Going green(er) with a Viessmann 200W (& heating efficiency home improvements)

Screwdriva
Forumite Posts: 967
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Following the success of our heating gas saving measures, we've decided to install polypipe underfloor heating on the ground floor of our semi-detached, paired with a new 19kW Viessman 100-W compact heat only boiler.
Reasons for going with Viessman boiler are:
1) Ability of the boiler to modulate down to < 3 kW (1:10 modulation ratio), which is quite incredible!
2) Weather compensation
3) Load compensation via Nest
4) Hot water prioritization
5) 10 year warranty for a relatively well made German boiler that is H2 compatible.
Absolutely nothing wrong with our 5 year old Worcester Bosch 18Ri heat only boiler but it cannot achieve this level of theoretical efficiency, as it lacks the above functionality. We also looked at air source heat pumps but came away feeling that these are simply too nascent a technology today.
Here is the episode of the completed project. Heating efficiency gains will be shared in the coming winter.
https://www.youtube.com/watch?v=UHD_-i5ulms
Reasons for going with Viessman boiler are:
1) Ability of the boiler to modulate down to < 3 kW (1:10 modulation ratio), which is quite incredible!
2) Weather compensation
3) Load compensation via Nest
4) Hot water prioritization
5) 10 year warranty for a relatively well made German boiler that is H2 compatible.
Absolutely nothing wrong with our 5 year old Worcester Bosch 18Ri heat only boiler but it cannot achieve this level of theoretical efficiency, as it lacks the above functionality. We also looked at air source heat pumps but came away feeling that these are simply too nascent a technology today.
Here is the episode of the completed project. Heating efficiency gains will be shared in the coming winter.

- 10 x 400w LG BiFacial Panels + SE P505 Optimizers + SE 3680 HD Wave Inverter. SE London (Zone 2).
- 40% of panels in an East/ West rooftop orientation.
- Viessmann 200-W on Advanced Weather Comp. (the most efficient gas boiler sold)
- 40% of panels in an East/ West rooftop orientation.
- Viessmann 200-W on Advanced Weather Comp. (the most efficient gas boiler sold)
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Comments
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WE have a 10 year old 19kwh 200-w, similar but I think only modulates down to 4kwh , hasn't been great tbh, seems to use more gas than you would expect and not as reliable as we would have hoped (fine until the 5 year warranty ran out but then pretty dodgy)I think....2
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1) I bought an Atag gas boiler in 1998 which could modulate down very low.
2) Likewise it had weather compensation and load compensation. This was 24 years ago
3) Load Compensation via Opentherm. Any Opentherm-compatible controller should do weather and load compensation. You'll need the right wiring though, not what you have at present.
4) Are there really boilers that don't prioritise hot water?
5) A 10 year warranty is good. I would be very surprised if you see any H2 in the next 10 years and you may find that you have to pay for the mod. that makes it work.Reed0 -
Reed_Richards said:1) I bought an Atag gas boiler in 1998 which could modulate down very low.
2) Likewise it had weather compensation and load compensation. This was 24 years ago
3) Load Compensation via Opentherm. Any Opentherm-compatible controller should do weather and load compensation. You'll need the right wiring though, not what you have at present.
4) Are there really boilers that don't prioritise hot water?
5) A 10 year warranty is good. I would be very surprised if you see any H2 in the next 10 years and you may find that you have to pay for the mod. that makes it work.- 10 x 400w LG BiFacial Panels + SE P505 Optimizers + SE 3680 HD Wave Inverter. SE London (Zone 2).
- 40% of panels in an East/ West rooftop orientation.
- Viessmann 200-W on Advanced Weather Comp. (the most efficient gas boiler sold)0 -
Screwdriva said:Absolutely nothing wrong with our 5 year old Worcester Bosch 18Ri heat only boiler but it cannot achieve this level of theoretical efficiency, as it lacks the above functionality.Not wishing to be a gloomster, but it seems a lot of work and expense to save maybe 5% on your annual gas consumption.(That's just my wet-finger estimate; your current boiler should be 85-90% efficient and so there's not much room for improvement.)N. Hampshire, he/him. Octopus Go elec & Tracker gas / Shell BB / Lyca mobi. Ripple Kirk Hill member.Ofgem cap explainer. Economy 7 cap explainer. Gas vs E7 vs peak elec heating costs.
2.72kWp PV facing SSW installed Jan 2012. 11 x 247w panels, 3.6kw inverter. 30MWh generated, long-term average 2.6 Os.1 -
Screwdriva said:
1) Ability of the boiler to modulate down to < 2kW (1:10 modulation ratio), which is quite incredible!
Beyond that, they are pretty similar in efficiency figures. Your WB is quoted at 86.9% efficiency for a high flow temperature, and 97.6% at 30 ℃ return temperature.
The 100-W is quoted as 88.2% and 98.0% in comparison, so maybe a 1.3% improvement at best.
I saw you mention low flow temperature in your other thread. Are you heating the DHW tank using the immersion heater or renewables rather than with the boiler?0 -
QrizB said:Not wishing to be a gloomster, but it seems a lot of work and expense to save maybe 5% on your annual gas consumption.
We're hoping the underfloor heating will increase comfort levels substantially for us as we're currently "under-radiated".- 10 x 400w LG BiFacial Panels + SE P505 Optimizers + SE 3680 HD Wave Inverter. SE London (Zone 2).
- 40% of panels in an East/ West rooftop orientation.
- Viessmann 200-W on Advanced Weather Comp. (the most efficient gas boiler sold)0 -
QrizB said:Screwdriva said:Absolutely nothing wrong with our 5 year old Worcester Bosch 18Ri heat only boiler but it cannot achieve this level of theoretical efficiency, as it lacks the above functionality.Not wishing to be a gloomster, but it seems a lot of work and expense to save maybe 5% on your annual gas consumption.(That's just my wet-finger estimate; your current boiler should be 85-90% efficient and so there's not much room for improvement.)Solar install June 2022, Bath
4.8 kW array, Growatt SPH5000 inverter, 2x Growatt ML33RTA batteries.
SSW roof. ~22° pitch, BISF house. 12 x 400W Hyundai panels0 -
Have you looked at electric boilers?
6.4kWp (16 * 400Wp REC Alpha) facing ESE + 5kW Huawei inverter + 10kWh Huawei battery. Buckinghamshire.0 -
Magnitio said:Have you looked at electric boilers?
N. Hampshire, he/him. Octopus Go elec & Tracker gas / Shell BB / Lyca mobi. Ripple Kirk Hill member.Ofgem cap explainer. Economy 7 cap explainer. Gas vs E7 vs peak elec heating costs.
2.72kWp PV facing SSW installed Jan 2012. 11 x 247w panels, 3.6kw inverter. 30MWh generated, long-term average 2.6 Os.7 -
QrizB said:Magnitio said:Have you looked at electric boilers?But there are some interesting new developments such as this:Not sure I would want to be an early adopter, but I quite like the concept.
6.4kWp (16 * 400Wp REC Alpha) facing ESE + 5kW Huawei inverter + 10kWh Huawei battery. Buckinghamshire.1
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