Load compensation vs Weather compensation (Viessman Vitodens related but general question)

[mrklawuk]

Hi,

 

I’m about to replace my combi boiler and the likely candidate is a vitodens 050-w 25kw. I have a question related to getting the most out of lower flow temperatures. I’ve done some calculations and had an initial visit for potential heat pump and the radiators and pipework seem ok except the spare room. The reason I’m going combi is mainly lack of space for a HW tank (its now our ensuite). In the next ‘some’ years I’ll look at getting a porch extension and carve out some space as a mini plant room. But not today.

 

I have a tado wireless thermostat and receiver, and smart TRVs on all radiators. Being a UK model it is only on/off, but Tado have just released the EU version in the UK which I could buy which would give me opentherm controls and should be ok on the vitodens. So I think I have two options and I’d appreciate your input.

 

1) keep the on/off tado, buy the external temperature sensor for the vitodens and ask the installer to connect to the boiler. This would enable weather compensation where the vitodens has a single fixed curve and would lower the flow temperature based on the outside temperature. 

 

2) replace the tado wireless receiver with the opentherm one and use that to enable load compensation driven by the tado. This will vary flow temp based on the delta between room temp and called for temp. 

 

I can’t tell if one is noticably better than the other. I can see ups and downs for both.

- with load compensation if the called for temperature gap is wide, it’ll use a high flow temperature even if outside isn’t cold which would be inefficient. However if it isn’t cold outside the gap shoudln’t be wide, right? so maybe a false use case? 

- with weather compensation if its cold outside the flow temperature will be high even if the call for heat is only small? could this lead to cycling? Although again if its cold outside the heat loss would be greater 

 

Maybe they’re both as good/bad as each other. I checked the viessman manual and they are one or the other - they can’t both be active at the same time. 

[billt]

You need to confirm that your opentherm controller will actually talk to your opentherm boiler. Although opentherm is supposed to be a standard it seems that different manufacturers have different accents of opentherm and don't necessarily talk to each other.

 

I have a Veissmann 100W system boiler which allegedly has opentherm but it won't talk to my Drayton Wiser which is also allegedly has opentherm. It also won't talk to an opentherm interface connected to an ESP8266 that I tried.

 

I also installed the Viessmann external temperature sensor to try to get weather compensation working which also didn't work satisfactorily. The low end Viessmann boilers have very little adjustment of the weather compensation curve and the only available curve didn't work well with the on/off control of the Wiser system.

[JohnMo]

The idea with WC is you set the flow temp so without TRV or thermostats the house temp is where you want it, it runs 24/7 during the heating season, but with setbacks of a couple degrees at night or when empty.  If a room is too hot/cold that room radiator/ufh is flow balanced to get the temperature right.  Some set up time is required, a bit of trial and error.  The temperature curve will need to be lowered or increased depending on how your house reacts to temperature changes.

 

Load compensation does a similar thing to WC, but without reference to the outside.  If you have UFH, load compensation is too slow, so would not use it (have tried).

 

If going for either route on/off/opentherm thermostats are ok, but not needed, they are just set a couple degrees higher than your required room temp as a limit stop.  When I decorate next, all my thermostats are being removed - they are not needed with WC.

 

Use the manufacturers controller and keep it simple.

[chris_x]

billt.

 

Did you have any success linking the Viessmann 100 boiler to a Drayton Wiser using opentherm?

[mjsx]

On 19/01/2023 at 14:32, chris_x said:

Did you have any success linking the Viessmann 100 boiler to a Drayton Wiser using opentherm?

 

@chris_x It's not listed on Viessmann's "Which smart thermostat to buy" so … maybe not?

[Adsibob]

@mrklawuk did you ever manage to get your Tado to speak with your Viessmann? I have a Vitodens 200W and tado thermostat, but it seems they can only speak binary to each other (on/off).