Heating coming on when hot water selected

[brenman]

When the hot water is selected the heating comes on. They have changed the mid position valve, programme, roomstat and cylinder and the problem still exists. Any ideas?

 

[fitz1]

seems to be wiring problem then.

 

[brenman]

When the hot water is selected ,the hot water comes on and so does the heating , i have had two seperate electricians they said the wiring is fine ,could it be sludge build up?

 

[croydoncorgi]

could it be sludge build up?

Definitely, absolutely not!

You're going to have to start from first principles.
- How did the problem first appear?

 

[brenman]

when we bought the flat it was like this .Everything in the airing cupboard has been changed . the boiler is a floor standing thorn , app the previous owners had changed it , the vent used to come from the boiler which has been blanked off and is now teed into the flow before the pump. could it be a one pipe system,running out of ideas ?

 

[fitz1]

just been on the pop so bear with me. other engineers will help understand. if the cyl is not the last tee and the by-pass is fully open it may back flow , ithink. ill think about it tomoz.

 

[Softus]

Sorry fitz1 - I don't quite agree with that one. If the DHW port is closed on the MZV then nothing can backflow through the coil.

I understand your thought about the bypass, but this would only account for backflow if the bypass was fitted in the wrong place, i.e. between coil and MZV, instead of between boiler and MZV.

Contrary to cc's post, I believe it's possible for debris to hold the MZV DHW port partly open. However, I completely agree with cc about starting from first principles.

I don't encounter very many systems that have two faults at the same moment - most have just one. In there was only one fault then it's ludicrous any component was replaced just as part of the fault-finding process.

Notwithstanding that, if all the new components are working, and all the wiring is correct, then the system must be wrongly plumbed.

Personally I'd remove the MZV powerhead and feel the condition of the valve, at the same time testing the powerhead.

 

[fitz1]

if cyl return isnt last tee in at boiler it can use bypass to circulate through rads before reaching boiler. wrongly piped is my guess.

 

[fitz1]

p.s dont forget im ****ed.

 

[meldrew's_mate]

Sorry fitz1 - I don't quite agree with that one. If the DHW port is closed on the MZV then nothing can backflow through the coil....

...Contrary to cc's post, I believe it's possible for debris to hold the MZV DHW port partly open.

I think Softus is refering to the Heating port here, not the DHW port which is normally fully open at this time of year. It's the heating port wich may have debris behind it, though looking at the evidence presented (new valve, twice checked electrically) that would seem remote.

As the problem may have existed before the OP moved in, and there's a history of modifications, it's probable that the previous occupier also had this problem (ask them?). It strongly points to system piping error - the return from the DHW cylinder should be the last connection on the return to the boiler. You may have some pipework mods to do this W/E

 

[brenman]

the cyclinder is the first tee on the return back to the boiler ,after that its the return from the towel rail and then the return from the central heating and then the bypass . initially there were two two port valves and when we changed everything we had a mid position valve fitted .

 

[Bamber gaspipe]

Reverse circulation..

 

[brenman]

thank you for your help what do you recommend to solve the problem ?