Radiators on when hot water is on

[Sester70]

I have a gravity central heating system. Just moved in to house. Rafa were on when hot water came on. So I replaced the Honey Well power valve as I tried just the motor and still had the issue. Old 3 valve looked very worn.
I tried to refill the system from header tank but it would not completely refill, rads upstairs were not filling up but downstairs did. So back filled it from downstairs rad, drain valve. Bleed all rads upstairs and downstairs, water started going back in header tank so I guessed it was full. Then I found a split in copper pipe coming off header tank, so guessed that was why it was not filling up. Replace copper pipe. Started system back up and it did sound like it had a fair bit of air in system.

The strange thing now is rads are still getting hot when hot water is on, but what's really strange is the pipe off the powerhead valve to radiators doesn't get warm until it's been running for a while but rads get very hot instantly, like the hot water is finding another way to the rads! How can this be possible?

System is running pretty quietly at the moment but still rads coming on. Could I have a blockage somewhere, I can't figure it out. I ran out of time so need to go back to it this weekend. Thinking of draining the system again and letting header tank refill it?

It's just so strange how the rads get red hot when only water should be on and the pipe out of valve to rads doesn't get hot, just warm.

I check the valve operated correctly with my wiring and it all look correct, hot water, central heating or both open. Motor had exactly same colour wires and same model so I know that's all correct.

Would really appreciate any advice and if anyone has seen this before?

Many thanks
Simon

 

[terryplumb]

So you have a 3 port motorised valve ,is the brass body and actuator head all brand new ?
If so ,and the pipe from the valve to radiators isn't hot,then there must be reverse circulation taking place through the radiators

 

[Sester70]

Yes there must be but how would that happen?
Motor and valve all brand new.

 

[terryplumb]

Pipework issue,most likely always been that way.

 

[Sester70]

That was my worry but seems to be worse now

 

[terryplumb]

I can only guess that the previous owner turned off all the radiators by their TRV ( or wheeled valves if no TRV) during summer.
If you run only central heating on the programmer,does the hot water cylinder also heat up ?

 

[Sester70]

I have not tried that yet. But will give it a go. Thanks for you help on this. I'm sure it seems worse since I changed valve.

 

[dal5band]

Also check which end of the radiators get hot first - is it the same as when the CH is on or the opposite end. If opposite then it's prob reverse flow.

 

[Sester70]

Great idea, thanks

 

[Alec_t]

Are you sure the 3-port valve is not mechanically latched open?

 

[Sester70]

Yes, I watched it move to all 3 positions before I plumbed it in, but had wired the motor in. I did this as a test before I even removed the old valve. Plus pipe that's rad side coming off valve just doesn't get hot, but rads a very hot!

Just had all new carpet put down, don't want to have all pipe with changed

Thanks for checking.

 

[Sester70]

Just checked upstairs as hot water was on, I have one flowing left to right and two flowing right to left. Can't get to third to turn on and check.

 

[ericmark]

I am an electrician, not a plumber, so may have things wrong.

I can't really see how heating comes on, even if the three port valve is faulty, as the TRVs should be closed. But suppose there must be a limit to the pressure the valves can handle? So would be looking at the settings of the by-pass valve and the pump speed.

Not sure why a three port valve on a gravity system, my own system is gravity, and it does have motorised valves, but these do not switch between central heating and domestic hot water, but turns sections of the heating on/off. Without the motorised valves, the flow reverses, maybe it would have been better with non-return valves, but I left the plumbing to the plumbers, and I only designed the electrics to suit what they fitted.