hot water will not come on without heating

30 May 2007
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United Kingdom
I have a fully pumped gas-fired system with motorised valves controlling hot water and heating. Hot water is stored in an indirect cylinder with a thermostatic control (which appears to be working correctly). Until recently it was all working correctly.

Now, if I try to heat the hot water by pressing the hot water 'on' button on the programmer, nothing happens. But as soon as I press the central heating 'on' button, everything bursts into life and hot water starts circulating both through the heating pipes and through the hot water cylinder. But if I then switch the heating off again at the programmer (leaving the hot water 'on') the pump stops and everything goes off.

At first I thought the programmer might have developed a fault, so I installed a new one. The same problem persists.

What can be wrong?
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namsag said:
Faulty hot water motorised valve
Thanks for your quick reply namsag - would this be an electrical fault or a mechanical fault?

Would a fault in this motorised valve also be what is preventing the pump starting when the hot water is switched 'on'?

The motorised valves I have are Honeywell 8901 - are they easy to replace?
Sounds like valve has mechanical fault when the HW is selected it gives power to a motor in the valve that opens it and at same time activates a microswitch that gives power to the boiler and pump .. When they start to wear and get a bit of scale on the shaft the motor will not move the insides far enough over to hit the microswitch......

Sometimes its just the motor and a new one will solve problem. But probably better to change the whole valve..

If you have a gate valve on the return out of the cylinder turn this off and if you have pump valves with spindles on them turn these off and any bypass valve beside it you should then be able to change valve without draining system.

There will be a bit off water so have some towels ready..

If your pump valves have slots don`t touch them and just drain system down
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The sequence of operation is :
The programmer (HW) sends a live to the HW thermostat.
If HW tank is cold, the HW thermostat sends a live to the HW valve.
The valve swings into action , opens the boiler pipe to the HW tank heat exchanger. At the same time, HW valve flips a microswitch which sends power to the Boiler+Pump which leap into action.

Do a few checks to make sure it's not just a loose connection or something trivial.
Once you have confirmed it is the HW valve, you can usually change the actuator (the box on top) without touching the water side of things (unless it is very old - over 10 years)

- Does the hot water get hot in which case the valve is opening but there is something wrong with the microswitch.
many thanks namsag and dal5band for your superb explanations

yes dal5band - the water is heating (as long as the central heating is on) so I think it has to be the microswitch which is not operating

namsag - from what you are saying it sounds as though the valve may not be opening fully, but when the hot water is 'on' I can feel no resistance at all on the manual lever (there is resistance when the hot water is 'off') - does this suggest that the valve is opening OK but that it is the microswitch that is the problem and replacing the head will be likely to solve it? or could it still not be opening far enough?

also - if the microswitch is not operating is there any sort of fiddling around I could attempt, in the hope of getting it working without replacing anything?
Very rare its the microswitch more often the cam that the motor operates doesn`t quite reach the switch and we are only talkink about a couple of mm at most. Take valve cover off and have a look at how it all moves as it works . You can use an insulated electricans screwdriver an press the microswitch button in to check this you will hear pump start and boiler fire if ok..

If the valve cover has a little pimple on it at one end then just the valve head can be changed if no pimple whole valve required

If just changing head make sure the spindle that comes out of valve body has not been leaking and scaled up. spindle only moves about a quarter turn so get pair off pliers and check it moves freely..
thanks namsag - assuming the cam is not going far enough to operate the microswitch is there anything I can fiddle with to help it get there?

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