I think I see one major difference already. The potable hot water (which would also be used for the bath) and the heating water are never (?) from the same source. It's been ages since I've lived in a house heated by hot water, but I seem to remember there being a separate boiler for the radiant heat. Now that I think about it, I could be desperately wrong, though. But I'm pretty sure that the heating water was kept in the tubing and not wasted, so I can't imagine how it could intermix with the potable water. But it might have been heated from the same source (which was an oil-fired furnace, in that case).

Anyway, usually, to reiterate, there's an appliance called a hot water heater that's an upright cylinder about 2 to 3 feet in diameter and about six feet tall. Those dimensions are estimates, and I'm sure there's some variation amongst this general common size, but that's the basic size. They're not on timers. I've never even heard of one with a timer. They try to keep themselves full at all times with hot water. They do have an adjustable thermostat. They're not in the living area of the house at all, unless you happen to have a finished basement, and, even then, it'd usually be found in an unfinished part or at least a utility closet. I've never seen the dishwasher-size in-the-kitchen one I used to have anywhere except in that apartment complex, which was fairly old. I doubt you could find a new one. In houses without a basement, they'd be in the crawlspace. I don't know much about UK home construction, but in the US, the crawlspace is the area within the foundation underneath the lowest floor joists. It's usually quite a cramped space, the floor being the soil and unfinished in any way. There can be entrances both internal to the house and external, though not usually both in the same house. Anyway, in houses with a crawlspace, a normal hot water heater would be too tall to fit, so they often use what's called a low boy heater, which is the same thing, except the tank is a squatter cylinder. It also usually has a lesser capacity.

As far as I know, there's no heat exchanger. The water that gets heated and stored in the tank is the water that comes out of the tap. I don't think that the heating unit and the tank are separate at all. I know on the electric ones that there's an immersion coil at the bottom of the tank. If you remove the coil without draining the tank, you'll get very wet. I'm less sure about the gas ones. But the heater and the tank are certainly in the same single chassis.

I'm sure you've come to the conclusion that it's a fairly wasteful system, and it is. There seems to be a slight trend in newer houses to use just-in-time heaters, but they're not at all common yet.

Edit: There's an illustrated listing of the hot water heaters available at Lowe's, one of the common home improvement stores, here. BTW, the acronym ``DIY'', while possibly understood in the US, is not a common term. But it should be.

is saying "other than two things" instead of "apart from" or "except for" standard US English? It looks very odd in UK English.

I'll have to admit that, looking back, it does read funny. But I can't say that it's at all uncommon in speech. Either of your options would be fine, too. Wait. Is it the ``two things'' part that worries you or the ``other than'' part? The ``two things'' part reads odd to me, but the ``other than'' is perfectly common.