UKC

Its that time of year when i make the move from heating & water to water only on the boiler. 

When I flicked the switch to water only mode, the bloody heating still comes on. 

System is a gas boiler with a water tank in the airing cupboard and a randell 102e control unit. 

any ideas? the control unit guide doesn't include a troubleshooting guide. 

Can't you just turn the thermostat down as far as it will go? Should stop the heating coming on.

There's normally a valve which switches between heating and hot water - that may have seized or the electronic actuator broken. You could try removing the actuator and operating the valve with some pliers.

In some "classic" (system boiler i.e. with hot water cylinder) systems like my parents' previous boiler there isn't such a valve.  The only difference in that case between heating mode and hot water mode is that the pump doesn't operate in hot water mode, but some water does circulate naturally and warm a few radiators.

Assuming it was fine last year then valve.  If you don’t know that it was fine before it could be wiring.  

Do you have a back up electric immersion heater?  You could use that instead if you are too hot with the heating on.  

Or just turn all the radiators off!

The thermostat and receiver have never work we just run then system on a timer 

What's an actuator? Where does it normally live? 

Sadly not 

I like this the simplest solutions are often the best

Sometimes there is deliberately one radiator you can’t turn off as part of the setup. 

Assuming you have a mid position 3 way valve, most likely is either a stuck actuator or let by on the rubber washer inside the Body of the valve. Watch the lever on the side of the valve as you turn heating on and off with hot water demand on as well, if it doesn’t motor over motor gone, if hw only on and lever returned to hw position but heat gradually comes down ch pipe then probably let by. Often the actuator will Burn out because the body of the valve is stiff or seized.

hope that helps, obviously turn off your rads on there own valves as an instant fix.

cheers Chris

It looks something like this:-

https://www.jtatkinson.co.uk/danfoss-hp22-complete-2-port-motorised-valve-a...

And will normally be found somewhere near the hot water tank and pump.

You can easily separate the motor from the valve. You may find the valve difficult to move so if you try to turn it a bit with a spanner or pliers to loosen things up (maybe with a bit of WD40 applied) you may be lucky. If you turn the heating on you should see the motor moving. If not then you've found your problem. For the summer, if you don't want the heating on then you can just leave the motor off the valve and set it to water heating only

Often with combi boilers there is although most have an internal bypass these days. On a conventional vented system there's no need for a radiator you can't turn off as the flow and return to the HW tank acts as a bypass and its an unpressurised vented setup anyway.

For the OP, continuing to heat water from the gas boiler will be cheaper and more controllable (i.e. on a timer) than by using an immersion heater until he can get the problem diagnosed and fixed. My guess is the motorised valve is stuck open that diverts the flow to/from the radiators and HW tank.

Some immersion heaters are on timer too, anyhow he doesn’t have one, so it doesn’t matter.  

Thanks for clarifying about the combi vs invented, it makes sense now I think about it.  

Sounds like the diverter valve letting though, what model boiler is it ? 

Boiler is baxi solo 18.

Had a look at the diverter seems fine. Moving freely circuit seems ok

From cold system turn on hw only, ch off and put your hands on the pipes leaving the 3 port valve, the hw pipe to cylinder should get hot pretty quickly and the ch stay cold. If it also warms up and the motor has stayed in the correct hw position then the rubber inside the valve is not sealing allowing let by.

replace valve.

cheers Chris