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

Screwdriva

Further significant update:

We swapped the Viessmann Heat Only 16 kW boiler for a Viessman 200-W 19 kW System boiler. Known as the most efficient gas boiler sold, it will modulate down to 1.9 kW and has a 98% efficiency rating. Impressive. 

Only downside is it will not fit in the cabinet area which would be an aesthetic setback. 

Just as we were about to pull the trigger, the installer asked if we would be willing to consider a Vaillant Aerotherm Air Source Heat Pump. Next steps are a heat loss assessment to determine if the heat pump will actually save us enough £ to pay for the £3-4K extra we'd need to pay upfront. 

Reed_Richards

Screwdriva said:

Further significant update:

We swapped the Viessmann Heat Only 16 kW boiler for a Viessman 200-W 19 kW System boiler. Known as the most efficient gas boiler sold, it will modulate down to 1.9 kW and has a 98% efficiency rating. Impressive. 

Only downside is it will not fit in the cabinet area which would be an aesthetic setback. 

Just as we were about to pull the trigger, the installer asked if we would be willing to consider a Vaillant Aerotherm Air Source Heat Pump. Next steps are a heat loss assessment to determine if the heat pump will actually save us enough £ to pay for the £3-4K extra we'd need to pay upfront. 

You may find that the 98% efficiency is only achieved by operating it at a much lower temperature than you would a conventional gas boiler.  You could do that easily if you have UnderFloor Heating throughout but if you have radiators and are swapping out a conventional gas boiler then you would need new radiators with a larger surface area.  The same is true if you get a heat pump.  If you have just UFH then you might well achieve something that is cost-competitive with gas.  I doubt that you would save the £3 -4k extra at current prices but you would be doing your bit to save the planet.

Edit:  Okay, you will have UFH downstairs, I see, so what I said about radiators applies to the ones you have upstairs.  They will perhaps need to be 3 times the surface area of the ones you have at present if those were specified for Delat T = 50 C. 

Screwdriva

Reed_Richards said:

Edit:  Okay, you will have UFH downstairs, I see, so what I said about radiators applies to the ones you have upstairs.  They will perhaps need to be 3 times the surface area of the ones you have at present if those were specified for Delat T = 50 C. 

Interesting comment. We have an 18X6 and 14X6 T22 radiators currently on the ground floor. Once UFH goes live, those will be "relocated" one floor up to replace 10X6 T21s. 

On the boiler, it's hard to find any boiler that will outperform the 200-W on efficiency, keeping flow temperatures constant. On the heat pump, I'm not sure we're ready to sign up to the additional complications of ownership just to achieve parity with gas costs, if that's what the heat loss calculation yields. 

mnbvcxz

Out of my depth but wonder if you include a heat pump does the whole thing become vat free? UFH on basic heat pump using solar electricity and leave the rest of it alone as it is? 

Screwdriva

mnbvcxz said:

Out of my depth but wonder if you include a heat pump does the whole thing become vat free? UFH on basic heat pump using solar electricity and leave the rest of it alone as it is? 

Spot on. VAT savings of 20% = roughly £2K saved on the overall job. 

Reed_Richards

Screwdriva said:
.... On the heat pump, I'm not sure we're ready to sign up to the additional complications of ownership just to achieve parity with gas costs, if that's what the heat loss calculation yields. 

I had a gas boiler in my old house, then an oil boiler followed by a heat pump in my new house.  The gas boiler and the heat pump got/get an annual service which does not take very long.  The oil boiler went wrong about every six months so needed a service contract.  The oil boiler was the most complicated since it was not reliable, the gas boiler and the heat pump I see as equally easy.

The only way the heat pump will win the heat loss calculation is if it turns out you can run your radiators at, say, 40 C max.  That should give you a high enough COP.    

Reed_Richards

I have been trying to find the technical specifications for your chosen gas boiler.  I'm not sure if these figures are for exactly the same model but it says:

Efficiency @ full load (80/60°C)    % 86.8
Efficiency @ part load (50/30°C)   % 96.7
Seasonal efficiency accordance to Part L2 % 97.5

You do realise that you need to run the boiler at 50/30°C to get the high efficiency you aspire to, don't you @Screwdriva ?  I don't think you have mentioned this explicitly.  So the best comparison would be with a heat pump run at 42.5/37.5°C so the radiators would be at the same average temperature.

Screwdriva

Reed_Richards said:

I have been trying to find the technical specifications for your chosen gas boiler.  I'm not sure if these figures are for exactly the same model but it says:

Efficiency @ full load (80/60°C)    % 86.8
Efficiency @ part load (50/30°C)   % 96.7
Seasonal efficiency accordance to Part L2 % 97.5

It is quite difficult to locate exact efficiency figures. I do believe the Seasonal efficiency % is accurate. 

You do realise that you need to run the boiler at 50/30°C to get the high efficiency you aspire to, don't you @Screwdriva ?  I don't think you have mentioned this explicitly.  So the best comparison would be with a heat pump run at 42.5/37.5°C so the radiators would be at the same average temperature.

Yes, fully aware. We ran the Worcester Bosch 18Ri at 50/30C which helped reduce our consumption by 25% this past winter. That boiler can only modulate down to 6kW.

Given the 200-W can modulate below 2kW in theory and has advanced weather compensation and is more efficient and combined with the increase in radiator surface area + UFH, I'm quite optimistic we will see a real world savings % in the double digits over an already efficiency setup. 

Reed_Richards

Screwdriva said:

...Given the 200-W can modulate below 2kW in theory and has advanced weather compensation and is more efficient and combined with the increase in radiator surface area + UFH, I'm quite optimistic we will see a real world savings % in the double digits over an already efficiency setup. 

I think you may be being over optimistic and double-counting some of the beneficial features.  The best you can achieve with a gas boiler is 100% efficiency.  Your old boiler, operated at 50/30 might well have achieved >90% efficiency.  If the new one has a SCOP of 0.98 (i.e. 98% efficiency) then that is probably because it has all the other features and faculties you list; they are necessities to the performance, not extras.

Screwdriva

Reed_Richards said:

Screwdriva said:

...Given the 200-W can modulate below 2kW in theory and has advanced weather compensation and is more efficient and combined with the increase in radiator surface area + UFH, I'm quite optimistic we will see a real world savings % in the double digits over an already efficiency setup. 

I think you may be being over optimistic and double-counting some of the beneficial features.  The best you can achieve with a gas boiler is 100% efficiency.  Your old boiler, operated at 50/30 might well have achieved >90% efficiency.  If the new one has a SCOP of 0.98 (i.e. 98% efficiency) then that is probably because it has all the other features and faculties you list; they are necessities to the performance, not extras.

You may be right on my level of optimism. What is clear is that the Viessmann boiler has a superior modulation ratio and advanced weather compensation, which my Greenstar 18Ri boiler does not. Installing UFH and increasing radiator surface area are additional improvements proven to improve low flow temperature heating.

I'll be sure to report back with real world consumption numbers.