Viessmann Vitopend 100-W WH1B heating not working. (Hot water in taps is fine)

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For some reason my boiler stopped heating the radiators, hot water running fine. A month ago when it happened i kinda played around with pressure, i drained few liters of water so it would be at 0, then put pressure to 1 it worked for 3 weeks, then happened again same trick didnt helped.

I drained the entire water from the system opened the valves on radiators to drain all the water. Then i put all the water back bled radiators from air. Add water again to keep pressure around 1.5 when heating. Got ressurected again heating worked for a day. That was 2 days ago, just now it happened again. Control is on for heating, but it doesnt start.
When its not heating for a quite the time when i turn off and turn on the heating on thermostat it will make a sound like its working, Control for flame will light up but after 10 seconds it will turn off. But the heating control is still on.

In the video you can see that it turned on, but then after few seconds went off, you can see on thermostat that its still ON. This will happen ONLY when being off for some time, if i switch OFF and ON on again, it wont even start like in the video, have to wait few hours.

Heating light is ON when thermostat is in heating mode, but it just not working. Hot water fine. So i studied on internet a bit, so as i understood it might be a diverter valve thats either not working or is stuck in hot water in taps, but im not sure.

I would like to investigate as much as i can before calling specialist, to avoid spending a lot of money to diagnose and fix problem since there is no error showing up and i was able to fix that with replacing water in the system, but was only temporary.

Appreciate all the help in advance :)
 
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Hard to tell from the video. Yes it could be the diverter. Is the temperature dial all the way up? Check the thermostat too, could be a fault on that. Hard to see what thermostat you have as the video is all over the place
 
Hard to tell from the video. Yes it could be the diverter. Is the temperature dial all the way up? Check the thermostat too, could be a fault on that. Hard to see what thermostat you have as the video is all over the place
It's older thermostat wasnt able to identify. Found few on marketplace but without any name or id. It's powered from 2x AA batteries, that i replaced and i think that thermostat should be ok. Its making a clicking sound when turning on and off heating. (radiators heating control on boiler also lights up when its on)

I do have a new one, i made other thread regarding replacement of thermostat, since i wasnt sure how to connect it, but since this issue popped again i postponed it.
HERE

I found a diverter valve for my model but it costs 150e in my country. Would like to know if it's indeed diverter valve. Not sure how to test that.
 
Not really a way to test it if it’s stuck in hot water position, usually involves removing the cover (not recommended) and checking the motor is moving and the pin is free
 
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Not really a way to test it if it’s stuck in hot water position, usually involves removing the cover (not recommended) and checking the motor is moving and the pin is free
Thanks, i hope that issue is indeed with the diverter since hot water is fine and i was able to fix him for few days after draining all the water out.
I saw on YT repairs of diverter valve and in most cases they just replaced the inner cartridge, not sure if i can do the same since i didnt find any catridge only entire diverter valve. HERE

But if i understood correctly there is a motor attached to it. So it can be a motor issue or a diverter valve ? I didnt find the motor on the internet so not sure.
 
This happened when i turned off the heating and the boiler and then turned back on.
After i turned on the knob for heating boiler started to make this circling water sound. Sounds remains even after i turn off the knob for heating or turn off the thermostat. When i turned off and on the boiler, sounds not present when heating knob is turned off.


Do you know what might be the problem ? I will try to drain the system again today if that will help, like it did before. I'll probably call the specialist on Monday but would like to investigate as much as i can till then. Thanks ;)
 
Sounds like it's overheating. The water sound when the boiler is switched on is the pump circulating the water and air moving around the boiler HEX/pipework whilst it's in its purge phase. What pressure is in the boiler, do you have to fill it up regularly? Boiler may need bled.

Are you sure the model is a WH1B as there doesn't seem to be that model variant. if you have the manual there should be a GC number in it.
 
Pressure is at 1 its ok. I bled the radiators and drain all water from system today. But after i started it same issue as in video. (Before it helped and worked for a few days) it was like 6 days ago when i drain the entire system first time, it worked for maybe 24hours.

I dont need to fill it regularly, boiler holds pressure good, no leaking.
You mentioned overheating, i noticed that when i started it and water sound began after a few minutes i noticed a temperature was around 80 celsius pretty high (but im not sure im not oftenly checking that and its not in red numbers :D so idk), and hot water wasnt on nor flame indication.
Yesterday it was normal around 60 since hot water in taps is working. So what could cause overheating ? if it is indeed this problem.
The sound is coming from the pump in the photo.

vitopend pump.jpg

EDIT:
regarding the model im not sure, manual is generic for multiple types on the cover there is just Vitopend 100. This model has no display just temperature and pressure monitor. It came with the house when i bought it.

Whole picture of boiler that might help identify model.
vitopend1.jpg
 

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