Viessmann 200-W Gas Boiler - The most efficient boiler sold in the UK?

MeteredOut

Bendo said:

Reed_Richards said:

I repeat, to me the issue is not how very good the new boiler and heating system is, but why the old one was so much worse.  If we knew that and there was something that could be changed to make improvements short of replacing the boiler,  then that could help a lot of people

Its pretty obvious isnt it. UFH with low flow temps means it will always be cool enough to condense. The big rads removed would have been only condensing a little or not at all.

Exactly my thinking. Its seems weird (if not naive) to attribute a 30% reduction in gas usage almost exclusively to a new boiler when the fundamental approach to heating the house has changed significantly. I suspect the latter would be a contribution factor as much as the the former.

Netexporter

It's the combination of lower flow temperatures, in parts of the system, and the ability of the boiler to switch flow modes, that is fundamental to the efficiency improvement.

FreeBear

Screwdriva said:

FreeBear said:

Viessmann 200, rated at 89% - https://www.homeheatingguide.co.uk/efficiency-tables?model=018017 - 

https://www.viessmann.co.uk/en/products/gas/vitodens-200-w.html

98% stated in the brochure as well. Their other boilers (V100/ V050) are rated at 94%.

Digging a little deeper in to the technical specifications, the 200-W is up to 98% based on a 40°C flow, 30°C return. The 050-W is the same.

The WB 18Ri will also return 97% efficiency at the same flow/return temperatures.

Like car mpg, in the real world, actual efficiency is very much dependent on settings and environment. What is important is that the boiler keeps your house warm with a running cost that you are happy with.

markin

FreeBear said:

Screwdriva said:

FreeBear said:

Viessmann 200, rated at 89% - https://www.homeheatingguide.co.uk/efficiency-tables?model=018017 - 

https://www.viessmann.co.uk/en/products/gas/vitodens-200-w.html

98% stated in the brochure as well. Their other boilers (V100/ V050) are rated at 94%.

Digging a little deeper in to the technical specifications, the 200-W is up to 98% based on a 40°C flow, 30°C return. The 050-W is the same.

The WB 18Ri will also return 97% efficiency at the same flow/return temperatures.

Like car mpg, in the real world, actual efficiency is very much dependent on settings and environment. What is important is that the boiler keeps your house warm with a running cost that you are happy with.

How is it tested, If they simply dump the heat to always have it run at the perfect temp for 24hrs and to always condense, on paper its great, Or is it 24 cycles with 24 dirty 'cold starts' and only in condensing mode 60% of the time. The was a study years ago saying most boilers don't hit the tested efficacy.

A high modulation number means its likely over sized for for job in 95% of homes.

Screwdriva

Bendo said:

Its pretty obvious isnt it. UFH with low flow temps means it will always be cool enough to condense. The big rads removed would have been only condensing a little or not at all.

What's less obvious is that the radiator circuit runs flow temperatures 5 degrees warmer than the UFH circuit (35 vs. 30 degrees C flow). Any decent boiler will condense at those temps.  

MeteredOut said:

Its seems weird (if not naive) to attribute a 30% reduction in gas usage almost exclusively to a new boiler when the fundamental approach to heating the house has changed significantly. I suspect the latter would be a contribution factor as much as the the former.

Perhaps equally strange to not give credit to boiler than can capitalize on a property's low heat loss and modulate its output to levels no other comparably sized boiler can. But hey, these days, everyone's a cynic! 

Reed_Richards

Screwdriva said:

Perhaps equally strange to not give credit to boiler than can capitalize on a property's low heat loss and modulate its output to levels no other comparably sized boiler can. But hey, these days, everyone's a cynic! 

As a total cynic, I would say that this just illustrates a fundamental flaw with combi boilers.  You need a large power output capability to supply the hot water, which may be much more than a well-insulated house needs to keep it warm.  That's why you need a very large modulation range.  The alternative is to store your hot water in a cylinder and use a heat source that's better matched to the needs of your home.   

Screwdriva

Reed_Richards said:

As a total cynic, I would say that this just illustrates a fundamental flaw with combi boilers.  You need a large power output capability to supply the hot water, which may be much more than a well-insulated house needs to keep it warm.  That's why you need a very large modulation range.  The alternative is to store your hot water in a cylinder and use a heat source that's better matched to the needs of your home.   

The old Greenstar was a system boiler paired to the same Megaflo cylinder on S plan

Reed_Richards

Screwdriva said:

Reed_Richards said:

As a total cynic, I would say that this just illustrates a fundamental flaw with combi boilers.  You need a large power output capability to supply the hot water, which may be much more than a well-insulated house needs to keep it warm.  That's why you need a very large modulation range.  The alternative is to store your hot water in a cylinder and use a heat source that's better matched to the needs of your home.   

The old Greenstar was a system boiler paired to the same Megaflo cylinder on S plan

Then why on earth do you need a boiler with such a huge output?  

Screwdriva

Reed_Richards said:

Then why on earth do you need a boiler with such a huge output?  

Actually,  I need a boiler with the capability to modulate effectively. 19 kW to recharge the cylinder quickly and 2.1kW to heat the house on most mild days. 

Cardew

Screwdriva said:

MattMattMattUK said:

Condensing gas boilers set up properly operate in a 95-97% efficiency range. Unless the pervious system was not set up properly then there is no way that a 30% reduction in like for like usage would be possible, that would have meant the older boiler would have needed to be operating at around 60% efficiency, which is low even for a non-condensing boiler. 

Two things stand out, one you had an entirely new central heating setup, underfloor heating generally creates a perception of warmth greater than the actual air temperature, secondly is the data does not make consistent sense, your usage in 21/22 is largely flat, where as 22/23 it is highly variable and whilst the unseasonably warm November appears to be a factor, the other months make little sense. 

1) The Worcester Greenstar Ri was definitely an inefficient boiler. It could not modulate below 6kW, 

2) The data is based on actual average monthly temperatures for both years. On milder days, the V200 modulates down to an output of 2.1-3kW for most days, which is half the old boiler's peak modulation capability. This is when savings near 50%. On the coldest days, observed gas savings are closer ~20% when it is outputting between 4-5kW to compensate for the property's heat loss of ~5.2kW at -2C.

Both in this thread and your other thread you appear to attach great importance to the ability of the boiler to modulate down to low outputs, and indeed in the other thread it appeared to be the main justification for disposing of a five year old boiler.

The advantage of low modulation is to prevent 'cycling'and whilst preventing cycling does improve efficiency you appear to be claiming this ability to modulate down to low levels accounts can give a 50% saving on milder days  and this cannot be true.

If your boiler can modulate down to, say, 3kW for a given heating demand and another boiler can only modulate down to 6kW for the same heating demand, this boiler simply switches off and on(i.e. cycles). They still produce the same amount of heat to satisfy the heating demand albeit your V200 will be marginally more efficient.

Indeed Oil boilers do not modulate:

'oil boilers generally do not modulate - they simply operate on an on/off basis. To incorporate modulating technology in an oil boiler would substantially increase the cost of manufacture and offer little gain in efficiency to the homeowner.'