Wiring diagram for conventional CH system

[BigHammo]

Heres one for you experts.

I have a conventional central heating system, potterton Netaheat boiler, standard 2 speed pump, tradional honeywell controller, (split for CH only, Water only), room themostat, and a two way valve.

Does anyone have a wiring diagram of how this is likely to look? Or can tell me a web site to look at?

I have an intermitant fault where central heating does not work. My pump comes on and then goes off, when it should be on. There was a a humming noise coming from the two way valve, but that has now stopped. So I have assumed that it is something to do with this, and I still get hot water.

I tried bleeding my radiators, but I got no air rushing out on any of them, I could take the whole key out, but still no hissing sound or water. I tried this on 4 of upstairs radiators, same result. Then all of a sudden after 10 mins it did, and the pump kicked in, but still no central heating.

I want to check the eletrical system for wiring defects, pump or valve problems. But it would be nice to see a wiring diagram. Any other thoughts?

 

[kevplumb]

look here y plan

 

[MANDATE]

I think you'll find your two way valve is a 3 port mid position valve. It is supposed to stay at or move to the 1st position (rest) when HW only is demanded. It is supposed to move and then stay at the 3rd positon when CH only is demanded and if the demand is for both then it should be in the mid positon.
Most problems seem to be associated with the 3rd position. Sometimes the motor in the actuator head fails to move the valve. Or it might in fact move the valve but due to a faulty microswitch it fails to supply power to the boiler via the output wire (orange) If you can remove the actuator head from the valve spindle, you can observe the bottom side as you try CH only to verify if the valve moves to 3rd positon. If it does you can locate the orange wire in the junction box and confirm there is no power.
What might be happening, is that that everything works ok while demanding both HW and CH because the power to the boiler is coming from the HW cylinder stat, but of course that state only lasts a few minutes because the HW will soon be satisfied and the heat loss is quite small compared thr CH side. This might make you think it's intermittent.
If this is the case, try running some hot water off when demanding both HW and CH.
hope this all makes sense.

 

[BigHammo]

8) Mandate and Kevplumb, thanks for ideas I will look at this, and thanks for the diagrams.

One thought though! Why isn't pump the pump permanantly on. I thought pumps were on if the controller CH is switched on. Is it switched on and off by the hot water thermostat?

AFTERTHOUGHT!!!

I should have read the wiring diagram before I wrote the above, as it does. Please ignore my gibbering.

 

[MANDATE]

Hi Bighammo. If it is a 'y' plan system with a 3 port mid position valve then pump would be running along with the boiler if there is a demand for either HW or CH which has not been satisfied. So the control is down to the cylinder stat and/or room stat, not the controller/programmer. As soon as the demand is satisfied there is no point in boiler and pump continuing, except some systems are designed so the pump stops a little later than the boiler.
My own system is gravity for the HW with a pump for the CH, so in my case the pump is independent of the boiler and controlled by the room stat alone.
If you are not getting CH under any circumstances then it sounds as though the valve is staying in HW positon and it would soon become satisfied and stop boiler/pump.
Perhaps the first thing that shoud be looked at, is the room stat. Power is fed to the room stat which is just a switch and when there is a demand for CH it allows power to go to the mid position valve (white wire) so that's a check that can be made with a voltmeter in the junction box.