by Chris Woodford. Last updated: January 8, 2015.
Are you a shower person or a bath person? Do you like to feel pulsing jets of water blasting your dirty skin clean? Or do you prefer to laze luxuriously in the tub until the water turns cold? If you wash for less than five minutes and your shower has a modern, low-flow head, you'll save over 50 gallons of water and a huge amount of energy compared to taking a full bath. Saving hot water with a shower helps to cut your energy bills and it's a simple thing you can do to fight global warming. Sounds good? Let's take a closer look at how showers work!
Photo: A simple Mira electric shower. There are two controls for adjusting the temperature. The upper dial sets the temperature broadly as high, medium, or low. Once you've set the temperature roughly, the lower dial lets you fine tune to get the water exactly as hot as you want it.
How do mixer showers work?
The simplest showers are called mixer showers and, as their name suggests, they work by mixing hot and cold water from separate pipes to make warm water whose temperature is somewhere in between. The most basic form of mixer shower is a Y-shaped rubber pipe that you fit over the hot and water faucets (taps) on a bath-tub. By adjusting the faucets, you create a single stream of water at exactly the temperature you want.
The trouble with mixer showers like this is that the temperature is hard to control. If someone switches on a cold faucet or flushes a toilet elsewhere in your home, the cold water supply is suddenly reduced. That means there's proportionally more hot water coming through your shower and, if the water's too hot, you could be scalded. The opposite will happen if someone switches on a hot faucet—you'll suddenly find the shower turning freezing cold!
Mixer showers that are plumbed into the wall overcome this problem by using built-in thermostats. They constantly adjust the temperature of the mixed water to ensure you're not boiled like a lobster or frozen like a penguin by water that's alternately too hot or too cold. Most mixer showers also have safety cut-outs that prevent you from turning the water up to dangerously high levels, which is good news if they're being used by frail elderly people or young children.
But there's still a basic problem with mixer showers: they typically run off hot water from a tank. Once the tank is empty, there's no more hot water and you have to wait for the tank to fill up before you can shower again.
Photo: Left: A typical, thermostatically controlled mixer shower. Hot water and cold water flow in through the two pipes in the wall (top), mix in the valve unit in the center, and flow out through the curly hose at the base, which connects to the shower head. This is a really simple shower: you just turn the knob to switch the water on cold, then keep turning it clockwise to make the water as hot as you wish.
Photo: Right: Take it apart, however, and it's a bit more complex! The magic component is that little black cylinder in the middle, which is a wax thermostat. It contains a sealed capsule of wax that expands or contracts, opening and closing the hot water valve and so keeping the overall temperature of the mixed water constant.
How does an electric shower work?
Electric showers overcome this problem by heating cold water with electricity. They never run out of hot water, so they're a great solution if you have lots of people in your house who like to shower one after another.
Electric showers work in much the same way as other electric appliances that get hot, including electric toasters and hair dryers. They send an electric current through a piece of metal called a heating element. This has a moderately high resistance, so it gets really hot when electricity moves through it. Cold water flows past the element, picking up heat and heading out through the nozzle where you're standing.
You probably know that water and electricity are usually a very dangerous combination, but electric showers are perfectly safe if they're properly fitted. That's because a heating element is a completely sealed unit. The electric current flows through the element, but not in such a way that it can give you a shock. No electricity comes into contact with the water that touches you.
You can adjust the temperature of an electric shower by turning a dial, which is usually marked with a scale running from blue (for cold) to red (for hot). In basic electric showers, this dial simply adjusts the flow so more or less water streams past the heating element, making the water that comes out of the shower head colder or hotter. In better showers, pressure-balancing valves, a built-in water mixing tank (which helps to avoid sudden changes in temperature when you turn the dial), and various thermostats and flow sensors keep the temperature and water pressure as safe and steady as possible. That's a tall order, because the water feeding into an electric shower is much more variable in temperature and pressure than you might think. In wintertime, the incoming temperature might drop as low as a few degrees Celsius, while in summertime it's very likely to be ten degrees hotter. It's a real challenge for a shower to maintain a decent water flow rate and a constant water temperature all year round. (That, incidentally, explains why, even if you never change the temperature control of your shower, the water that comes out of the hose might feel hotter or colder than it was last time you showered.)
For an electric shower to work effectively, you need a cold water supply with reasonably high water pressure to begin with, because the shower heating unit will reduce the pressure of the water as it flows through. If you don't have enough water pressure for an ordinary electric shower, the solution is to fit a power shower. It takes in and heats cold water just like an ordinary electric shower, but it also uses an electric pump to increase the water pressure so it leaves the nozzle with greater force and higher speed. If saving water and energy is important, bear in mind that power showers use 3-5 times more than ordinary showers. That means a lengthy power shower can easily cost you as much as a generous bath-tub full of hot water!