3 port / mid division

stidge · Aug 28, 2014

Hi, Can someone please explain to me why on a Y system I have just altered( for pump overrun on new boiler). Why does this system have to have water on for the heating to work ?. The 3 port valve through WHITE cable will only feed ORANGE cable to fire boiler when the controller asks for water as well.??????? can the valve be changed so we can control independently ???

davesparks · Aug 28, 2014

A Y plan system does not require hot water to be on for heating to work.
Are you sure it is a Y plan system?
It could be that the programmer is set to gravity mode, have a look on the back of it to set P or G with a switch or jumper.
A gravity system requires the hot water to be on for heating to work so the programmer will operate that way if set to gravity mode.

stidge · Aug 28, 2014

Thanks davesparks, apparently the system has Allways worked like this ! The grey isn't connected, am I right in thinking perm L to grey, then white and orange flick the microswitch between them ? I checked back of controller and no dip switch! What system is it called where it needs water to be on for heating?

davesparks · Aug 28, 2014

No, grey is DHW off on a 3 port mid position valve, white is CH on and orange is boiler fire.

Are you sure it is a Y plan and that it is a 3 port mid position valve not a 3 port diverter valve?
It could be a priority system which uses a diverter valve to heat the hot water first and then switches to heating once the hot water is up to temperature.

stidge · Aug 28, 2014

That sounds good dave, how will I know if it's a diverter valve or a 3port ? If it is a diverter can the plumber change to a 3 port ? Your advice is greatly appreciated cheers

davesparks · Aug 28, 2014

If the label is still legible then that will tell you, but failing that just observing exactly how the system works will do it.

I think diverters only have 3 core cable, but have seen it many times where a plumber has just changed one for a mid position valve without realising the difference and just bunged the wires in anywhere!

A mid position valve can be connected such that it works the same as a diverter though by joining grey and white together.

Electrically it is easy to convert to a Y plan, but I can't say how it affects the hydraulic side.