CH Not Working. HW OK.

Hi,

I have a similar problem to a previous poster on this site, but with enough differences to make me need to post this.

I have a system that consists of a boiler (potterton but still working), indirect cistern, single pump and 3 port valve ( Siemens MA-V322C). The HW is working fine but the CH is not. There is power from the room thermostat when it is turned up high enough. I suspect the 3 port valve as the problem but am confused as to how the system works.

For example, when I turn the CH on with the thermostat up, nothing happens - no pump, no valve operation. If I manually open the valve with the small lever beneath it, the radiators get partially warm but the pump does not operate. I know the pump is working so why does the pump not operate when the heating is turned up and the valve is open? If I can understand that, then I'd be confident it is the valve, otherwise, I guess it could be a control problem.

Can anyone help please?

Cheers,

Andrew

The valves normal (under spring tension) position is HW only, so for this, the valve doesn't need to operate at all. Then it moves through HW&CH and CH which operate a micro switch which informs the boiler that they have reached their desired position and all fires up. If after a predetermined time (usually about 5 secs) no signal is received, nothing happens. The valve could be seized but more likely in these circumstances the small motor that drives the valve gives out. Take the cover off the valve and check if it is turning enough to make the micro switch when you have CH selected. If not, first call is to change the motor, £10'ish, even Q & B stock them.

Thanks,

I already took the cover off the valve and the motor doesn't turn at all. When should the pump kick in, in relation to the valve operation? If the valve is set to CH or CH and HW, I would have expected the pump to be operating?

Cheers,

Andrew

If the syncrom motor is faulty try this..

push the valve into manual but take the cylinder stat and lay on the floor
because when hot water satisfies the c/stat everything stops

I looked at the valve again last night. As well as the manual lever to open the valve, there is a switch that seems to trigger the pump and the boiler. When I hold both open, the central heating kicks in.

So it does seem to point to the synchron motor. When there is power to this motor it makes a small noise and then doesn't move at all. It does however get very hot. If the motor had burnt out, presumably it would not get hot at all? I also took the motor off and checked the resistance across the terminals. It has one of the triangular connectors with black, white and earth wires. There is a resistance of 2.2kohms across one pair of terminals but open circuit across the others.

The fact that the motor gets hot and that there is resistance across the terminals would seem to say the motor is ok, but then it doesn't seem to be doing its job.

Could the problem be something other than the motor?

Thanks very much for the help,

Andrew

Motors can partially fail -they just loose power. The real test is to grip the cog and feel the "strenght". I don't advise you to do this since the casing is no longer earthed when removed from the valve actuator. Just replace the motor (only £10 - £15).

I already threw caution to the wind and tried turning the cog with the power on and off last night. With the power off it turns, albeit very slowly but when the power is on, it doesn't budge at all.

I'm going to pick up a replacement motor at lunchtime and try it tonight. Will let you know how it goes.

Cheers,

Andrew

Hi,

I installed the new synchron motor last night and everything is now working.

All didn't go smoothly though, since the cog on the new motor was only half the length of that on the original. This meant that it didn't engage properly with the ratchet on the valve unless I physically pulled it together. In the end I fitted a number of washers under the plate that holds the motor to bring it forward slightly and now it works perfectly.

Thanks again for your help!

Andrew