Choice of viessmann vitodens 222 19kw-25kw-32kw

[shaya]

After a lot of research I thought viessmann vitodens 222 would be the best as we have a high water usage and this comes with 100l tank also its very efficient and have modulation ratio of 1:17.
We have a 5 bed 3 bathroom and 19 radiators and our water flow is about 30, when I asked the plumber I was told not to go for 32kw and go for 25kw as its all depend on insulation of the house and our gas bill would be higher, unfortunately I do not know the exact figure for our house but does it make much of difference if I go for the biggest one to be safe? as the boiler usage ratio is 1.9 kw to 32kw and the one below is 1.9 to 25kw.
my question is if we go for the biggest one would that increase our gas usage unnecessary, wouldn't this boiler adjust its usage to what we need regardless of its size?

 

[Johntheo5]

It won't increase the gas usage as the minimum output is 1.9kw in both cases, also, if this is a combi boiler the 100 litre store can be quickly used up and the 32kw unit will still give 13.5LPM (at 40C) in the winter and ~ 16.4LPM in the summer vs 10.5LPM and 12.8LPM for the 25kw unit.

 

[shaya]

So why would anyone go for smaller one as the princes isn't that much different

 

[Johntheo5]

Because there is generally a mismatch between CH requirements and DHW requirements. You may only require say 2kw of CH with zoning and increased levels of insulation etc after the house is up to temperature but even a 40kw combi will still "only" give ~ 16.9LPM in winter and 20.5LPM in summer enough for one rain shower or just about 2 X 8LPM showers, quite a lot of 40kw gas boilers will not modulate lower than say 8kw resulting in boiler cycling when on CH and extra fuel and maybe more wear and tear, a 25/30kw might modulate to say 5kw on CH resulting in less cycling. The Vitodens 222 gives the best of both worlds but probably at a good price increase over the normal boiler. Also, there may be practical considerations like gas suppy limitations which will determine the boiler output selection.

Also one should beware of claims that a 32kw boiler will give a flow rate of say 20LPM, it will, but only until the heat store is exhausted.

 

[shaya]

John thank you very much for such a detailed explanation very kind of you indeed, I also had another reply from Vissman which they said "The 32kW will require a supply pressure of 3.69 m3/h for maximum output but will modulate down as heating demand reduces, As with any Combi, or storage Combi, the output will have some influence on the Hot Water delivery (l/h) so the larger 32kW will perform better in its DHW flow rate than the 25kW "

As far as I know there is no limitation in gas supply, considering all the factors should I just go for 32kw in that case in your opinion?

 

[Johntheo5]

Yes, definitely, IMO.

 

[Mister Banks]

Is a combi boiler even suitable for a 3 bathroom house? I have read that a combi is suited more to 1 bathroom properties where space is tight and a DHW tank can’t be installed.

 

[Johntheo5]

A combi is a compromise in most situations, a stored combi far better, the vitodens 222 with 100 litre storage will give ~ 150 litres at 40C, enough to fill a bath and a half, it will then take ~ 10 minutes to reheat 100 litres to 60C. 20LPM is possibly the normal max flow rate that the mains will supply whether to a combi or a unvented cylinder. If the above is replacing another combi then a good choice IMO, if a new build or replacing a system boiler with a DHW cylinder then a bit of thought required.