Problem with Viessman Vitodens Heat Only boiler / system

[Kentguy]

Hi
Have a problem with Viessman Vitodens 100w WB1B boiler just 3 years old.
System was cleaned again in May this year but boiler cycles off before reaching max temperature on central heating. On max position 6 on boiler thermostat dial the CH flow wil only reach about 65 degrees with return circa 40 degrees. Also system very slow to heat up. Return seems particularly slow. New pump fitted with no real change. System will work very well in Service mode with all radiators getting hot throughout the house. Albeit in normal mode they do not get hot enough and take a long time to heat up.
The boiler has been thoroughly vented of air, no air in pump or rads either.
Any clues please from good Heating Engineers ??

Many thanks

 

[ianmcd]

when you put the boiler in service mode it overrides the sensors in the boiler that tells the boiler to stop running at maximum, if everything working good in service mode then circulation sounds OK so you are looking at a temp sensor fault but these boilers have lots of setting regarding max heat input etc and might just be a setting.

 

[Phill Peck]

@Kentguy when the system was cleaned again recently was anything removed and cleaned from the boiler?

The compact is very susceptible to problems with dirt.

I have rectified this problem many times but am not near Kent as I presume you are.
Is your heating engineer familiar with the boiler?

 

[Kentguy]

Hi
When the system was cleaned in a May this year, I don't recall anything being removed from the boiler. The boiler was serviced just a couple of months ago. My heating engineer is planning on doing a magna cleanse of the entire system in a few weeks time as he can't fit it in before then. He thinks that dirt or sludge in the system is slowing down the return which in turn is restricting the max flow temperature. The burners do not modulate very well either

 

[Phill Peck]

Hi
When the system was cleaned in a May this year, I don't recall anything being removed from the boiler. The boiler was serviced just a couple of months ago. My heating engineer is planning on doing a magna cleanse of the entire system in a few weeks time as he can't fit it in before then. He thinks that dirt or sludge in the system is slowing down the return which in turn is restricting the max flow temperature. The burners do not modulate very well either

Does the boiler sit in low fire?

Tell him he needs to remove the flow pipe from the boiler and clean the dirt from around the flow switch and thermistor.
There will also probably be dirt baked into the heatexchanger.
These flakes may have been removed with powerflushing but could well be lodged in the flow pipe.

 

[Kentguy]

Hi Phil
Many, many thanks for your great reply.
I will pass on your recommendations to my heating engineer.
It is unlikely that he will be able to work on the boiler now until mid January
due to Xmas hols and other jobs booked in.
You are very correct in that yes the boiler does tend to sit in low fire mode
the return temperature and speed remains too low.
On central heating, the burners only seem to modulate up to the third bar
but spend a lot of time in just the first bar.
Thank you once again Phil and all the very best to you and your family
for Xmas and 2018
Best wishes
Kentguy

 

[Kentguy]

Hi Phil
Update . System thoroughly flushed again after adding cleaning agent for 10 days.
Drained completely, reflushed and refilled. You were correct there were lots of small rust flakes
caught up in the heat exchanger. Magnetic filter thoroughly cleaned again.
F& E tank cleaned before and after cleaning and flushing.
System working beautifully again now - thanks again for your help
Kentguy

 

[Phill Peck]

Hi Phil
Update . System thoroughly flushed again after adding cleaning agent for 10 days.
Drained completely, reflushed and refilled. You were correct there were lots of small rust flakes
caught up in the heat exchanger. Magnetic filter thoroughly cleaned again.
F& E tank cleaned before and after cleaning and flushing.
System working beautifully again now - thanks again for your help
Kentguy

Did he remove the pipe as suggested?

 

[JohnD]

The pipe in the pic looks very small. What diameter is it?