Worcester 42 or 37?

[dave27]

I am moving into a 1960's house in couple of months. It is currently a 3 bed house but I will be putting on big extension. It will have 2 showers (boths mains pressure) and about 13 radiators.
I am planning to put new radiators on throughout but not renew the pipes.
I was thinking if the Worcester 42CDI combi but a couple of things I have read on the forums make me this this is overkill.
As I am a complete novice I would appreciate any advice.
(My reason for the 42 rather than 37 is purely on wanting a really good shower - this may very well be a naive statement so apologies!)

Many thanks
dave

 

[ianniann]

You should first spec your "really good shower" and then work out how much boiler you need to heat water for it. The number you need is the flow rate, determined by water pressure, pipe sizes, and the shower itself. Once you know that, you'll be able to work out the power of boiler needed to heat that much water to a useful shower temperature. The 37CDi will heat 13l/min by 40C and the 42CDi 15l/min, either of which would be called a good shower my most people. In practice you would probably have a mix of slightly hotter water than that with some cold water, but the end flow rate is the same. Note that when the incoming water is colder from mid-winter onwards, you may have to drop the flow rate a little to get the same shower temperature.

The WB 37CDi and 42CDi both supply the same power to the central heating circuit, and it is hard to imagine that this wouldn't be more than enough for your needs. You should run it through a heating calculator though, just to be on the safe side. Both boilers will modulate down to around 10kW and this may well be higher than your steady state heating needs on all except the very coldest winter days. That's not ideal but also not the end of the world.

You mention two showers. Do you see a need to run both at once? The combis you mention are barely powerful enough to heat two showers that I don't think you'd call good. Of course your mains supply probably couldn't provide for two showers at once anyway.