Potterton Kingfisher Boiler Continues Running After Nest Thermostat Turns Off

[apna]

Hi all,

I have a Y-plan central heating system with the following:

1. Potterton Kingfisher MF CF 100 boiler
2. Potterton EP2002 programmer
3. Potterton PRT2 room thermostat
4. Honeywell V4073A 3-port mid-position valve
5. Honeywell L641A cylinder thermostat
6. Grundfos UPS 13 pump (connected to the boiler)

The EP2002 programmer developed a fault, so I've replaced it with a 3rd-generation Nest Learning Thermostat and Heat Link.

I've also removed the original room thermostat and replaced both the 3-port valve and cylinder thermostat.

Current Nest Heat Link Wiring​

• L – Permanent live
• N – Neutral
• Terminal 1 – Not connected
• Terminal 2 – Linked to permanent live
• Terminal 3 – Connected to the white wire on the 3-port valve
• Terminal 4 – Connected to the grey wire on the 3-port valve
• Terminal 5 – Linked to permanent live
• Terminal 6 – Connected to common (C) on the cylinder thermostat

Honeywell V4073A 3-Port Valve Wiring​

• White – Connected to terminal 3 on the Nest Heat Link (heating on)
• Grey – Connected to terminal 4 on the Nest Heat Link (hot water off) and terminal 2 on the cylinder thermostat (hot water satisfied)
• Orange – Connected to the boiler switched live and terminal 1 on the cylinder thermostat (hot water call)
• Blue – Neutral
• Green/Yellow – Earth

Cylinder Thermostat Wiring​

• C – Connected to terminal 6 on the Nest Heat Link
• 1 – Connected to the boiler switched live and orange wire on the 3-port valve
• 2 – Connected to the grey wire on the 3-port valve

The Nest thermostat is configured for a system boiler with hot water enabled.

The Problem​

When I switch on the power, the Nest Heat Link powers up normally.

When I call for central heating or hot water, the boiler fires up and both appear to work correctly.

However, when I turn off the heating and hot water demands through the Nest thermostat, the boiler continues running and does not switch off (It's not just the pump running).

I've checked the wiring several times and everything appears to be connected correctly.

With both heating and hot water switched off, I've measured approximately 80V on the switched live/orange wire of the 3-port valve. I'm not sure whether this is a phantom voltage or whether it could be contributing to the boiler continuing to run. This only happens when the demand is switched off.

I've also tried replacing the 3-port valve, but the problem remains.

My Questions​

• Does the Heat Link and cylinder thermostat wiring look correct for this Y-plan setup?
• Could the voltage on the orange wire be causing the boiler to continue running, or is it likely to be a phantom reading?
• Could the boiler switched live be returning the voltage back to the 3 port valve?
• Is there anything else I should check to identify why the boiler isn't switching off when both heating and hot water demands are removed?

Any advice would be appreciated.

Thanks!

 

[ericmark]

My Nest Gen 3 does not change to orange when heating DHW, nor does it record it on the software, so first guess is firing to heat DHW not central heating.

I remember once working with a three port valve, and did get some odd readings.

 

[CountryFan]

Can you post a photo of wiring of Nest and cylinder thermostat?

 

[Razor900]

This used to cause really annoying issues on Worcester boilers. The fix is to put a capacitor across the switch live and neutral which gets rid of the unwanted 80V. Can't remember what value capacitor it was but I imagine Google will bring it up

 

[ericmark]

These capacitors were used with LED lighting and would likely work with a boiler in the same way.

However the three port valve has been a standard method used for years, so correctly designed boilers should not need one.