1. If you have a combi boiler of around 35 kW or bigger, and your shower flow requirements aren't outrageous, you could have two showers using around 8 litres per minute each without hot water storage. However, anyone turning on another tap could well cause one or both shower to run cooler.
2. If you have a smaller combi you could consider using an electric shower (say 10.5 kW) for one bathroom. As well as avoiding installing hot water storage, this gives you some hot water if the combi is out of action. The flow from an electric shower won't be great, but many people manage with one as their only shower. I'd consider the flow adequate for a teenager!
3. If you have a system or heat only / regular boiler, you will need a hot water cylinder (HWC). This could be:
3a. Unvented, which means you wouldn't need a cold water storage cistern (CWSC).
3b. Vented, which means you would need a CWSC.
4. Given that you have a loft conversion, you may not have the room for a CWSC, in which case your options are big combi or unvented.
5. If you have room for a CWSC (which must be higher than the HWC) you could use a pump or pumps to provide better flow to the showers, subject to careful sizing of the tanks, pumps and pipework,
6. If you opt for an unvented HWC:
6a. They can only be installed by a G3 registered plumber.
6b. They must be notified to Building Control. This is normally done by the G3 installer.
6c. They require an annual service, again by a G3 registered person.
6d. You can get one with one (or two) immersion heaters, which again can provide hot water if the boiler is out of action.
6e. You will almost certainly need to get the cold water inlet to the cylinder piped in 22 mm from the incoming mains if you want a decent flow out.
7. Your existing boiler can be used to heat an unvented cylinder, whether it is a combi, system or heat only boiler.
8. If you think it likely that there will be several showers taken simultaneously / in quick succession, have a look at fast recovery cylinders, but be aware these require substantially more heat (for less time) from the boiler.
