NEST v3 thermostat and Heatlink with Honeywell ST6400C

27 Jan 2018
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United Kingdom
Ive just installed a nest v3 Thermostat to control heating and hot water on my existing system which is
Potterton Suprima 100L Boiler
Honey well ST6400C Programmer
Honeywell v4073A mid position diverter valve

The old system didn't have a main thermostat , just TRV's on every radiator.

I've installed the heat link as follows

Nest 1 CH OFF to Programmer 2
Nest 2. linked to Live
Nest 3 , CH ON to programmer 4
Nest 4 HW Off to prgrammer 1
Nest 5 linked to live
Nest 6 HW on to Programmer 3

Everything works fine, as long as the programmer is not plugged in, with both heating and water going on and off as expected.
The nest site says you can leave the programmer connected as long as it is set to continuosly ON.
However if i do this, setting both water and heating to ON on the programmer, even with NEST not calling for heating or water (Thermostat lower than room temp and heating off ), the boiler fires up and the heating comes on.
I did try setting the programmer with both heating and water set to off but in this case if the nest calls for hot water the boiler fires up but the diverter doesn't switch so the hot water goes to the radiators.
I can work quite happily with the programmer not plugged in, but if that's the situation I'd have to build a box to cover the exposed terminals on the back plate of the programmer, hence I'd rather have it plugged in.
Any suggestions ?
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Ok, I've spent time drawing out the circuit diagrams and I've answered my own question. The NEST is never going to work with an ST6400 also in parallel circuit.
If, as suggested by NEST, the ST6400 is set to water continuously ON, Then that puts power on pin 3 of the programmer, which is connected to diverter Orange and Boiler ON, even if the nest isn't calling for boiler on. So the boiler fires up.
On the other hand if ST6400 is set to water OFF that puts power to pin 1 of the programmer which puts power to grey on diverter, water off. So even when the nest calls for boiler on, the diverter will never send that hot water to the water circuit.
So the ST6400 HAS to be disconnected and can't be left in circuit
In my case I opened up the 6400 and removed the water relay as that seemed easier than building a box to cover the ST6400 backing plate with it's live terminals. Then I can leave the ST6400 in place with everything OFF.
Problem solved.

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