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Nest Thermostat and Worcester Bosch Heatslave 15/19 ?

Postby Stferr » Thu Nov 07, 2019 3:14 pm

Hi Folks, Just wondering if anyone knows whether or not a Nest Thermostat would work with my old Worcester Bosch Heatslave 15/19 oil boiler? Any advice or alternatives would be much appreciated!
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Postby ericmark » Fri Nov 08, 2019 10:02 pm

I think my boiler is a heat slave, but not sure, it is Worcester Bosch, and I am using Nest.

With Nest the relay contacts in the heat slave can be wired any way you want, they are volt free, and the com is not internally connected, so you can wire it as you want.

The C plan has altered through the years, many oil boilers a plumbed with no tank thermostat and no motorised valve to control DHW since the contacts are independent it can be wired line in to N/O contact, and line from CH thermostat to the N/C contact, and Com feeds the boiler. Very few controllers or thermostats will allow wiring this way around.

So if the hot water time calls for water the boiler is fed direct from hot water relay, and if hot water not timed, then the pump feed also feeds boiler. So Nest is one of very few that can allow the old C plan to work.

Instructions only give Y plan and S plan, but C plan can be used, as can W plan. So I can't think of any boiler that Nest will not work with.

Standard C plan has three versions, first had no tank thermostat, or motorised valve, and the programmer turned to pump on if central heating wanted, and did not turn on pump if only domestic hot water required.

In the summer domestic hot water was purely controlled by time.

Next a tank thermostat was added, this did nothing in winter, but in summer it means boiler only fires up when water has cooled.

And last added a motorised valve to the system, this means in winter you can limit the temperature of the domestic hot water to less than central heating hot water.

Nest will work with all three versions, but there is no wiring diagram in the instructions. My system is the old one where in summer DHW is purely controlled by time. The old programmer was set up so it has 6 options.
1) Off
2) On
3) DHW once a day
4) DHW twice a day
5) CH and DHW once a day
6) CH and DHW twice a day
The clock set how long it ran for.
With Nest I have ability to turn off/on DHW for a set time in 1/2 hour increments 7 days in the week, I have it set in summer to 1/2 hour every other day, and an hour on Monday to kill legionnaires.

In winter I turn off DHW which can't really be turned off, it heats up when ever central heating runs. And I set the hall temperature to 20 deg C in the day and 17 deg C at night, but in morning it lifts the temperature by 0.5 deg C every hour not all in one change, this reduces the hysteresis and the TRV in every room control room temperature spot on. All house TRV heads are programmable electronic heads.

I have one problem, in theory the Energenie MiHome electronic TRV head should pair with Nest so all change together, I found it was intermittent, some times the TRV heads would follow Nest, other times they remained where they were, as soon as I opened app on phone they would follow, but since they did not follow all of the time, I have set them to the same schedule as Nest, but not following Nest, so I could have used the much cheaper eQ-3 electronic heads down stairs as well as up stairs.

But to answer your question yes it works, in my case also a flat below the house, so I have motorised valves on the central heating so the flat and house are independent controlled from same boiler.

The plumber and oil central heating guy could do the pipe work and fit valves, but I had to design the wiring, which because splitting house and flat includes two relays and a second thermostat. Well at moment relay pulled. So flat central heating only works when house has fired up boiler, picking up second relay this weekend.

Your welcome to my wiring diagram.
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