by ericmark »
Tue Apr 09, 2019 2:16 pm
It seems as if the link in Nest is missing, however Nest can be connected as OpenTherm but Bosch where there is access to boiler bus as far as I am aware has to use Wave as the boilers are not OpenTherm enabled.
Nest can be connected to boiler bus, or to 230 volt or to any other voltage i.e. 24 volt, there are loads of options.
I seem to remember the junction box in the Bosch is in a gas tight area, so I think to work on boiler side you have to be gas safe registered, personally think it is daft to put terminals in a sealed part of boiler.
However as far as I understand the Wave thermostat requires a large lump to be swapped in the boiler.
May be I should explain, years ago boilers and thermostats were digital i.e. either off or on, however a new boiler was designed which extracted more heat from gas called condensing, for this to work, the return water needs to be cool enough, so the boiler is designed to modulate, that is turn up and down, so it's output is between say 7 kW and 28 kW and originally this was controlled with the return water, and instead of wall thermostats we used thermostatic radiator valves (TRV) and the boiler sensed the temperature of the return water and turned up or down as required.
However once at 7 kW it could go no lower, so then started to switch off/on to control, and even in middle of summer it would continue to switch off/on. So we added a wall thermostat, often in the hall, to completely turn off boiler in the summer.
However as we progressed the idea of turning the boiler up/down electrically came in, so we swap the TRV heads with Nest to Energenie MiHome and these are either controlled by Nest or Nest controls the valve depends how set, then Nest with the OT1 and OT2 terminals tells boilers with OpenTherm to turn up/down.
With some the so called thermostat was really a hub reading the temperature in every room so each room can be a different temperature, the likes of Tado and Evo Home can be very complex and cleaver the way they control each room.
However as far as I am aware Bosch never signed into this system, they produced their own called Wave, however it only controls one room, so you have to adjust the lock shield on radiator to ensure that is last room to get hot, which seems daft as likely you went best control in your main room, so actually want that room to get warm first.
And it seems all the cheaper systems have some draw back, even with Nest and MiHome you can set the TRV head to be same as wall thermostat, but idea is you can have kitchen cooler than living room so you still end up with some fiddling to get each room the temperature required.
If you use off/on there is always some over shoot, or hysteresis as it's called, so thermostats started to include anti-hysteresis software, this turned boiler off/on before and after it gets to correct temperature altering as it is called the mark/space ratio. However when you turn the boiler off, any heat is sent out of the flue, so one you want the boiler as cool as possible before turning off, and two turn it off only when already at lowest output so switching off/on wastes energy, and can defeat the whole idea of condensing boilers.
However how much it saves or looses is unclear, the Energenie MiHome TRV heads and Nest can be set using GeoFencing i.e. turns off when is senses your phone has left the house and on again when it is within a set distance. However when I turned whole central heating off over night in heart of winter, in morning still at 17 deg C 8 hours latter, so with such a well insulated house is it worth it?
With oil central heating the boiler goes between 18 kW and 22 kW output and one has to ask was it really worth it? Yes ticks box as condensing boiler, but unlikely to really save much.
So do you just get extra wire added and boiler resealed, or do you install Wave instead, or simply use electronic TRV heads? If it was me, I would check boiler is 230 volt control, and if so just add the link wire.
