3 port valve

truffle pig · Nov 21, 2024

Hi, got an issue with a Y plan system. after the heating has stopped calling for heat I am getting around 30v on the CFH at the boiler - its coming from the orange on the zone valve and also back feeding the white (CH on) with the same voltage. it only does it with the HW off (so 230v down the grey from the programmer. the plumber has replaced the programmer & room stat before calling me ( all wired correctly) and I have replace the zone valve head - no difference. The programmer / tank stat and room stat all appear to be switching correctly, and there is no voltage present on the neutral. anybody else come across this? Back there tomorrow to test wiring etc

ProDave · Nov 21, 2024

It sounds like the boiler "call for heat" input is way too sensitive and being triggered by a low stray voltage. I would start by connecting a snubber across the call for heat input

truffle pig · Nov 21, 2024

ProDave said:
It sounds like the boiler "call for heat" input is way too sensitive and being triggered by a low stray voltage. I would start by connecting a snubber across the call for heat input

Thanks Dave, I will give that a go. the system has been working fine for the last 8 years, not sure why it should change now

SPECIAL LOCATION · Nov 21, 2024

Have you split the orange from the valve, and the "water-on" feed from the cylinder stat, (at wherever they join before feeding the boiler CFH), to confirm 100% it is coming from the Valve orange, and not the Cylinder stat water-on, (whatever colour it is?)..

Remember..
"Water On / Heat Off" is the default position where the 3-port valve doesn't need to move.
(held in the home position via the return spring, no trigger voltages required, So the boiler CFH bypasses the valve).

"Water On & Heating On" has to stall the 3-port valve in the mid-position.
(only one voltage trigger from the heating thermostat, out to CFH via the valve orange).

"Heating On / Water Off" has to drive to motor to the furthest position.
(two voltage triggers from heating ON and water OFF, out to CFH via the valve orange).

How long is the cable run run from cylinder to valve/control centre box? Could this be a source of phantom voltages?

Whenever I encounter "strange" happenings with heating operation, I normally start splitting off bits of the control wiring, to ensure I'm not chasing my own tail up a wrong path.

I also sometimes use a bog-standard pendant bayonet lamp holder plus flex, with a GLS lamp wago'd across key points to verify the expected voltages are where they should be... As it can save a load of faff sticking meter probes in various screw terminal connectors trying to work out what's happening.

I presume this is the same physical boiler for the whole 8 years?

truffle pig · Nov 22, 2024

Thanks special locations, that's very helpful. I did disconnect the tank stat & did an IR across the terminals and was ok but am going to investigate that a bit more this morning. Also disconnecting the LR on the boiler showed it was coming from the wiring centre. it only appears to be present when the "water off " from the programmer (grey wire) is powered. I have noticed the pump is fed separately - gonna investigate that and try connecting that into the CFH to see if that helps

Yep, same boiler - the system hasn't changed unit this fault appeared .

Tempted to just get it changed to a 's' plan - much easier!!

Nicky Tesla · Dec 5, 2024

Back feed voltage explained here at around 16:40.

truffle pig · Dec 6, 2024

ended up putting a relay in the grey ("hot water off") cable activated by the "heating on" cable. Solved the back feed to the boiler and also means when not in use the 3 port valve goes back to its rest position.