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.
I have seen some designs in books here for solar water heating, either by feeding water from the cistern through the solar panels on its way down to the cylinder, or by including the solar panels in the closed circuit of an indirect cylinder (i.e. incorporating into the boiler circuit).
Even in this far-from-always-sunny land, it's possible to collect a fair amount of heat in this way, less so as one goes farther north.
The common South African system is an immersion heater in the attic on a dedicated 30A circuit and with a double pole isolation switch next to the water heater. The heater works at mains pressure so it makes for great showers. I can understand why the UK is bath country as I could never get a decent shower there where the hot water was at 2 psi and the cold at 15 psi!
The drawback of mains pressure are the leaks when one of these units fails and I question the quality of some. My last one lasted only 5 years before my wife noticed drips of water running down the walls – luckily before any damage or staining resulted. The plumbers had to take a large section of roof tiles off to replace it as well!
Would have tackled it myself if it wasn’t for the broken arm at the time and being away on business – these things always coincide, don’t they. Had to tackle the wiring they installed though later. No flexible conduit, unsheathed conductors exiting sharp and broken off rigid conduit and hanging freely in space – I was appalled.
Cylinder in the attic, eh? Most of ours are put into an airing cupboard to make use of the heat (although many such cupboards are desparately lacking in the vents that are needed to provide air flow to aid drying).
In contrast to U.S. practice, we also use double-pole isolators (switched neutral) on immersion heaters, and on fixed heaters in general.
I meant to mention the shower aspect a little more.
Where the hot water is fed from an attic cistern, it is normal to feed the cold water to the shower from this cistern also so that the pressures are the same (equally low!). In fact, the standard arrangement with these cisterns is that all cold outlets in the bathroom come from the cistern. The water bye-laws in some places (e.g. London) required this to ease demand on the ancient water distribution system in use.
If the cold to the shower is at mains pressure, then it would be normal to install a pressure-balancing valve to the shower to reduce the cold pressure to that of the hot.
The prevalence of this antiquated hot water arrangement probably accounts for the growing popularity of on-demand "instant" electric showers which couple straight to high-pressure cold only and incorporate a 7 to 10kW heater.
Paul, How popular are these "instant Hot-Water" heaters,in England?. I have installed a large number of these, along with thier large cables, for people having showers in new ensuites(Bathroom next to a bedroom), plumbers over here love them,as they only have to lay on a cold water supply, to these units, OK, if you don't have to pay the power bill. Just Curious?.
Let's face it, these days if you're not young, you're old - Red Green
I'd say the instant electric showers have become very popular, particularly for add-ons to existing properties. As you say, the fact that the plumbing needs only a single cold pipe run from any convenient mains-pressure line makes them an attractive option where minimal damage to existing decorations is a consideration.
Even in some major remodels, a lot of people these days seem to prefer these to a fully plumbed non-electric version. Where hot and cold are fed to a conventional shower from the low-pressure cistern in the attic, the cold feed is generally run as an entirely separate pipe from the cistern. If it were tapped off some other low-pressure cold pipe there would be the danger of someone turning on a tap elsewhere in the house and causing the shower temperature to rise suddenly.
I'm sure that the complexity of adding all the extra pipework has contributed to the popularity of the instant showers, especially for DIY installations (Hot topic at the moment, but the DIY mess that arises from some people trying to do the wiring on these high-current units is another story!).
Re the cost of running them, a 9.6kW unit used during normal daytime rates in the the U.K. would cost about 70 pence per hour, including tax.
A lot of older propeties here were fitted with a bathtub, but no shower. I guess when people decide to renovate the bathroom themselves, they figure that the instant electric shower will be easiest for the reasons we've discussed above.
One of the problems I see is they sometimes don't allow for the extra cost of the wiring. Some of the cheapest units are selling here for as little as £50 (about U.S. $75 or N.Z. $160). I've come across people who buy one thinking that the whole shower installation can be done for £100 or so. Sometimes they do even realize the cost of 10mm cable and buy that along with an isolating switch. But that's often as far as it goes.
The typical older place that these DIY projects take place in already have distribution panels with no spare fuseway. I did an estimate for one such place a couple of months ago when I was asked "How much to just connect it up if I install the shower?"
I had to explain that I would need to replace the main panel, complete with all new C/Bs, etc. The quote came to around £290 (N.Z. $950) for the whole job.
Some folk seem to think that you can just tap a 9kW shower into an existing ring circuit.