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SwitchMaster400 and Nest Learning Thermostat v2 with an old Y-Plan valve heating system

Pixil8
Community Member

Ok - so I've just purchased a v2 thermostat - and got it working from eBay - relay is working.

Was disappointed to realise that the v2 only does heating but not Hot Water - but it's not a biggie.

The SM400 had only the Call Heating and Call Hot Water connected.

So I removed the SM400 and wired the Nestv2 ports 2&3 into the backplate Live and Call heating.

The Nest wakes up and I have set it up - and the relay turns on and off as expected.

But when it puts live into the Call Heating - nothing happens (Yes I put the old thermostat on full so it should work).

I also tried shorting the Call Heating to Live on the backplate and still nothing happens.

But when I put the SM400 back - everything works!!!!!!!

I'm wondering whether there's some cleverness in the Y-Plan wiring that is catching me out.

Or something that the SM400 is doing that is unexpected.

1 Recommended Answer

Pixil8
Community Member

Ok - I think I've figured it out.

If the Heating circuit is on, then the Hot Water circuit is on and can't turn off (ignoring thermostats).

The controller doesn't have a HW Off connection which is the problem.

So - I'll be connecting the Nest to the junction box and then connecting the HW On/Off to the Heating On/Off on the SM400 (using the old Heating On cable for HW Off (rewiring the junction).

This makes it easier to remove the old Thermostat and use the cable to deliver +12v power the new nest thermostat.

Phew!

View Recommended Answer in original post

5 REPLIES 5

Pixil8
Community Member

Ok - the first question I need to get to the bottom of is: How does this system work with two controls to the timer - when a normal Y-plan needs three (HW On, HW off and Heating)?

Either I dig deeper and decipher the rats nest of the wiring - or I take the timer apart to see what it's doing.

Neither option fills me with joy.

Then there's the third option - redesign from the ground up - but if I don't understand the current system, then I probably don't know enough to design a new one. 😞

Pixil8
Community Member

Ok - I think I've figured it out.

If the Heating circuit is on, then the Hot Water circuit is on and can't turn off (ignoring thermostats).

The controller doesn't have a HW Off connection which is the problem.

So - I'll be connecting the Nest to the junction box and then connecting the HW On/Off to the Heating On/Off on the SM400 (using the old Heating On cable for HW Off (rewiring the junction).

This makes it easier to remove the old Thermostat and use the cable to deliver +12v power the new nest thermostat.

Phew!

DragosC
Community Specialist
Community Specialist

Hi there @Pixil8,

 

Thanks for reaching out and thanks for the detailed description. Here's the installation guide for the Nest Learning Thermostat. The Y plan is at page 23: https://bit.ly/3nt2N4t 

 

Let me know if you've wired it correctly and if you need more help! 

CristianC
Community Specialist
Community Specialist

Hello again. Just checking in. Do you still need assistance? 

AlexD
Community Specialist
Community Specialist

Hey there, I haven't heard from you in a while so I'll be locking this thread. If you have any new issues, updates or just a discussion topic, feel free to start a new thread in the community. Thanks, AlexD.