Modified Y plan for unvented cylinder

grumpyjohn01 · Oct 4, 2023

I had to wire in an extra 2 port valve today to protect a newly installed unvented cylinder (replacing the old gravity fed one) in a standard Y plan setup. However the wiring centre adjacent to this tank didnt have a permanent live available as the programmer and boiler with supply spur are remote and it wouldn't have been needed in the old setup. I connected the brown and grey together off the tank stat call and it all works as expected. Can anyone think of a reason for not doing this? It seemed like it would work in all circumstances so figured it was ok. Getting a P LIve there wouldn't be disastrously difficult so will do this if nesessary..Thanks..

grumpyjohn01 · Oct 4, 2023

just for reference here is the modified diagram courtesy of JW, obvs as stated with boiler, spur & prog remote from wiring centre, 6 cores between positions with 1 core taken up by pump live from heat only boiler.

davesparks · Oct 4, 2023

grumpyjohn01 said:
I connected the brown and grey together off the tank stat call and it all works as expected.

If the tank stat switches off whilst the heating is running the boiler switched live will backfeed to the two port valve motor and effectively latch it open.

grumpyjohn01 said:
Can anyone think of a reason for not doing this?

Yes, when the backfeed occurs it prevents the 2 port valve from closing and thus preventing this safety device from working.

If you connect the live to the valve switch (grey) to the hot water on signal from the programmer (3) then the cylinder stat will break that feedback loop and prevent the dangerous situation from occurring.

Pretty Mouth · Oct 4, 2023

Just thinking this through...

When the cylinder thermostat calls for heat, the grey and brown wires of the 2-port valve go live. The valve opens, the internal switch connects the grey and orange wires together causing the orange to go live, which then sends a live to the boiler and pump to start up.

When the thermostat stops calling for heat, brown, grey, and orange are no longer live, so no signal to the boiler + pump. The valve closes, and disconnects the grey from orange.

All good so far. However...

If, when the cylinder thermostat stopped calling for heat, the boiler was receiving a live from another operation, eg. central heating, this would backfeed through the orange wire, internal switch, grey, and brown. I think this would keep the valve open until the other operation stopped signalling the boiler.

Edit, @davesparks beat me to it

grumpyjohn01 · Oct 4, 2023

davesparks said:
If the tank stat switches off whilst the heating is running the boiler switched live will backfeed to the two port valve motor and effectively latch it open.

Yes, when the backfeed occurs it prevents the 2 port valve from closing and thus preventing this safety device from working.

If you connect the live to the valve switch (grey) to the hot water on signal from the programmer (3) then the cylinder stat will break that feedback loop and prevent the dangerous situation from occurring.

Thanks much appreciated, I had missed that...will rectify as you suggest tomorrow !!

grumpyjohn01 · Oct 4, 2023

Pretty Mouth said:
Just thinking this through...

When the cylinder thermostat calls for heat, the grey and brown wires of the 2-port valve go live. The valve opens, the internal switch connects the grey and orange wires together causing the orange to go live, which then sends a live to the boiler and pump to start up.

When the thermostat stops calling for heat, brown, grey, and orange are no longer live, so no signal to the boiler + pump. The valve closes, and disconnects the grey from orange.

All good so far. However...

If, when the cylinder thermostat stopped calling for heat, the boiler was receiving a live from another operation, eg. central heating, this would backfeed through the orange wire, internal switch, grey, and brown. I think this would keep the valve open until the other operation stopped signalling the boiler.

Edit, @davesparks beat me to it

Thanks for out thinking me, should have spotted this...end of day brain fog is all I can blame..!

Last plumber · Oct 12, 2023

What do the ports A & B on the 3 port feed?

Last plumber · Oct 13, 2023

grumpyjohn01 said:
Thanks for out thinking me, should have spotted this...end of day brain fog is all I can blame..!

What do the ports A & B on the 3 port feed?

davesparks · Oct 13, 2023

Last plumber said:
What do the ports A & B on the 3 port feed?

One feeds heating and the other the hot water cylinder, I can't remember which is which though.

The two port valve under discussion is installed after the three port valve as a positive shutoff in the case of an overheat situation.

Last plumber · Oct 13, 2023

davesparks said:
One feeds heating and the other the hot water cylinder, I can't remember which is which though.

The two port valve under discussion is installed after the three port valve as a positive shutoff in the case of an overheat situation.

I see. I was thinking something was no longer fully controlled temperature wise but I suspect you know what I was thinking!

grumpyjohn01 · Oct 13, 2023

Thanks to everyone for contributions, this is now resolved after following advice from Davesparks.

davesparks · Oct 13, 2023

Last plumber said:
I see. I was thinking something was no longer fully controlled temperature wise but I suspect you know what I was thinking!

Nope, no idea what you were thinking.

Last plumber · Oct 13, 2023

davesparks said:
Nope, no idea what you were thinking.

I was thinking that when the new two port opens and turns boiler and pump on, the normally open port (priority) on the three port is uncontrolled.

Modified Y plan for unvented cylinder

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