Viessman vitodens 100

[Joshua Lightowler]

I have a viessman vitodens 100 and the heating will work and all of a sudden it will turn off. It is not a set time when it turns off. But it seems when the gas burner is not needed it will not re-ignite. Its usually when it hits 69c.

Once it cuts out the heating will not come back on. either the system has to cool down or its the next day when it will start and the problem will just repeat its self.

I have wired 3 different thermostat controls to the boiler and they do not change the problem.

After the boiler turns its self off I can put it into service mode where the boiler will run burning full gas for 30 minutes. This is what I cant understand as the boiler is running the pump and all radiators are getting hot.

Any advice is appreciated as I am not a plumber and getting one is a long wait. Its becoming really frustrating and want it sorting as soon as possible.

Thanks.

 

[ianmcd]

these are terrible for the hoses from the heat exchanger blocking, not a DIY repair though

 

[Joshua Lightowler]

these are terrible for the hoses from the heat exchanger blocking, not a DIY repair though

But if it was the hoses why would it run fine as soon as I put it in service mode. Service mode runs it on 5 bar gas and it runs fine. I'm puzzled

 

[JohnD]

In normal running, the boiler compares the incoming water temperature "return," and outgoing hot water temperature "flow."

It expects to find that the Flow will be hotter than the return, as the water absorbs heat as it runs through the heat exchanger. If the difference exceeds (I think) 25C, it deduces that flow is insufficient to carry away the amount of heat the burners are delivering, and turns it off. In normal running, it tries to modulate the gas flame down for greater efficiency, so it would not reach a 25C difference unless there was something wrong, such as a partial blockage. There is a tight bend where the pipe enters the heat exchanger, I understand this tends to accumulate obstructions, the bore is quite small. I suppose it would have to be taken apart to remove the blockage, if a chemical won't do it.

During Engineer mode, this control seems to be either disabled or given wider parameters, to enable the boiler to fire at maximum (or minimum) power for the test. Mine only cuts out in Engineer mode if the flow reaches about 85C. If you keep running it in engineer mode it may be that you are putting it under thermal stresses in excess of the design intent, I don't know. I am a householder not a boilerman.

 

[ianmcd]

But if it was the hoses why would it run fine as soon as I put it in service mode. Service mode runs it on 5 bar gas and it runs fine. I'm puzzled

no it doesnt if your boiler was at 5 Bar gas you would be in very serious trouble

 

[Joshua Lightowler]

no it doesnt if your boiler was at 5 Bar gas you would be in very serious trouble

I don’t mean 5 bar pressure. Just 5 bars on the screen meaning it’s burning the full amount of gas. I think it’s called being in service mode.

 

[ianmcd]

yes and when your boiler is in service mode it ignores the thermistors which tell the boiler to start modulating down, untill the Hi Limit stat turns the boiler off for safety,when the hoses are blocked the boiler runs as normal and cant get the heat out fast enough , but the thermistors will operate as they should when not in service mode

 

[JohnD]

how are these blockages cleared?

 

[Joshua Lightowler]

yes and when your boiler is in service mode it ignores the thermistors which tell the boiler to start modulating down, untill the Hi Limit stat turns the boiler off for safety,when the hoses are blocked the boiler runs as normal and cant get the heat out fast enough , but the thermistors will operate as they should when not in service mode

Ok thanks. So what would you suggest doing. Because it will work and run say for an hour and then it won’t fire up usually until the following day. But as I say it will run fine in service mode.

 

[ianmcd]

the hoses are best replaced, they can be cleared but not recommended, and cant be done by you as it means removing the heat exchanger

 

[JohnD]

mine is the conventional which has copper pipes, not rubber hoses. 2008 vintage.

I don't know what Joshua's has.

I'm wondering if the cost of repair is worthwhile. Mine is still going but I think it has that blockage accumulating.



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