Hot water coming on when ch on

[M4NNY]

Hi, it seems as though my HW tank is being warmed up when CH is on. I had a new unvented system installed back in January and all seemed to be working fine until last week when we switched the heating on for the first time since March.

My setup is a Y-Plan with an additional 2 port valve near the HW tank, I believe this was installed as a safety measure? Both 3 port and 2 port valves seem to be working and are switching on/off when needed. But I’m not sure how to test this.

So,
HW only: 3 port switches to W & 2 port opens (I assume it’s open as the manual lever is free to move).
CH only: 3 port switches to H, 2 port springs back if HW was on.
HW & CH: 3 port switches to M.
Whenever HW is turned off 2 port springs back.

When only the CH is on, the flow pipe to the HW tank is hot, therefore I assume heat is going into the tank warming the water? Howeverthere is a 2 port valve between the HW tank and 3 port valve which as far as I know is closed so how or why would the flow pipe into the HW tank be hot??

As a test I have today turned off the HW and have only the CH running so will see what happens. I would really appreciate if anyone could offer up some advice as to what the cause could be and what I could do to fix it.

Also, my system is controlled by 3rd Gen Nest.

I’ve attached a simple drawing of what I mean regarding the valve positions.

Many thanks

 

[Chuck]

It seems unlikely that a 3-port valve and and a 2-port valve would both fail independently. I suppose it's possible that the 3-port valve has been installed with the DHW and CH outlets switched, either due to a plumbing or wiring error, and the 2-port valve is passing, possibly because it was installed backwards. That's a lot of mistakes for one installation, however.

Another, more likely possibillity is that thermal conduction along the copper pipework is heating the section from the 3-port to the cylinder. This could be from the CH pipes or or the HW cylinder itself it it is hot. If you want to test this hypothesis, soak a face-flannel in cold water and wrap it round the pipe, for a minute or so. If the pipe is cooled and stays cooled for a few minutes then there is unlikely to be any flow inside it and all is well. If it can't be cooled or returns to 'ouch' temperature immediately the cold cloth is removed it would seem that the wiring / plumbing need investigating.

Maybe some photographs of what you actually have would help us figure out what's going on. Diagrams only show what the artist thinks is going on...

 

[M4NNY]

Thanks for your reply Chuck.
I’ve attached some images of the pipe work for you to see.

So the CH was on over the weekend and HW off all weekend. Good news is that HW was running cold on Sunday and flow pipe was no longer boiling hot, so CH isn’t warming up the tank.

I have since turned both HW and CH on. HW was on for 45mins today from 3.30pm and have just tried your soaked flannel theory out and it does cool the flow pipe down, it then gets hot again, really hot but the pipe work between the 3 port and 2 port valve is barely lukewarm so I think you’re right in saying it’s thermal conduction from the HW tank. The return pipe is cool.