Webasto Air Top heating problem

[Carduelis]

The heating system is currently only producing luke warm air. You can comfortably hold your hand in front of the vents, wheras last winter it was too hot to do so for more than a few moments.

The heater had a full service from Deisco last year and has only done a few hours since, so I'm reluctant to believe it's coked up again.

What I have noticed is that adjsuting the cabin thermostat seems to make very little difference although I can't remember how this is supposed to operate. When the thermostat is 'off' (i.e. up to temp) is it supposed to switch off the fan, or does it just turn down the heat inside the unit?

What would be the best way of testing the thermostat?

Can anyone advise please?

 

[FWB]

When up to temp the fan still runs.
Best to ring Toby at KETO he is the expert
01202 690 900

 

[pcatterall]

When did it start to go wrong? Gradually or all at once? Had you moved/changed anything? Does it run hot after start up then slow down? Does it smoke?
This can occur when the heater uses its internal thermostat and the outlet is near the inlet.... the heater is fooled into thinking it has reached the right temp.
The fan ( as stated ) should not cycle on and off when set temp is reached, the heater should slow down and maintain required temp.
You could by pass the thermo for a while which would make the heater run at high power ( join the Grey? wire)

 

[maby]

When did it start to go wrong? Gradually or all at once? Had you moved/changed anything? Does it run hot after start up then slow down? Does it smoke?
This can occur when the heater uses its internal thermostat and the outlet is near the inlet.... the heater is fooled into thinking it has reached the right temp.
The fan ( as stated ) should not cycle on and off when set temp is reached, the heater should slow down and maintain required temp.
You could by pass the thermo for a while which would make the heater run at high power ( join the Grey? wire)

Our Webasto Airtop (just a year old and professionally fitted) shuts down completely when the boat is up to temperature - always has. It does cut the output back a bit as it is approaching the thermostat temperature, but it certainly shuts down when significantly over temperature. I leave it switched on all winter with the thermostat set very low to protect against frost - could not do that if the fan was running continuously.

Note that it takes a couple of minutes to cycle down once it is up to temperature - it needs to pass some cold air through the combustion chamber to bring things down to a safe temperature before cutting the fan.

 

[Carduelis]

When did it start to go wrong? Gradually or all at once? Had you moved/changed anything? Does it run hot after start up then slow down? Does it smoke?
This can occur when the heater uses its internal thermostat and the outlet is near the inlet.... the heater is fooled into thinking it has reached the right temp.
The fan ( as stated ) should not cycle on and off when set temp is reached, the heater should slow down and maintain required temp.
You could by pass the thermo for a while which would make the heater run at high power ( join the Grey? wire)

Nothing has changed, we just just haven't really used it since about last March when it was working fine. No smoke, other than a puff of buish smoke at start up. My current thinking is that the thermostat has stuck 'off' (i.e. up to temp) and so the heater is permenantly in low power mode.

I agree that the best test is to try and bypass the thermostat. Can you explain a bit more about the "grey wire" please? Is this on the thermostat or the controller unit?

Many thanks for your inputs BTW.

 

[FWB]

Just ring Toby !

 

[pcatterall]

Thermostat wire. I am an eberspacher man so not sure what the webasto set up does.
Grey is in fact the eber colour wire for the thermo. On the eber system the main switch still controls the desired heat setting and the room thermostat ( when used) just feeds into that so there are just two wires to it. I GUESS that when the temp is reached the circuit is broken and the heater shopuld slow until the circuit is 'made'' again. I cant find any diagrams showing a webasto thermostat set up here and in any case yours may be a 'one off' fitting. Sorry I cant help more.
I am suprised that Maby's system shuts down completly at temp. This can happen in an eber when the required heating is set so low that the heater cant actually run at that temp and will then cycle on and off, but even then the fan will run for some time to prevent the heater body overheating.

 

[maby]

...

I am suprised that Maby's system shuts down completly at temp. This can happen in an eber when the required heating is set so low that the heater cant actually run at that temp and will then cycle on and off, but even then the fan will run for some time to prevent the heater body overheating.

I did report that it runs the fan for several minutes after the burner has shut down to cool off the chamber. It certainly gets into cycling on and off at relatively high temperatures - around 20 degrees by the cabin thermometer.

I must say that the Webasto control box is not the most sophisticated I've ever seen - the temperature setting on the thermometer for a given cabin temperature is a bit hit and miss. There are times when I'm feeling cold and it has not switched on and other times when we have the oven on and the boat is like a sauna, but the Webasto is still running like a jet engine.

 

[Kurrawong_Kid]

I did report that it runs the fan for several minutes after the burner has shut down to cool off the chamber. It certainly gets into cycling on and off at relatively high temperatures - around 20 degrees by the cabin thermometer.

I must say that the Webasto control box is not the most sophisticated I've ever seen - the temperature setting on the thermometer for a given cabin temperature is a bit hit and miss. There are times when I'm feeling cold and it has not switched on and other times when we have the oven on and the boat is like a sauna, but the Webasto is still running like a jet engine.

Our warm air central heating at home displays the same facets. Sometimes we feel frozen and have to turn it up to get it to blow and at other times we are boiling and it is still blowing! I've come to the conclusion that humans measure temperature coupled with humidity;whilst thermostats just measure temperature!

 

[thalassa]

The heating system is currently only producing luke warm air. You can comfortably hold your hand in front of the vents, wheras last winter it was too hot to do so for more than a few moments.

The heater had a full service from Deisco last year and has only done a few hours since, so I'm reluctant to believe it's coked up again.

Can anyone advise please?

Perhaps some dust has accumulated in the cabin sensor, if it is an external one.
It is a very simple device, a NTC thermistor, mounted in a small open cage. The contacts are quite close together, and conductive dust (salt air) can lower the resistance, and correspondingly simulate a higher temperature.
Just measure the resistance at the contacts, it should be about 14.1 kOhms at 18 deg.C.

 

[Carduelis]

Perhaps some dust has accumulated in the cabin sensor, if it is an external one.
It is a very simple device, an NTC thermistor, mounted in a small open cage. The contacts are quite close together, and conductive dust can lower the resistance, and correspondingly simulate a higher temperature.
Just mesure the resistance at the contacts, it should be about 14.1 kOhms at 18 deg.C.

Excellent - that's exactly the sort of advice I was looking for. I'll try it.

Thanks!

 

[David2452]

I am suprised that Maby's system shuts down completly at temp. This can happen in an eber when the required heating is set so low that the heater cant actually run at that temp and will then cycle on and off, but even then the fan will run for some time to prevent the heater body overheating.

I assume this is a pukka marine heater, just one of the differences between vehicle heaters and marine ones, Webastos anyway (yes there are ! ) is there is no control idle so it will sometimes shut down.
I would strongly suggest that the issue here is the temperature probe which should be an external one, it thinks its warm when it isn't and not an unusual fault connecting to a thermotest diagnostic would show the target temperature and confirm the problem, it certainly is not coking.

Edit to say I could drop by and connect up the thermo test on my way home from Shotley on Sunday if you like and bring along a spare probe.

 

[Carduelis]

Very many thanks for the offer.

Unfortunately I'm not around this weekend. I've managed to find a wiring diagram online and I think I'm capable of testing the thermostat. So I'll try that first and get back in touch if no joy.

 

[David2452]

OK, I'm on the boat in Shotley most weekends.