Hot Water only when CH is on

[RidleyRumpus]

Hi,

For sometime, I have been having a problem where the hot water will only heat up if the central heating is on.

I thought it might be related to my controller, which was definitely faulty, so I replaced it with a ST9400 (https://www.diynot.com/diy/threads/swapping-a-hw-st699-for-a-hw-st9400.503463/) and that seems to be working OK, thanks for everyones help.

But the hot water problem still remains so I suspect the problem is elsewhere, one of the valves?

Any ideas where to look would be appreciated, thanks.

 

[stem]

You need to provide more details about your system and what is controlling it. For example, do you have two, 2-Port Motorised Valves, one for hot water and one for central heating? Something similar to this:

Untitled

• stem
• 26 Mar 2010
• 6

....or do you have a single 3-Port motorised valve supplying both heating and hot water? Something like this:

Untitled

• stem
• 1 Apr 2010
• 8

 

[RidleyRumpus]

Thanks for getting back to me.

It looks like the former

 

[stem]

Thanks. In that case, the valves are powered from the programmer via the associated room and hot water cylinder thermostats. When they reach the full extent of their travel to the open position, they operate a small internal microswitch which starts the boiler.

The most likely scenario in your case is that the hot water valve is opening OK, but the microswitch is not operating to start the boiler & pump, so nothing happens. Then, when the heating valve opens it starts the boiler & pump and because the hot water valve has actually physically opened it will also receive water from the boiler.

If you are competent with a multimeter, or know someone who is it, can be easily checked, the orange and grey wires from the motorised valve are those connected to the microswitch.

 

[RidleyRumpus]

Thanks for that. Yes, I am competent. Well, I am with electronics.

I presume you should check in the actual valve and that the valve to check is the one upper one in the pic?

 

[stem]

Yes the upper valve is the one for the hot water. It will be opening because the hot water does heat up when the heating is on, but is probably not operating the switch.

One of the wires will be connected to a permanent live and the other to the boiler. Normally you can hear the microswitch 'click' as the valve opens (there is a 10 second or so delay as it winds open quite slowly) If you don't hear the click it isn't operating. You may hear the click and it may still not be working if the contacts are worn out, but the multimeter will tell you. Make sure that the heating is off when you do the test otherwise there will be a live fed from that.

 

[RidleyRumpus]

OK, thanks for the input.

I have powered down the sytsem completely and then I checked the lever on the valve head. I can push it across to the manual position without feeling any resistance and if I let go it does not return to the default position.

When I do the same on the CH valve you can feel the motor/gearbox rotating and when you let go the lever returns to the home position.

It seems to me that the HW valve is knackered but from what I have described wouldn't this mean it was jammed open? If so the HW should work all the time, shouldn't it?

More to the point what should I do? Take the head off and check for a jammed valve? (Make sure it isnt the 4 screw version first) or is the fault in the motor/gearbox?

 

[stem]

I have powered down the sytsem completely and then I checked the lever on the valve head. I can push it across to the manual position without feeling any resistance and if I let go it does not return to the default position.

That's not good, you should feel resistance.

When I do the same on the CH valve you can feel the motor/gearbox rotating and when you let go the lever returns to the home position.

That's well and good and as it should be.

It seems to me that the HW valve is knackered but from what I have described wouldn't this mean it was jammed open? If so the HW should work all the time, shouldn't it?

Not if the valve is open to some extent, but not enough to operate the microswitch and start the boiler & pump. In which case water would only flow through it when the heating motorised valve starts the boiler & pump.

More to the point what should I do? Take the head off and check for a jammed valve? (Make sure it isnt the 4 screw version first) or is the fault in the motor/gearbox?

As a test you could disconnect and remove the actuator from the central heating valve which you know to be working (I assume you aren't using central heating presently) and put it on the hot water valve and see if it works as it should. If it does, then actuator was faulty. The little bump on the top of the valve actuator indicates that it can be removed from the valve.

 

[RidleyRumpus]

That's not good, you should feel resistance.

However, there is resistance when the system is powered up.

Not if the valve is open to some extent, but not enough to operate the microswitch and start the boiler & pump. In which case water would only flow through it when the heating motorised valve starts the boiler & pump.

Ahh so it is open and jammed at all times, just not operating the microswitch?

Prior to swapping heads could I take the HW head off and manually try to open/shut the valve to free it?

 

[Hot&Cold]

Seems the idea of testing with a multimeter has been shelved .

Have you tried the test,last paragraph from Stem in #8 ?

you could remove the powerhead from the heating valve and note the movement of the valve stem,it should turn with finger& thumb pressure (it does not fully rotate). Now do the same on the hot water valve and note the valve stem movement with finger & thumb it should be the same.If not its seized.
Dont force the valve stems with a spanner,they could leak water .

its a process of elimination,the fault could be the valve body,powerhead or its wired up incorrectly

happy tinkering,