Cindy's recent questions about a faulty water heater got me to thinking that I should post some notes on how electric water heating is carried out in British homes.
Although "on-demand" heaters have gradually been appearing over the years, the ubiquitous copper cylinder is still the most common system here. Unlike their U.S. counterparts, these are not usually pressurized, but are plumbed as an "open" system from a cold storage cistern in the attic. In those homes with central heating, the cylinder usually contains a copper coil fed from the boiler and the electric is used as a boost/back-up.
The simplest arrangement has a single element mounted almost vertically from the top of the cylinder. These elements are almost always 3kW, and are thermostatically controlled. It's unusual here to leave the power turned on all the time, so it's either switched manually or by a time-switch. The latter is particularly handy for homes on "Economy 7," a tariff which offers low-rate power overnight.
Next step up is a twin element system, usually with two shorter elements mounted horizontally at top and bottom of the cylinder, and both fitted with a thermostat.
When manually switched, it was common to have a double control switch, one for on/off and the other often marked "sink" and "bath." The "bath" position applied power to the lower element to heat the whole cylinder, and the "sink" position powered the upper element to keep just a couple of gallons in the top hot for general dish-washing etc.
This system used to be very common in homes which used electric water heating exclusively, but it is gradually disappearing as central-heating is taking over.
The twin element arrangement also lent itself to Economy 7 usage, where the lower element was timer controlled to provide a a full hot cylinder in the morning, and the top element was switched on manually to boost the top part if necessary later in the day.
"Over sink" water heaters hold 2 or 3 gallons and discharge directly into the sink. Most are an open outlet type with the tap on the cold inlet side. They are also generally 3kW as well, which made them very popular for economic dishwashing as it takes only a few minutes to get piping hot water.
In old properties much larger versions of these were sometimes fitted over the bathtub if there was no other piped hot water. These things were real monsters!
Small pressurized systems for under-sink mounting have been getting more common in recent years, although still far from widespread, and although big 8 to 10kW on-demand units are available they are not particularly common.
