I am about to install a second bathroom and want two showers to work at the same time. I am replacing all plumbing but there is only 15mm feed to flat. What type of boiler etc is needed to run two showers and washing machine at the same time?

You didn't give you location, but my guess is that you are East of the Atlantic ocean. If so, I am not sure any of the regulars here can help much. About a year ago we welcomed a plumber from England, but unfortunately, he didn't stick around. There are times we need him.

I think the tank style water heaters more common here might work for you. Even the large ones don't have that high of a current draw, but will have a good supply of hot water ready in the morning. You can buy faucets that automatically adjust to hold the same temperature when the other shower starts and stops.

Hi John,

Let's see, one washing machine = 20 to 30 gallons per washing. Two showers at 5 gallons per minute = 10 gallons per minute.
And you wish to know what type and size of boiler, (water heater) it would take to supply all of this at one time with the house supply of 15mm,(1/2").
My friend, you can not get more out of a system then you put into it. For a water system to work you must have two things, pressure and volume. You must have both. One will not work without the other. You ran out of both the minute you ran all of these fixtures at the same time. Your question should be "how can I increase the volume of water coming into my flat" Not how much of it do I have to heat. If you had the volume to supply all those fixtures you wish to operate azt the same time I would say a 52 gallon water heater would serve your purpose. Unfortunately you don't have the volume entering your flat to sustain this. To supply all your needs I would increase the water service to 3/4, (19mm) or 1",(25mm) and if the pipes in the flat were no larger then 1/2" I would increase the house main to 3/4" with 1/2" branches with no more then three fixtures on each branch.
Good luck, Tom