MaeMobley
PigletJohn, with a combi, can you never run 2 showers at the same time? I assumed it was our poor useless boiler
Sorry didn't see that until today.
With a combi, the total amount of water that comes into the house from the main is the total amount of water that can come out of the taps, hot and cold added together, at any one time.
It is probably between 8 and 12 litres per minute.
So turning on a second tap will always reduce the amount of water that can come out of another tap.
Depending on its power, a combi can probably produce about 10 lites per minute of water that's hot enough for a shower. If you're blessed with a big incoming water main that can deliver, say, 20 litres per minute, the boiler will not be able to make it hot enough. Bath taps are usually tinkered with to reduce the flow when a combi is fitted.
Installers like combis because they are easy and quick to fit, especially in a small flat with no space for tanks. They are popular with some householders who think that having an extra cubic metre of space in the airing cupboard will not mean that they just fill it up with more junk.
IMO they are not satisfactory for baths, as they take so long to fill up (100 lites at 10 litres per minute = 10 minutes), and they are not satisfactory for a house with two showers that may be in use at the same time as they will only get half the flow each.
A cylinder, or a Megaflow (which is a modern version of a cylinder that runs at incoming mains pressure) heats up the water in advance of use, so you are not contrained by the speed the boiler can heat it, and can fill your bath as fast as the water will come through the taps. A Megaflow does need a modern, large incoming water main with a lot of flow, though, to get the full advantage.