Issue with Salus Controlled Viessman 100 System

30 Nov 2005
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United Kingdom
I hope someone can help please.

We have a Viessman Vitodens 100W conventional boiler, just over 3 years old. It is controlled by a Salus RT310i thermostat that also works with an app.

This past few days, the red and green lights on the Receiver are on (solid) when on Auto, but the heating is not coming on.

The boiler works and heats the hot water.

If we switch the Receiver to Manual, the heating still doesn't come on (red and green lights on).

Oddly, if we switch the hot water on while the Receiver is set to Manual, it heats the radiators.

I have followed previous threads where it was suggested to reset the thermostat and Receiver, but that hasn't solved it.

Also if I manually changed the Sunvic SZ2301, the radiators heat up. I thought that may be the issue (when it broke previously years ago, before the boiler replacement, I replaced it), so I changed it for a Danfoss HPA2, but it hasn't solved the issue.

Has anyone got any other suggestions please?

My final port of call will be getting a heating engineer out after Christmas, but I was trying to solve it myself first.

Many thanks
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Just to check your layout.

Do you have a hot water cylinder?

Are there two motorised valves?
If you have test equipment, you could check for voltages.
Thank you for your replies. I am a novice so no test equipment for voltages.

It is set up as a conventional system with boiler, hot water cyclinder in the airing cupboard and next to that there are two motorised valves. The valve I changed out was the top one (that is on the pipe to the pump).

Oddly after days of not working it did start heating the radiators after increasing the temperature manually on the thermostat (Receiver set to Auto). Whether it will do it again next time is the worry/mystery.

Thank you
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Thank you for your replies. I remembered our house insurance has a home emergency add on so am going to investigate further. Do we think they would sort this issue under the policy? It covers up to £500 of repairs including parts.

We have just noticed one of the radiators is leaking too, but assume that is not connected to the heating issue . I think I will mention both when I phone in the home emergency and see if they can sort both at the same time.
Reporting back as I know it is annoying when someone asks for help then doesn't say what happened. We have had hot water and also heating if we also switched on the hot water, so could make do.

The home emergency engineer came out and identified a two port valve was at fault (not the one I had changed previously). However, they checked the colour of the water in the system and said as it was discoloured it would not be covered on the insurance policy They said it would be a few hours job and quoted £400 to replace it .

Working on the basis of nothing to lose, and suspecting it was the head not the complete valve (as it still moved for the hot water) I found the part online at £60. So after also buying a very short ratchet screwdriver (as it would otherwise mean taking the hot water cyclinder out due to how inaccessible the valve was!), I have managed to change the two port valve head/actuator Myson MPE 222 myself (the valve itself still moved very easily). The heating now seems to be working fine .

Thank you for your advice on here and other threads which helped we narrow down the issue.
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What do you mean by a 2 way valve ? never heard of such a beast
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