In my last two homes the shower water was heated by gas, one used a pump at the shower point as with an open vented system the pressure was not good enough without the pump, when the boiler was changed to a combi boiler the pump had to be removed, regulations don't permit pumping from the mains supply in case it sucks in water under fault conditions, the other house always had mains pressure hot water since around about 1981 and worked well, with a few small problems.
The house using the Main 7 gas water heater for DHW (different boiler for central heating) had a minimum output, so needed a large rose as if not using enough hot water it would go cold, the combi boiler far more modern, but it had a reservoir of hot water, so when using shower it started cold, went hot, then cold again and then hot and stayed hot.
All used loads of water and with some force, and were good showers, and likely cheaper to run than electric, but expensive to install, and the one with the pump had to have a cold water supply from the header tank so both hot and cold at the same pressure.
I had used an electric dribble unit in the past, and they were rather pathetic, but cheap to install and only required a cold water feed, however when I moved into this house, two already installed, and they seem to work a lot better than I remember, and I see no reason to change to gas heated, well in this house oil heated.
I will guess it needs the rose carefully matching to the shower unit, and in the past likely not matched.
However 40 amp is a lot, when the whole supply to house is 60 amp, clearly if both showers used together there would be an overload. Also when I moved in the shower was not RCD protected, and really with electric in the cubical like that it needs RCD protection.
The other is both the showers are in a shower cubical, so the doors seal at bottom, so moisture is retained in the cubical, but in last house it was over the bath, so the chimney effect resulted in the moisture being distributed around the room, and so we had a mould problem as bathroom always damp. Maybe down to only being a mile from Mold but think more due to air circulation.
But using the shower a few times a week is not going to cost that much, and not worth worrying about amount of electric used, but when I was working I had daily showers, and the cost does mount up, so gas or oil is far better.
As to Part P, here in Wales it is £100 plus vat for first £2000 worth of work, plus any third party inspections, the LABC inspector did in the end accept my qualifications, C&G 2391 and a level 5 degree, but he was not happy letting us DIY without having an inspector selected by the LABC to check I have done it right, so in real terms no option but use a scheme member, or of course break the law.
When the power shower was fitted we selected a plumber who assured us he would do all required, we could just leave him to it, however it seems he had some out of date tile cement which was not setting, and he ended up running off, thinking all being done above board I informed the LABC I was taking over, only to find he had not submitted the planning application, I did not even know it was required, but the room was not originally a shower room, only a toilet, so it seems we should have submitted an application.
Lucky being fitted for mother after she lost he leg, so no charges, but had that not been the case likely would have needed to pay LABC £200.
Yes the tank fed and instant gas showers used more water, but we never ran out, and the showers were better, however as to if enough better to bother running new pipe work, likely involving ceiling removal to route pipes, so I would guess at least a £1000 if not more to use the DHW, I don't think for us worth it. At least not at my age.
