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

Screwdriva

QrizB said:

What are you doing to your system to stop the boiler condensing when heating DHW?

With a just-above-50 degree C return temp, your boiler should be in the condensing regime for the whole time.

If run under the same conditions, your old WB would have condensed too.

What am I doing to my system? Nothing really. It's hot water priority, so fires up to heat the cylinder with the burner often at 100%. The 250L cylinder is recharged in ~30 mins. 

Screwdriva

Netexporter said:

A big source of inefficiency of the old set up, shown in the video, was cold water constantly flowing through the HWC. Could that amount to 30%?

Yes, that was a problem when recharging the cylinder at preset times. That said, very unlikely, as the cold water mixing was a slow process and the usage pattern prior to the fix, involved recharging the cylinder just prior to use to minimize this. 

Bendo said:

Yes, but that's with UFH. Your K2 rads are a different matter. Unless they were seriously big in a small room then I can't see how they were doing anything.

Is 1800 X 600 seriously big? Definitely not a small room ( double ceiling height living room) but we remain quite comfortable at 35-40 degrees radiator flow temperature. 

QrizB

Screwdriva said:

QrizB said:

What are you doing to your system to stop the boiler condensing when heating DHW?

With a just-above-50 degree C return temp, your boiler should be in the condensing regime for the whole time.

If run under the same conditions, your old WB would have condensed too.

What am I doing to my system? Nothing really. It's hot water priority, so fires up to heat the cylinder with the burner often at 100%. The 250L cylinder is recharged in ~30 mins. 

OK, so (per the idealised HeatGeek curve) your boiler should still be condensing while heating your tank.
Possinly not right at the end of the heating cycle, if your return is significantly above 50C.

BikingBud

Reed_Richards said:

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.   

Do we not have a fundamental flaw with how we try to blend the differing requirements, CH and DHW into one piece of equipment. 

In terms of optimising the set up how would having:

• Low power boiler to deliver heat only, constantly in the condensing zone, as a  more direct comparison to heat pumps
• DHW driven off peak power and by solar PV.

Yes there may be issues with sufficient "free" power for DHW on cloudy days but purely on a efficiency basis how would this stack up?

MultiFuelBurner

Bendo said:

Screwdriva said:

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. 

You ran your rads at 35 degrees?

Are you a wind up,  you may as well have just lit a candle.

I think you need to reset the idea in your head of what you need to heat a house. (Think heat pumps but using your gas boiler the same way)

On for more time low and slow like a heat pump 35oC is more than possible infact ours has regularly run below that on the weather compensation curve this month keeping the house at 21oC-22oC all over.

No you cannot heat a house quicky getting in from work at those temperatures so useless for those that like to wack everything to 70oC to quicky feel the radiator burn.

Bendo

MultiFuelBurner said:

Bendo said:

Screwdriva said:

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. 

You ran your rads at 35 degrees?

Are you a wind up,  you may as well have just lit a candle.

I think you need to reset the idea in your head of what you need to heat a house. (Think heat pumps but using your gas boiler the same way)

On for more time low and slow like a heat pump 35oC is more than possible infact ours has regularly run below that on the weather compensation curve this month keeping the house at 21oC-22oC all over.

No you cannot heat a house quicky getting in from work at those temperatures so useless for those that like to wack everything to 70oC to quicky feel the radiator burn.

No problem with the concept, but with K2 rads unless they were huge it wouldn't have worked to well. In fact the OP later aluded to having to run higher temps when it was cold earlier in the month, and that was after the rads were replaced with UFH.

Either way it's moot, its clear his savings boil down to a number of things,  not soley the boiler as implied.

MultiFuelBurner

Bendo said:

MultiFuelBurner said:

Bendo said:

Screwdriva said:

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. 

You ran your rads at 35 degrees?

Are you a wind up,  you may as well have just lit a candle.

I think you need to reset the idea in your head of what you need to heat a house. (Think heat pumps but using your gas boiler the same way)

On for more time low and slow like a heat pump 35oC is more than possible infact ours has regularly run below that on the weather compensation curve this month keeping the house at 21oC-22oC all over.

No you cannot heat a house quicky getting in from work at those temperatures so useless for those that like to wack everything to 70oC to quicky feel the radiator burn.

No problem with the concept, but with K2 rads unless they were huge it wouldn't have worked to well. In fact the OP later aluded to having to run higher temps when it was cold earlier in the month, and that was after the rads were replaced with UFH.

Either way it's moot, its clear his savings boil down to a number of things,  not soley the boiler as implied.

Cheers for the extra info.

I agree it was a whole CH swap and concept change to achieve the savings not the single item swapping.

MeteredOut

Bendo said:

MultiFuelBurner said:

Bendo said:

Screwdriva said:

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. 

You ran your rads at 35 degrees?

Are you a wind up,  you may as well have just lit a candle.

I think you need to reset the idea in your head of what you need to heat a house. (Think heat pumps but using your gas boiler the same way)

On for more time low and slow like a heat pump 35oC is more than possible infact ours has regularly run below that on the weather compensation curve this month keeping the house at 21oC-22oC all over.

No you cannot heat a house quicky getting in from work at those temperatures so useless for those that like to wack everything to 70oC to quicky feel the radiator burn.

No problem with the concept, but with K2 rads unless they were huge it wouldn't have worked to well. In fact the OP later aluded to having to run higher temps when it was cold earlier in the month, and that was after the rads were replaced with UFH.

Either way it's moot, its clear his savings boil down to a number of things,  not soley the boiler as implied.

It wasn't implied. It was stated as fact, which IMO was pretty disingenuous.

Screwdriva

MeteredOut said:

It wasn't implied. It was stated as fact, which IMO was pretty disingenuous.

If we had replaced the K22 radiators with UFH while preserving the Worcester Greenstar 18Ri boiler, would we have achieved anywhere close to the shared savings? (IMO, it is disingenuous to suggest that would be remotely possible). 

However, if we had replaced just the boiler with the V200 and changed nothing else, flow temperatures would have remained 40 degrees (5 degrees higher than the UFH)  for the radiator circuits and savings would have come very close to the above stated 31%. This is based on flow temperatures for the existing radiator circuit.

MeteredOut

Screwdriva said:

MeteredOut said:

It wasn't implied. It was stated as fact, which IMO was pretty disingenuous.

If we had replaced the K22 radiators with UFH while preserving the Worcester Greenstar 18Ri boiler, would we have achieved anywhere close to the shared savings? (IMO, it is disingenuous to suggest that would be remotely possible). 

Agreed. It would be silly to suggest replacing a boiler with a more efficient one would do anything other than reduce usage/costs.

Screwdriva said:

MeteredOut said:

It wasn't implied. It was stated as fact, which IMO was pretty disingenuous.

However, if we had replaced just the boiler with the V200 and changed nothing else, flow temperatures would have remained 40 degrees (5 degrees higher than the UFH)  for the radiator circuits and savings would have come very close to the above stated 31%. This is based on flow temperatures for the existing radiator circuit.

To reduce the flow temperature to 40C (compared to your <60C previously), and keep the same radiators, would obviously either mean a colder house (although I note the EPC A rating) or the boiler requiring to run for longer to maintain the same temperatures as before.

I'm not claiming the new boiler is not giving you savings - it patently is - I'm stating you cannot, with the evidence you have provided, state is almost exclusively the reason for the 31% savings.