Retro install Thermostat (or Hive/Nest or similar) to viessman 200 without weather compensation

[rizsher]

Hello,

Returning to the UK after 8 years with very limited knowledge of how the home heating system works - so please bear with me.

I got a Viessman Vitodens 200 WB2b Combi boiler installed in my house 10 years ago. Owing to budgetary constraints, I didnt go for the "Weather Compensation" option. The boiler was not installed with any thermostat anywhere in the house, and I was told this model works on return temperature of the heating circuit - once the return hot water is within a certain delta to the outlet water temperature, this means all the rooms have reached the desired temperature, resulting in the respective radiator valves going closed, thereby the hot water doesnt lose any heat thru the radiators, returning to the boiler still hot. So - that is how the boiler has been working. Whether i paid higher gas bills than I needed to, I dont know.

Now, 8 years on, I see there are lots of wifi enabled boiler controllers available - Hive, Next, Tado, Drayton etc etc. They all seem to want to replace the existing boiler thermostat panel. I have none to replace.

Is there any such option available for this boiler?. Anyone has had any luck?.

Thanks for any help.

Regards.
Riz

 

[ericmark]

Two methods to control a boiler on/off or with ebus I did a google for your boiler here can't find anything which says ebus or opentherm so seems on/off is only option.

The boiler modulates, that is it turns flame up and down to match output, and the way you think it works is correct, as the thermostatic radiator valves (TRV) close the water is either forced through the other radiators faster or it lifts the by-pass valve and so the return water temperature controls boiler output.

Two problems, one if you switch boiler off and then back on again, it returns to full output, and second even when the whole house is warm it still cycles on/off as that is only way it knows house is still warm.

The Hive thermostat has a heat on demand function, so if any TRV sends a signal it needs heat the wall thermostat keeps the boiler running, however the big problem with Hive, EvoHome, Wiser and Tado is the TRV heads cost around £40 each, so great idea but expensive if like me you have 14 heads. So most of us look for a cheap option.

Nest is an odd one out, it does not have a dedicated TRV head.

Most of the wall thermostats when used in on/off mode try to predict when the boiler needs switching off, and on, doing it as little as possible, if the boiler switches itself off/on that's OK as it switches on at min output, but don't want the thermostat to turn boiler off until it can remain off for a few hours. There are some thermostats designed for non modulating boilers that use a mark/space ratio (they switch off/on) to reduce output as the room approaches the target temperature to reduce hysteresis (stop over shooting) you don't want one of these as each time they switch on boiler back on max output.

However in the main we can use the programmer to turn off boiler at night and when at work, and we are not helpless, we can manually turn it off in good weather, so likely the best control upgrade is a TRV head which acts faster and is programmable. The Eqiva eQ-3 starts at £10 each, I have the more expensive bluetooth at £15 each, however these will not work with the wall thermostat, there is a big jump to £35 each to get wifi versions that work with a wall thermostat.

There is nothing to stop you setting a wall thermostat to match the schedule of the TRV head, but if you decide the eQ-3 is not good enough, then moving to Wiser means they are no longer any good. So you have to decide how much you want to spend on automating. However I use a mixture, the idea was my energenie would work with Nest and control down stairs, and eQ-3 would control upstairs, but the linking between Nest and energenie did not work, so set to same schedule not linked.

So I in hind sight would have been better with eQ-3 all around, as cheaper, and my boiler is not modulating it is oil so a cheap system would have done.

So theory really cheap thermostat in hall set higher than the TRV so it never in winter turns off, and hall keep reasonable cool so when summer arrives it will be warmer than TRV setting so turns heating off, so programmable wall thermostat and programmable TRV head set to work together. Rest of house programmable TRV heads so no room over heats. May not need wall thermostat, you can likely manually turn off heating in summer, so no wiring, simply swap the TRV heads to programmable type, Terrier i30 or Eqiva eQ-3 very similar, if you can't reach the TRV easy then the bluetooth version (note will only pair to one device so wife controls some and I control some we both can't control the same head) if easy access to TRV head pressing on button swaps from eco to comfort and you set what those temperatures are, so I walk into bedroom press the button and it goes from 17°C to 21°C until either pressed again or the schedule changes it.

I find I can understand how hot 20°C is but not *123456 so I can now set either with dial or button easy, walking into a rooms set to 2 I knew I wanted it warmer but between 3 and 4 did not really work for me, and always forgot to turn down again, so I find the programmable TRV great, OK a pair of AA cells per year per TRV head, should last 2 years but a pain when they stop working are designed to fully open when battery goes flat, but simpler to change start of each Autumn.

Since I have to use PC, tablet or phone to alter my energenie but can simply turn dial on eQ-3 I actually think they are better, and at £10 each cheap enough too, clearly they can't tell boiler to start or stop, but I am not helpless yet I can still flick a switch when summer arrives.