Nest for underfloor heating, only one zone

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Hey guys,

Not long moved into my house and have been getting used to the central heating system.

I bought a couple of nests (multiple zones) I've installed one of them but I'm just doubting myself a bit on the second install

The heating system was like this

Downstairs - Underfloor heating in 3 zones (4 actuators)
Controlled by 3 thermostats (manual honeywell t6360b run from a single programmer ST9100A)

Upstairs - 4 Radiators (no thermostat but run off of a honeywell ST6400C)
Hot water - From the same honeywell ST6400C

I replaced the upstairs and hot water with a nest 3rd gen, easy enough. added the nest thermostat to a single room upstairs (all that i need) and removed the st6400c, that's all good.

Now to the problem

the underfloor heating downstairs i only want to replace one of the zones with the nest (maybe i will replace the other two zones at a later date with the cheaper nest-e thermostats when they're out in the uk) this means it has to work in conjunction with the current setup.

BIgv6D2.jpg


The zone i wish to change is marked 3 in the actuator control box and has the two actuators it needs connected beneath.

If i leave the honeywell st9100A on "always run"
can i remove the current thermostat wiring from zone 3 (I'll then use two of these for the 12v on the nest thermostat in it's current wired position)

can i then connect the live and neutral of the heatlink to that zone in the box and instead of the switched yellow do i put the heatlink's call for heat? this is the bit i'm second guessing about

this would leave me with a system controlled by the honeywell st9100A (set to always on) and the thermostat for zone 3 would be controlled by the nest

the other two zones would then be always on (probably set these to a lower temperature as the rooms aren't used much.)

Did i get anything wrong, or any problems i may not have forseen?


--- EDIT

just for anyone who may find the solution useful.
It was all fine, I installed the nest heatlinks into the control board zones and used the existing thermostat wires to run to t1 and t2 of the nest.

the only issue as predicted is the programmer needs to be set to run 100% of the time or the heatlink turns off, this was expected.
 
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Also on a side note

there's a pressure reducer in my system and the cartridge is leaking a little bit

https://imgur.com/a/F8kkq

I'm assuming I wont be doing this one myself.

I found a replacement cartridge for £69, how much is a good price for someone qualified to fit it and is it even worth changing it? seems to have been leaking for a long time.
 
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