Combination boilers provide heat for CH, for up to about a 6 bedroomed house, and hot tap water on demand.
They range from about 23 to 42 kW, and are usually chosen according to the hot water requirement. More kW = more or hotter hot water.
Pro's
Unlimited hot water, though the CH stops while the tap is on.
Save space - no need for any storage tanks
Cheap to install
No pumps needed for mains pressure hot water
Con's
If they go wrong, you have no backup such as immersion heater for hot water.
You do need a good mains supply.
If you want to benefit fully from a higher output combi, you need an unusually good mains supply!
Even then the taps lower in the house will be favoured if two are on together, so you might go short in the shower upstairs.
A thermostatic shower valve should stop you freezing, but the flow may go right down.
Even in the best situations a combi will only supply about two taps at once, with careful plumbing, and devices such as flow restrictors. Most plumbers won't know much about these.
