Central heating keeps turning off

[Scoob72]

Hopefully someone can provide some insight....

My heating is working very poorly on my combi boiler. Hot water is fine. What is happening is that when the thermostat fires up the boiler, the 2 way motorised valve (Salus MV222) moves to the halfway point and the heating starts up. The radiators start to get hot (only at the top, cold at the bottom), and then around 3 minutes later the valve closes and the heating turns off. The thermostat at this point is still requesting heat, but the boiler is no longer fired up and the valve is closed.

If I manually force the valve to open and keep it open then the heating works fine. All the radiators get nice and hot and the house warms up.

My first thought is that something is wrong with the motorised valve, but i'm hesitant to jump to conclusions, and my google research doesn't reveal any similar symptoms associated with failed valves.

Thoughts and advice much appreciated.

Thanks

 

[gas_man_gaz]

what boiler ya got?

 

[bhm1712]

i think you have discovered the fault yourself....

If the thermostat is calling for heat, and it works if you force the valve to open, then id say its the MV at fault.

However , you need to be sure the thermostat is calling for heat - just because its indicating, or clicking on does not mean the relay contacts are closed inside it.

Double check with a multimeter if your competent to do so. If the stat is ok, then i would still point to the MV

 

[Scoob72]

what boiler ya got?

It's a Baxi Platinum 28HE

 

[Scoob72]

However , you need to be sure the thermostat is calling for heat - just because its indicating, or clicking on does not mean the relay contacts are closed inside it.

Would the same hold true given that when I manually force open the MV everything works fine?

The bit I find odd is that the MV is closing after just a few minutes. I've read about them not opening, getting stuck open, but never faulting in a way that they close themselves after a few minutes.

 

[bhm1712]

well you need to verify the output from the roomstat. if the valve is closing then its possible that the roomstat output is dropping off when it shouldnt, but you say in your original post that the stat is calling, so i assumed rightly or wrongly that you had already tested and verified it.

If the roomstat is not calling and you force the valve open, then the heating would still fire providing it is an S plan, but you say its a combi, but the valve is only 2 port?

need more details as to what the system is as nothing makes sense at this moment from your description.

 

[Scoob72]

need more details as to what the system is as nothing makes sense at this moment from your description.

Sorry, I should have explained a bit better. It's a combi with two circuits - one for the rads, one for some underfloor heating. Each circuit has a 2 way motorised valve controlled by separate thermostats. The UFH works fine, so it very much appears it is either the MV on the rads circuit or the thermostat that controls it. I haven't tested the thermostat - merely verified that it is visually (symbol on the stat) and audibly (clicking) calling for heating. The MV is closing after 2-3 minutes every time and turning off the heating even though the thermostat is both visually and audibly (hasn't clicked to go off) still calling for heating.

 

[bhm1712]

right,

you need to use a multimeter to verify the thermostat output is really on when it says it is.

if it is, and the valve is still closed then it would seem that the synchron motor is faulty. potentially it is working ok to begin with, but after a few mins is overheating or similar and closing. thats my guess without being there and fully understanding the way its wired up. however, if the valve is somehow controlled from the boiler PCB then i may be entirely wrong.