Ohm meter against a known good ground to check which wire is actually grounded. Also, check for in-line fuses in the power supply or thermostat.

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I actually did that, and it is definitely grounded. There are no in-line fuses I could find, and the wiring is pretty short. There is a pop up fuse at the terminal block, but I tested both sides of that to a good known ground and it is not blown, and is working correctly.

Both the old and new thermostats do not have a fuse.

AKDoug said:

Possibly you bypassed the internal sensor with the new thermostat installation, and need to install an external sensor for it to work properly. The basic heater measures it's heat in the heater, and I installed an external sensor so it measured the room temperature instead of the temperature at the heater so the room would be the temperature I set the thermostat for.

It's worth looking into for you. The optional external sensor comes with a new cover for the heater and plugs into a port in the heater. My original wire harness also has a connection at the thermostat for an external sensor but I could only locate the sensor that plugged into the heater itself, not one that plugged into the thermostat unit.

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Hmm maybe, but I don't think this would have caused this. It ran for years before with a simple 3-wire thermostat that basically told the unit to go on when needed, and didn't rely on an external temperature sensor.

At the minimum, I would assume the heater would still power up.

I did the same mod. Changed the 20 year old analog for a new digital thermostat. Swapped wire for wire and had no issues. Does the ambient temp show on your thermostat? Does the set temp change when you push the buttons? Have you tried putting the old thermostat back and see if it works?

wwestman said:

I did the same mod. Changed the 20 year old analog for a new digital thermostat. Swapped wire for wire and had no issues. Does the ambient temp show on your thermostat? Does the set temp change when you push the buttons? Have you tried putting the old thermostat back and see if it works?

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Yup, if you read my original post, I put the old one back, same symptom. New thermostat works just fine in terms of its mechanics.

This has been solved thanks to some advise from Sure Marine. It turns out some of the wiring harnesses have the white/black wires switched, which was the case with mine. Swapping those so that the red goes to +, black goes to W, and white goes to ground made everything happy again.

At some point swapping the wires to the new thermostat put it in lock down mode as well, so putting the old thermostat back didn't fix things.

Came back to the boat today, reversed the wires, and the new thermostat is working like a champ! So much better with a digital thermostat, and a reasonable dead band. Thanks for everyone's suggestions.

I think you have answered your own question. If you re installed the old thermostat and it still doesn't work, something in the heater has changed. The odds of you buying a bad thermostat, and your old one simultaneously failing, are way too high. The problem will be in the heater. In a home furnace there is a flame sensor that will not allow fuel to flow unless it verifies that there is a flame present. Is there any kind of sensor like that in yours ?

I guess I didn't read far enough ahead...glad its solved

stevemitchell said:

This has been solved thanks to some advise from Sure Marine. It turns out some of the wiring harnesses have the white/black wires switched, which was the case with mine. .

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This just echoes previous posts about Sure Marine being a trusted and reliable business to deal with. I've had similar experience where they are experts and are very generous with giving advice/troubleshooting info to those of us doing our own work. It's one of those cases where I'll shop with them for my heating needs because of that support.

fractalphreak said:

This just echoes previous posts about Sure Marine being a trusted and reliable business to deal with. I've had similar experience where they are experts and are very generous with giving advice/troubleshooting info to those of us doing our own work. It's one of those cases where I'll shop with them for my heating needs because of that support.

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I completely agree. I've worked with them a number of times before, and a month or so ago to get some parts, and they were super nice and even offered to deliver it to me at the marina, which is rare with any company nowadays.

This morning, someone called me back, chatted with me for 15 minutes and we troubleshot for a while, but he also said he'd be happy to stop by no charge to take a look at this, just after the Seattle Boat Show (which starts in a day) which I can understand!

Good service is hard to find, and for specialized things like heaters and such that we don't all work on daily, its nice to find a place like Sure Marine.