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

Screwdriva

Cardew said:

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.'

In both posts, I have stated that the V200's ability to deliver the 31% reduction in gas consumption is due to several factors, including its class leading modulation ratio and ability to deliver lower flow temperatures across both circuits.

Forgive me for not believing the benefit of preventing boiler cycling is in any way marginal.

https://www.heatgeek.com/what-is-boiler-modulation/

To flip the coin on its head, do you believe the old Greenstar 18Ri could have delivered anywhere close to the gas savings I have shared if I had simply replaced K22 radiators with UFH? I think we both know the answer is an emphatic no. 

Bendo

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.

Screwdriva

Bendo said:

You ran your rads at 35 degrees?

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

35-40 degrees at -2C even today. Welcome to low temperature heating. 

The house is warm and confortable as it is well insulated. 

FreeBear

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.

A 35°C flow temperature is quite acceptable for a modern heating system - If you are running a heat pump, it is within the typical range. The downside is you need radiators some five times larger than you would with a 70°C flow temperature (a traditional number for gas/oil CH). The flow & return pipes also need to be much bigger as you need to pump a greater quantity of water around the system.

Reed_Richards

Screwdriva said:

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. 

You need 19 kW to recharge your cylinder quickly because the coil inside your cylinder hasn't got a large enough surface area.  It's like trying to heat a well-insulated room with an undersized radiator, you can only do it quickly by making the radiator very hot. 

You are going to have to replace that cylinder when you eventually get a heat pump, a decade or so hence.  Either that or you will briefly run your heat pump at a high water temperature and put up with the fact that that is not so efficient.  Presumably that also applies to your gas boiler now, you have to greatly increase the output temperature to recharge your hot water cylinder?  Do you think you are making any savings on hot water heating or is it all coming from heating your house?  

Screwdriva

I Reed_Richards said:

You are going to have to replace that cylinder when you eventually get a heat pump, a decade or so hence.  Either that or you will briefly run your heat pump at a high water temperature and put up with the fact that that is not so efficient.  Presumably that also applies to your gas boiler now, you have to greatly increase the output temperature to recharge your hot water cylinder?  Do you think you are making any savings on hot water heating or is it all coming from heating your house?  

We have a Megaflo and I agree it’s got to go.

Savings on gas for PDHW are around 30-35% (measured) as the v200 measures cylinder temperature and heats to 50C only (with a weekly Legionella cycle). All due to lower storage temperatures, as the boiler can’t condense for most of that cycle. 

Current preference is to install a Mixergy iHP and have the built-in heat pump handle hot water autonomously. 

https://mixergy.co.uk/mixergy-ihp/

Reed_Richards

My Atag gas boiler, installed 1998, always measured the DHW cylinder temperature and we set it to 50 C (with an automatic weekly Legionella cycle).  I've had this capability for such a long time that I forget that for many people it's an innovation.  

QrizB

Reed_Richards said:

My Atag gas boiler, installed 1998, always measured the DHW cylinder temperature and we set it to 50 C

Every gas CH system with a HW tank should have a tank stat that only calls for heat when the tank is below the set temperature. If the OP didn't, that's a clear shortfall on their old installation and not the fault of the boiler.

Screwdriva said:

Savings on gas for PDHW are around 30-35% (measured) as the v200 measures cylinder temperature and heats to 50C only (with a weekly Legionella cycle). All due to lower storage temperatures, as the boiler can’t condense for most of that cycle.

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.

Reed_Richards said:

You need 19 kW to recharge your cylinder quickly because the coil inside your cylinder hasn't got a large enough surface area.  It's like trying to heat a well-insulated room with an undersized radiator, you can only do it quickly by making the radiator very hot. 

That doesn't seem to follow?

A small coil can only transfer a small amount of power, and so will only need a smaller boiler output.

If Screwdriva needs 19kW to "recharge his cylinder", it must have a large coil? Otherwise there's nowhere for the heat to go?

I don't know how big the cylinder is, but heating 300 litres of water from 10C to 40C only needs 14kWh. Unless you're a family of six having 100-litre baths back-to-back, I don't see a use case?

FreeBear said:

The flow & return pipes also need to be much bigger as you need to pump a greater quantity of water around the system.

Sorry but that's nonsense.

If your flow temp is 10C higher than your return temp (a common design goal), you'll get the same heat transfer at the same flow rate regardless of the actual water temperatures.

For a delta-T of 10 degrees C, 1 litre of water per second is 42 kJ/s - 42 kW.

OPs 19kW boiler is notionally capable of heating 452ml of water a second by 10C. This is the same for a 35/25C flow system as for a 50/40C or 80/70C one.

Netexporter

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%?

Bendo

Screwdriva said:

Bendo said:

You ran your rads at 35 degrees?

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

35-40 degrees at -2C even today. Welcome to low temperature heating. 

The house is warm and confortable as it is well insulated. 

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.