A four times larger boiler is generally cheaper to install and to run than four small ones, that’s a general rule. But it works in the technical dimension, while the legal and marketing ones may change the game, if you’re about to offer the flat to let, then you have to take them into account. Probably you are unwilling to provide utilities to your tenants instead of the usual service companies, probably they’re unwilling to buy them from you either preffering to have a separate account with the aforesaid providers, that’s something that only you can decide on.
@Practical_Severard, I don't think one boiler for the whole house is a customary solution here. I have never heard of it.
So you have several options which you can compare against each other in terms of capital costs, time to install, running expenses, running time and safety wise. Enlarging a bathroom might be not so expensive as laying a gas pipe there, I guess. Here we have to employ a service provider's workers only for anything concerning the pipeline gas, so it’s expensive and time consuming, but your situation may differ. But the pipeline gas kW*h (or a BTU a kilocalory or whatever) of thermal energy is way cheaper than the one produced with electric power, but again it all depends on your local prices. The district hot water is the safest option, followed by a tank electric heater then by a tankless (larger current > higher fire danger) while the gas firing one is most dangerous, not least because of the fact that tenants aren’t usually the most accurate people.
To get municipal hot water I would need an extra installation that goes through chimneys. It could turn out that my chimneys are not in a good condition either (I mean inside ).
Well, gas installation may be working in the attic. I have to ask a plumber. If so I would install a gas heater. They are cheaper.
Oh no, I don't want to have a tankless electric heater - I am trying to make out which options I have.
I could enlarge a bathroom but it's not the best solution either for my nerves or my purse.
While I don’t quite grasp the difference between the connection and contracted power I guess that you could find this information in your contract with the electric company. Or even from the writing on the incoming rocker switch. But a question isn’t a bad thing. But you don’t need a 3-phase connection, you just need a connection of enough power. Having a separate meter for the tenants may be reasonable not because of the water heater only, but because your prospective customers wouldn’t be afraid of you cheating them on utilities bills.
4kW is contracted power. But we used to have four meters and four bills so it may potentially be even 4*4. I don't know. It's contracted power versus connection power. I have just sent an official question concerning the possibility of three-phase connection to my provider. They have to answer irrespective of the fact that I am a lady. And I will know at last what connection power I have.