Nest 3rd Generation and Installing Heatlink With a Condensing Boiler

Postby Jcagreen » Sun Nov 12, 2017 10:19 am


We have bought a nest thermostat and are attempting to install. Our boiler is a Vaillant condensing boiler. We currently have a UP1 programmer (wiring attached).

The wiring seems relative straight forward (live, neutral, call for heat (into 3) and call for hot water water (into 6)) except I believe you need to install a relay link between the live (2) and call for heat and call for hot water? Is this right? In some wire diagrams there are two relays and others only one I’m confused whether we need one or two.

My Dad is helping with the install and is a lot more electrically minded than me so apologies if any of the terms above are wrong. But he’s a bit confused by the relay so I’m troubleshooting for him.



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Postby Mr White » Sun Nov 12, 2017 12:55 pm

The number of relays depends on how many the Nest stat you have purchased has, you can't just add another relay.
A wiring digram would also help.
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Postby ericmark » Sun Nov 12, 2017 3:35 pm

Step one is read boiler instructions so Boilers like Bosch Worcester have special thermostats and can't be used with Hive, Not sure on Nest, but it all depends how the thermostat tells the boiler what to do.

Pre condenser boiler the boiler runs at a set output, So any timer, programmer, thermostat or switch simply turn the boiler on/off, and near every thermostat can work.

When the condenser boiler came out, then the whole method of control changed, for the first time the boilers had a variable output, within a range, so for example 10 ~ 28 kW and with early versions the only way to control the boilers output was by varying the temperature of the return water. So in real terms the thermostat is the thermostatic radiator valve (TRV) there are some cleaver systems which take this a step further and the "Thermostat" actually is a hub which corrects info from electronic heads on the TRV and can control each room independent and also tell the boiler when all rooms are satisfied, this is what EvoHome does.

But unless that is done the hysteresis can become silly for a simple wall thermostat so instead of the wall thermostat switching boiler on/off a new system has been developed where the thermostat connect to the boiler buss and tells the boiler what output is required, so boiler output goes up/down as it does with TRV's not on/off. One system is called OpenTherm and Nest will work with OpenTherm, however not all manufacturers have adopted the standard and some use their own system, this is what Bosch Worcester did so you have to use their special thermostat.

In the main the Nest replaces the programmer with pre-condenser boilers, and there are two extra connections for boilers with OpenTherm. There are also another extra pair which can be used to power the thermostat sensor,

Nest will work with Energenie TRV heads using IFTTT with follow commands, however that means every room at the same temperature so not really a very good design. I think Hive is the same.

There are others like Wiser and Tado each seems to have different ideas it seems likely Tado will work similar to EvoHome but the info is a bit poor.

When I looked into doing mothers house it seemed best option was Energenie heads which allow you to alter temperature using your phone or PC and if later you want you could add Nest. However I found the electronic heads have anti-hysteresis software, and if the thermostat also has anti-hysteresis software then when you try to raise temperature to a new figure for example to get house warm for getting home, then they fight each other, so actually better if the thermostat does not have the software.

In fact I have to cheat, at 6 am I set eTRV to 24 degs then at 8 am back down to 20 degs to warm house up in the morning, without that boost it would be 10 am before house got to 20 degs. So the whole idea of the system monitoring the smart phone and turning up the heat automatic as you start travelling home does not work, as it takes too long the heat the house with the anti hysteresis software, so may as well use simple timers, and if using simple timers not the smart phone GPS then may as well save a lot of money and fit stand alone eTRV at around £20 each. And also use a cheap programmable thermostat which does not have anti hysteresis software built in. (Not wireless as wireless cheap is not reliable)

If I was starting again either whole hog and fit EvoHome or super cheap and use the Terrier stand alone i30 heads and a simple thermostat, the half hearted Nest or Hive are not really worth it, they are neither good smart thermostats non cheap thermostats with no anti-hysteresis so learn from my error either get the proper EvoHome or get super cheap. Half way is really no good.

Yes Nest does have OpenTherm so will work with boilers with OpenTherm and it does have IFTTT but for IFTTT you need internet which never fails as no internet means no heating if you are using IFTTT.
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