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Nest 3rd Gen and Removing old Analogue Wall Thermostat

Postby Jcagreen » Sun Dec 03, 2017 9:31 pm

We have a condenser boiler and have installed the nest 3rd generation thermostat which is working well.
We still have the old analgogue dial thermostat on the wall that has to be turned up to give the nest control.
If we want to remove it what is the best course of action? Find the thermostat wires from the boiler and remove them?
Is there any way we can use the wires to power the nest and mount it on the wall in its place e.g use them as the 12v supply?
Many thanks!
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Simply Build It

Postby ericmark » Mon Dec 04, 2017 10:29 am

Most old central heating systems have a junction box, in theory it should follow the honeywell plan, in practice often it is a load of connectors in a double socket back box with a blanking plate.

If it followed the Honeywell plan then likely terminals 4 and 5 are the thermostat, but it would need very careful following of the wires to confirm this.

Since with a condenser boiler we should be connecting the thermostat to the eBUS (OT1 and OT2 on nest) the thermostat connections at the boiler would be likely the best place to alter wiring, reconnect the link across the thermostat, however that will only work if the boiler is OpenTherm and you are using the OT1 and OT2 terminals.

It would be nice if every boiler used OpenTherm, however that is not the case, my mothers boiler is a Bosch so uses a Wave thermostat not Nest, there are a whole host of different ones, depending on boiler make, in an ideal world we want the TRV head to talk to the wall thermostat nest uses IFTTT to talk to the Energenie MiHome range of TRV heads.

I had intended to install nest before I found out my mothers boiler uses Wave, so I have had to abandon the idea of a eBUS connected thermostat and use the return water temperature to control the boiler instead, in fact some boilers don't have an option, there is no eBUS connection, you have to use the TRVs to control the boiler.

If the moderator will allow it this link explains modulation it is USA but it is also one of the best I have found to explain how it works.

I have found the cleaver thermostat which has anti hysteresis software is a disadvantage when using smart (smart means connects to phone, nothing to do with being smart) TRV heads which also have anti hysteresis software, the two combined means turn up the heat from 16 to 20 degs C and it takes 5 hours to reach the new temperature, by cheating and asking for 24 degs C it will heat up in 2 hours, and if the wall thermostat did not have anti hysteresis software likely would heat up in an hour.

In hind sight, and hind sight is easy, I should have bit the bullet and fitted EvoHome, but I tried to save money.
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