Central heating shuts off once watertank is up to temperatue

[gorsey]

I recently changed the pump as it was blowing the fuse and since then i've had a few problems.

Initally the diverter valve was getting stuck on hot water and wouldn't return to heating. This has now been replaced and functions correctly. However!

With both heating and hot water ON and heating thermostat requesting heat - the valve is in the middle position which is great. Once the hot water is up to temp the whole system turns off. So the heating is still not up to temp yet but this isnt satisfied. If you shut the hot water off, then the system fires up and diverts to heating.

The thermostat on the tank works - so i'm confused. As if the hot water totally overrides the heating.

 

[mysteryman]

There must be a link from the cylinder thermostat satisfied terminal to the grey on the diverter valve. This will parallel the wire from DHW off at the programmer to the grey wire.

Refer to Honeywell for more info.

 

[gorsey]

Yeah i've just been checking that wire.

I assume there should only be voltage on this when the cylinder thermostat is requesting heat.

I turned the thermostat right down (heared a click) and there is still voltage on the gray wire.

perhaps the thermostat is faulty? any way to test?

 

[mysteryman]

On a Y Plan, the grey wire must be LIVE for the valve to motor fully across to the central heating only position. The white needs to be live also via the room 'stat.

 

[gorsey]

okay, thats okay then.

Checked white wire and no voltage when requesting heat.

turning hot water off however sends the diverter valve to CH only?!?

 

[oph]

Initally the diverter valve was getting stuck on hot water and wouldn't return to heating. This has now been replaced and functions correctly................

It cant do

You need to go back to the wiring centre. You've placed a wire in the wrong terminal

 

[mysteryman]

Liven up the white and it should fire.

 

[Space cat]

There must be a link from the cylinder thermostat satisfied terminal to the grey on the diverter valve. This will parallel the wire from DHW off at the programmer to the grey wire.

(My italics.)

I assume there should only be voltage on this when the cylinder thermostat is requesting heat.

I don't have the thing in front of me so i'll take mysteryman's word for it that the relevant wire is grey. The point is that it should have voltage on it only of there is no request for heat. If the programmer says HW-off or the cylinder stat says "I'm hot enough" (or both), that's when that wire goes live. If you turn the stat down until it clicks, it sais "I'm hot enough" so there should be volts on that wire.

If the programmer and room stat are both calling for CH, the valve will drive to its middle position. This will give you both HW and CH - which you say you have. If you want HW only, voltage on the grey wire can take the valve all the way over to the CH only position - but only if it's first driven to mid-position by a CH demand.

 

[gorsey]

humm, think i've lost the plot.

What voltage's should going through these wires?

There seems to be no voltage on anything yet the central heating is on and working??!?

turning hot water on puts voltage on the gray wire which puts the valve in the mid position.

confused.com

 

[gorsey]

Ah so when there's voltage its actually OFF. Makes sense.

More testing ...

 

[oph]

Silly question but the motorized valve isnt manually locked in the open position is it?!

 

[gorsey]

no the valve is working as it should.

The problem is when HW and CH is left ON the HW comes up to temp and then the system shuts off. Never goes back to CH.

The gray wire seems to be connected to the CH thermostat...

 

[D_Hailsham]

Read How a mid-position valve works.

 

[mysteryman]

Space cat's reply was a bit wrong. He meant 'When you want CH only, voltage on the grey wire...'

Basic modus operandi:

White live, cyl stat calling = mid position, boiler and pump live on orange
White live, cyl stat not calling, grey also live = full travel, boiler and pump live on orange
White dead, cyl stat calling, grey dead = spring return to DHW only, boiler and pump live on cyl stat call terminal [also back-feeds to orange]
White dead, grey live = no output from orange to boiler and pump

 

[gorsey]

Thanks for all your help. All good info even if I don't understand all of it.

Is the below situation correct? I'm beginning to think its the programmer. Everything seems to work but put it all together and it doesn't.

HW and CH on. Cylinder temp low and satisfied. CH requesting heat.

In middle position

voltage accross HW thermo stat

No voltage accross CH thermo stat

voltage on white 240v
voltage on gray 88v
voltage on orange 42v

pump and boiler is off.