Increasing size of two radiators

[angel150]

Can anyone tel me if changing 2 radiators to larger double radiators will work with my existing boiler. We have a potterton profile 40e output 11.72kw. I think total output with present radiators is 8326 watts and I read I should allow 3kw for heating our hot water tank, so that makes a total output of 11.32kw less than boiler. Increasing the two radiators will add minimum of 958 watts taking our requirement to at least 12.28kw and ideally I'd require it to be 12.51kw so roughly 0.5 to 0.75 kw greater than the boiler output. What will happen if the boiler isn't powerful enough? Is there any chance it will work, really want to go ahead and do this because the 2 radiators I'm changing are very undersize for the rooms.

 

[Breesey]

Do you have all of your rads and the hot water on full wack all at the same time?

 

[angel150]

Don't think so- we'd normally have all radiators on at once in the winter on full wack, but we have a cylinder for hot water so guess boiler heats water up for that as and when we use it, which may or may not coincide with heating coming on. It would be very rare we empty it.

 

[Breesey]

You've answered your own question then!

Do you not have a spare room that has its rad off? TRVs would help in any case what you want to do should be fine

 

[angel150]

Thank you
I haven't turned any of our rads off yet- we use all the rooms although I suppose that is a bit wasteful! I think the radiator in the hall is probably oversized 8m2 hall with a 1400watt radiator- no outside walls so I suppose we could change it for one of the small 588 watt radiators we take out if we had to. Our hot water cylinder is on our very small landing probably a fair assuption that that's heating that space as much as a radiator. Was 3kw a fair estimate for the hot water?
Actually you've just given me a good idea. I'll turn the one in the hall off before we change anything and see how much difference it makes.