Tuesday 16 October 2018

UK - Dumb Boilers vs Smart Thermostats

It is increasingly common these days for home owners to buy a 'smart' thermostat to control their central heating. Indeed arguably smart thermostats are the number one category of smart home device. The leading member of this category is of course the Nest Learning Thermostat. (Now version 3.)

Originally such smart thermostats whilst indeed having various additional smartness actually worked in the same way as original dumb thermostats in that they basically sent a signal to the boiler asking for heat or saying stop I am warm enough, i.e. a basic on or off control. This approach involves the boiler either running at 100% power or 0% power i.e. fully on or fully off.

However newer models of smart thermostat including the aforementioned Nest Learning Thermostat v3 also support an alternative approach which allows setting a target temperature for the boiler so that the boiler can adjust the level it needs to run at to keep at that target temperature. This means that instead of constantly starting and stopping the boiler it will run continuously at a lower power level to keep the temperature more even. This can create additional energy savings on top of more efficient schedules and might add an additional 5% savings. This approach is referred to as modulating control.

Figure 1 - Traditional on/off control


Figure 2 - Modulating control


As you can see from these diagrams with traditional on/off control the boiler runs at 100% until it reaches the desired temperature and then turns off, it however will overshoot the desired temperature as the heat is released by your radiators, it will then undershoot as the radiators cool down whilst waiting for the boiler to heat them up again. With a modulating control the amount of power (heat) the boiler produces is reduced as it approaches the desired temperature meaning it does not overshoot and instead reduces power to the level needed to keep it at that level.

Now in order to benefit from this more efficient modulating control you need both a smart thermostat that supports this feature and a boiler that also supports this feature. As mentioned the Nest Learning Thermostat v3 supports this, as do various Honeywell Evohome smart thermostats and so does the Tado Thermostat. There is an official open standard called OpenTherm which was original devised by Honeywell and later released as an open standard. This OpenTherm standard is supported by the Nest v3, Evohome and Tado amongst others. Even Drayton offer an OpenTherm compatible thermostat. There seems to be also another alternative standard generally referred to as eBus aka energy Bus, however only Tado support this as well as OpenTherm. (It is not supported by Nest or Evohome.)

Unfortunately here in the UK many of the various boiler manufacturers are proving very unhelpful. Most do now provide at least some boiler models that support modulating control as well as the traditional on/off control but only support modulating control with their own proprietary thermostats. Whilst they do not say so it seems their proprietary thermostats are using the eBus standard. As such this precludes using the Nest etc. in modulating mode although the Tado would still work.

What is even more annoying is that Vaillant a leading brand actually sell their boiler with OpenTherm support in the Netherlands, they do this by selling their own eBus to OpenTherm bridge module - VR33 to convert their eBus signals to OpenTherm signals. However Vaillant do not sell this module in the UK and if you get one and have it fitted even by an official Vaillant engineer they will invalidate the warranty on your entire Vaillant system. Remember this is an official Vaillant part and one that does work on UK boilers.

Worcester-Bosch are a little better, they have their own proprietary variation on eBus called EMS. Bosch own several brands throughout Europe, Worcester-Bosch in the UK, Nefit in the Netherlands, Junkers in I believe Portugal and of course Bosch in Germany. Since OpenTherm is very common in the Netherlands Nefit have produced a module to convert Bosch's EMS to OpenTherm. See - this. However there is also another interesting possibility. Worcester-Bosch also sell an adapter to allow connecting their EasyControl smart thermostat which speaks only their proprietary EMS protocol to OpenTherm boilers, it should also work with their older Wave smart thermostat. This adapter however is described as bi-directional which might mean it can also do the reverse and allow an OpenTherm smart thermostat to connect to a Worcester-Bosch EMS boiler. The Nefit module is not sold in the UK but the Worcester-Bosch adapter is officially available.

Both OpenTherm and eBus have additional benefits, they can provide error diagnostics to your smart thermostat so that you can be far better informed of either a potential problem or an actual fault, they also allow a smart thermostat to not only control the central heating but also to control your hot water scheduling as well. I have not seen anything official but Tado at least suggest that the eBus standard is technically superior to the much older OpenTherm standard. I also get the impression eBus maybe a purely European standard at this point - hence the fact Nest and Honeywell aka Evohome do not support it.

To summarise -

  • UK boiler manufacturers try and lock you in to their own proprietary 'smart' thermostat
  • Most UK boilers do not support OpenTherm and do not say they support eBus (but in reality many do)
  • Vaillant who do at least in the Netherlands support OpenTherm are deliberately refusing to do this in the UK and even go as far as punishing anyone who gets their own OpenTherm bridge module

So either you have to run your boiler in old fashioned dumber on/off mode, or get the Tado Thermostat or accept being locked in to the boiler manufacturers own proprietary 'smart' thermostat.

Note: If your Vaillant boiler is out of warranty you could consider using that VR33 module.

A list of potentially OpenTherm compatible boilers is available here.

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