Boiler is circa 1996 (Potterton if that matters). The CH works as such but I can’t have it on without also having the water heating on at the same time.

Any ideas as to why this might be? Google talks about the possibility of it being a three way valve near to the water tank upstairs that has failed – does that sound right?

Diverter valve (or three way valve as its also known) has packed up most likely. It’s electrically driven so its probably only the actuator that need changing.

It’s not necessarily near the tank. You’re looking for a small silver box at the junction of 3 pipes with a cable to it too. Probably.

Ours was designed to run that way according to the British Gas man.
Maybe yours is similar?

More information needed.
Have you lived there long? Because….Have you been able to control them independently in the past?
Do you know if you have a y plan system?
What are you hoping to gain by asking this question?
Can you fix it yourself if you get an answer you can understand?
I you don’t know the answer to any of these questions, call a plumber.

And. It’s probably the 3 port valve. You can replace the actuator on some and some you can’t. It won’t necessarily be a silver box at the junction of three pipes.

It could also be a priority hot water system. This needs the hot water on for the heating to work. Means you always have hot water but really we need more information.

We had one like that in our last place.

System boiler with convection flow of the hot water up to the tank above. When we turned the CH the pump would start to pump hot CH water round the rads. If we had the heating on a lot and didn’t use much water it got bloody hot! (no tank thermostat)

I found a silver box as described by squirrel.

Chojin – maybe but seems odd to me

Gears – lived here eight years. Never been able to control them separately but until last week we had a very simple timer unit below the boiler, thermostat (basic twist dial in the hall).. The timer failed and was replaced with a more modern one with more functionality. Similarly, the thermostat was upgraded. It was explained to me that the thermostat would turn the CH on independently and the timer would control the water only. I am guessing that having seen the system, the engineer thinks the CH ‘should’ be capable of that.. It doesn’t which lead me to try and find out what is wrong. I am guessing that the fault has always been there..

By asking the question I am hoping to gain a bit of knowledge, no more that that.. I can at least make a suggestion as to what may be wrong to the plumber when he gets here. At least I now know that it possible that my system is designed like that and it may not be faulty. If that is the case, the new 60 thermostat in my hallway was not needed and serves no function, which the plumber should have known…

There is a thermostat on the hot water tank as well..

Thanks for replies

just the way its wired.. no laws broken of either physics or build regs type..

A little more. Info and it becomes much clearer.
It would appear that the plumber who replaced your timer and stat doesn’t really understand what you’ve got, didn’t look properly or simply wired your components incorrectly.
Very often I get calls from people who say something is not working and when I get there, the story unfolds.
It’s notoriously difficult to replace programmers because although standard wiring schematics exist and are very useful, a lot of electricians and installers fail to use them.
Your system is outdated and very inefficient if you are unable to control heating and hot water independently. It would pay to have it modified to do what the plumber said had been done.
Often it takes a little figuring out, but it’s doable if your fitter is competent.
The alternative is to remove everything from the wiring center and redo it using the correct diagram. This is sometimes quicker than trying to figure out what was done in the first place.
Good luck

Thanks Gears. Pretty sure I’ve still got all the wiring diagrams in the ‘house’ folder! I’ll make sure he reads them when he comes back!

i shouldn’t post this for fear of the backlash but

chojin – Member

Ours was designed to run that way according to the British Gas man

never trust the British Gas Man – if you search for British Gas and their “expertise”, you will find many a horror story..

Anyway, with my limited knowledge, I’d go for knackered valve/actuator.