Viessmann Weather Compensation

[tech749]

Wonder if you can advise at all, I've had a Viessmann 050 25kw combi boiler with Weather Comp installed to replace my old hot water tank system. The installer has fitted an EPH CP4 RF thermostat and receiver connected to Open Therm, and says that they work together to control the flow temperature according to the room temp and outside temp. I thought they might clash, as Open Therm will effectively control the boiler as far as I can tell, but I may be totally wrong here.

I'm struggling to understand how that can work, I have voiced my concerns to the installer who is very knowledgeable and a Viessmann registered installer, and he is looking into it. I just wanted to get my head around the system first, in case I have any questions, and be able to understand the answers. Any help appreciated. Thanks.

 

[Swwils]

Load Compensation and Weather Compensation are both ways of making your heating system more efficient by more closely matching the heat output of the boiler to your needs, but they do so in different ways:

1. Load Compensation: This is a function which adjusts the flow temperature (the temperature of water leaving the boiler to heat your home) based on the difference between the set temperature on your thermostat and the current room temperature.

For example, if your room temperature is close to your thermostat setting, the boiler won't need to work as hard and so the flow temperature will be reduced. Conversely, if your room temperature is significantly below your thermostat setting, the boiler will increase the flow temperature to heat your home quicker.

2. Weather Compensation: This function adjusts the flow temperature based on the outside temperature. It requires a sensor located outside the home to detect the current external temperature.

For example, on a cold winter day, the boiler will produce hot water at a higher temperature than on a mild day. The theory is that the colder it is outside, the greater the heat loss from your home, and therefore the harder your boiler needs to work to maintain the internal temperature.

By adjusting the boiler's output in response to changes in the outside temperature, weather compensation can help to maintain a more consistent internal temperature and improve the overall efficiency of your heating system.

In essence, both load and weather compensation aim to enhance the efficiency of your heating system. Load compensation does so by responding to the difference between the set and actual room temperatures, while weather compensation responds to the outside temperature.

You are right in thinking that load compensation does also really account for the weather, but in a less direct way than an actual weather compensation sensor. For instance, very large south facing windows may make pure weather compensation overheat such a room - whereas load compensation would account for that better.

 

[tech749]

Thanks for that. So anyone know if both can be used together, the way my system has been set up, or will Open Therm override the Weather Comp anyway? Weather Comp would be the ideal I think.

 

[Swwils]

Yes they can be both used together.

Ultimately it is up to you to "know" your heating requirements and controls and adjust it to suit.

In *general* weather comp is the more suitable strategy for UK homes, load comp is good for people without regular heating habits.

You should not be really turning off your heating at all, unless you live in a tent.

 

[tech749]

Update - installer got back to me and said he'd contacted Viessmann, and they said you have to have one system or the other and not both, as on Open Therm the boiler will be a 'slave' to the room temperature controller. Options were disconnect weather comp, and use Open therm to control the boiler, or reconfigure the connections so the room temp controller becomes an On/Off control controlled by the set temperature, and the weather comp then controls the temperature of the house up to or down to whatever the setpoint is set to on the room thermostat. I have opted to try and use WC and see how that goes. Obviously I will set the stat to above the optimum setting of the WC (20deg) during the day. Not sure about night time settings though.

Yes they can be both used together.

Ultimately it is up to you to "know" your heating requirements and controls and adjust it to suit.

In *general* weather comp is the more suitable strategy for UK homes, load comp is good for people without regular heating habits.

You should not be really turning off your heating at all, unless you live in a tent.

My only query is, if I have the heating on during the night with a setback temperature of say 17/18 deg for comfort, the weather comp will try and raise the house temperature according to the outside temperature, the room stat will then switch boiler off at 18 deg and WC will try to heat up more to keep the temperature steady at the optimum setting (20deg), will this cause the boiler to 'cycle', which I understand is not good? Or have I got that completely wrong.

 

[Johntheo5]

Can you configure it so that the WC controls the boiler flow temperature and the roomstat switches the boiler On/Off, you can use a stat with a very low switching differential of say 0.3C, the boiler will cycle a little more maybe on Setback but this shouldn't be a big deal

 

[Swwils]

They can be both used together. It's probably some combination of your particular controls and boiler is why they can't.

WC does not try to raise the house temperature, it simply adjusts the flow temperature depending on the outside temperature.

Ultimately as I suggested everything is determined by accurate room temperatures.

 

[Johntheo5]

Yes, but if I am interpreting it correctly, isn't the Open Therm also trying to control the boiler flow temperature which just won't work as the OP suggests?.

 

[Swwils]

Depends on the implementation of OT.

In its most basic essence OT has the following information available to controller and boiler and essentially in most implemention is aiming for a targets temperature window with some PID.

Flame
PID lockout
PID Autotune
Actual temperature interval
Boiler Temperature
External Temperature
Return Temperature
Heating Target Temperature
DHW (hot water) current temperature
DHW (hot water) target temperature
CH OpenTherm current temperature
CH OpenTherm target temperature high
CH OpenTherm target temperature low
PID Climate Controller current temperature
PID Climate Controller heating
PID Climate Controller target temperature

ALOT of manufacturers fudge it completely.

 

[JCK1]

The Viessman 50 cannot work with both wc and opentherm, it is either not both? Both are setting the flow temperature. For me opentherm is the more logical as i want my rooms to heat up fast which opentherm will do by demanding high temp setpoint until the target desired room temp approached, only then will flow temp set point start to lower . i dont care about the outside temp which and indirect way of trying to set the setpoint . Although my understanding is that Viessman do WC better than others.

 

[Swwils]

If you are trying to "heat up your rooms fast" you are likely still doing ON/OFF heating which is completely the wrong strategy with a modulating system.

 

[JCK1]

Depends on the implementation of OT.

In its most basic essence OT has the following information available to controller and boiler and essentially in most implemention is aiming for a targets temperature window with some PID.

Flame
PID lockout
PID Autotune
Actual temperature interval
Boiler Temperature
External Temperature
Return Temperature
Heating Target Temperature
DHW (hot water) current temperature
DHW (hot water) target temperature
CH OpenTherm current temperature
CH OpenTherm target temperature high
CH OpenTherm target temperature low
PID Climate Controller current temperature
PID Climate Controller heating
PID Climate Controller target temperature

ALOT of manufacturers fudge it

If you are trying to "heat up your rooms fast" you are likely still doing ON/OFF heating which is completely the wrong strategy with a modulating system.

Nope, i have opentherm on Viessmann 100 combi, it is not on/off once room where thermstat apprached user desired temp then controller (in my case evohome) will instruct lower flow temp from boiler which cause boiler to modulate down burner output , when set point reached min modulation is 8% fir the Viessmann 100w , i believe same for the 50w .

 

[Johntheo5]

I thought it was the other way round, that opentherm might control the boiler flowtemp and evohome the room temperature, evohome may work very well but once the room temps and the SPs are very close then evohome just fires the boiler on and off even if the heating demand is greater than the minimum boiler output, the cycles per hour can be set.

WC and EH should work OK?

 

[vulcancontinental]

Listen to the manufacturer, if they say not together then OK, make your choice.

Wait till winter, whatever your occupancy lifestyle and preference run it OT for a week reading the gas meter and noting comfort before during and after then switch to WC doing the same thing the following week and decide on comfort or economy.

I can't believe Veissmann do a version of WC without two temp choices, comfort and setback, and OT offers the same.

Some manufactures do or can combine the two; frustratingly some models from the same manufacturer can and some cannot.

 

[needyahelp]

Tech749 now the weathers getting colder hows the weather comp going? I have the same boiler and originally chose it specifically for weather comp but my installer was eventually told by Viessmann techies that you cant have a a roomstat/timer and WC together (Viessman sales people were not aware of this either and the viessmann brochures covering the 050 are misleading in the way they market weathercomp especially for typical uk homes and traditional use of timers) so ive gone with room stat/scheduling for now as i was concerned with leaving it permanently on (gas cost) but im keen to hear if WC is worth a try.