Calling Viessmann Experts: Flow Switch

[CMairiD]

The situation:

One of the first versions of Viessmann Vitoden 100 18kW regular boiler, powering ten rads in a two down, three up home. The boiler struggles to heat the home (when it's working) and the rads appear appropriately sized.

Three times in the last nine months our engineers have had to go out and clear the flow switch. The last time I saw the boiler in this state, the burner was cycling on/off and the boiler couldn't get above 60C over 45 minutes of calling for 30C heat.

Engineer came the same day, cleared the flow switch and heating returned to it's semi-working condition -- meaning the rads don't get as hot as you'd expect.

When the engineers clear the flow switch, they are removing black, sludgy like stuff. Given the rad temperature and black gunk, you might think the system's sludged up and in need a jolly good clean. Well, I was with an engineer this evening, we drained some of the system into the kitchen sink and the water was clear as mains. Even used a Magnaclean test stick and minute amount of metal was was attracted.

The pump was changed this evening for a larger one which appears to have fixed the issue with the rads not heating up properly, but I am still very concerned about this flow switch and the black gunk.

Where's it coming from because it appears we have a clean system? Has anyone seen this on Viessmann boilers?

The kicker? This is not the only Viessmann we have that is displaying these symptoms, so trust me any advice you can provide would be much appreciated.

 

[gasmarc]

yes I have seen this problem a few times ,it is due to dirt in the system and cannot be anything else , to prove a point remove the worst rad and you should get a nice bit of crude out

 

[Blackcatgas]

They changed the flow switch to a Grundfos Vortex [DLMURL="http://net.grundfos.com/doc/webnet/directsensors/int/sensor_technology.html"]Grundfos Direct Sensors - Sensor technology[/DLMURL] on the Mk II 100 series which is esp resistant to clogging, So it must have been a weak spot, the MKII has a completely different all Grunfos Hydroblock.

 

[gasmarc]

very interesting blackcatgas , i have had numerous problems with the old vitodens but know if your engineers have been out 3 times and they are getting dirt on the switch ,it requires a powerflush as the rads are not heating up as you say semi working ,good luck,

 

[Blackcatgas]

Maybe try putting in an inline strainer?

 

[gasmarc]

and a magna cleaner may help

 

[CMairiD]

gasmarc, I take you point; everything screamed sludge in the system until I saw with my own eyes that the system water was clear. Checked at the drain-off on the flow from the boiler and again at the pump location when the pump was changed to a larger one which is now heating the all the rads to a suitable temperature.

Spoke to the last engineer that cleared the flow switch and he describes the stuff as a grey graphite-like substance, not the muck that usually comes out of a powerflush.

So since we appear to have this mystery substance that's not sticking to a magnet, what about going for the Magnaclean TwinTech rather than the normal Magnaclean?

 

[gasmarc]

that does sound abit odd ,but the switch would either fail or a blockage due to dirt in the water, yes i think your suggestion is the way forward make sure its put on the return though lol we dont want another flow switch failing,good luck