No hot water without heating

[cbrbenjy]

Hi all, I know this might have asked/answered already but I couldn't find it. We recently discovered that we don't have any hot water unless the heating is on.
We have a gravity fed system with 2 water tanks in the loft. 1 large one filled with cold water and a smaller one. A hot water tank in the airing cupboard along with the pump, 2 control valves and the controls for the heating and hot water. There is also a thermostat in the living room downstairs.
When the heating is on by itself we have no hot water. We need to have both hot water and heating on at the same time for a while to allow the water to heat up. After a while there is no hot water again. The temperature on the thermostat also seems to affect it. It needs to be turned right up to allow really hot water but that means the radiators are all really hot.
Is it possibly one of the control valves? Would I need to change just the control part or the full thing? Is it easy enough to do yourself or do I need to get an engineer in?
Please help. Ta.

 

[stem]

When you say "control valves", do you mean.....

Untitled

• stem
• 26 Mar 2010
• 6

If so, these motorised valves have a small microswitch inside that when the valve opens, that switch operates and turns on the boiler. My guess would be that the hot water valve is opening, but not operating the switch. Then when you operate the heating the switch in that valve starts the boiler. It can be easily tested if you are able to use a multimeter. Some types of valve allow you to swap the actuator head without draining the system down. Others don't.

BTW it's not correct to call it a "gravity system" (a gravity system is where the water circulates by gravity, ie without a pump) If you have two valves and a pump, it's known as an 'S Plan' and is described as fully pumped. Not gravity.

Untitled

• stem
• 17 Apr 2013
• 2

 

[cbrbenjy]

Yes those are the types of valves that are fitted. Im a car mechanic to trade so the multimeter isn't an issue, it was more how to go about checking them and if it was easy enough. The valves have on them "for replacement part use 40003916-001" so im thinking the head might be able to be replaced.
"BTW it's not correct to call it a "gravity system" (a gravity system is where the water circulates by gravity, ie without a pump) If you have two valves and a pump, it's known as an 'S Plan' and is described as fully pumped. Not gravity." Sorry about the confusion. We had another radiator installed last year and the heating engineer told me that's what type of system we had due to the tanks in the loft, even though it is pumped. Thanks

 

[stem]

There will be grey and orange wires that go to the motorised valve. These are the boiler switch wires. One will be permanently live (usually the grey one) and the other becomes live when the valve opens (usually the orange one)

Remember that the microswitches in both valves feed the boiler, so when you test the one suspected to be faulty make sure the other is closed. Otherwise you will get a live feeding back from the other valve.

Click here for a diagram of what you are looking to test.

And you can replace the powerhead an example can be found by clicking here