Building Construction, Renovation & Maintenance

Electric Water Heaters


Electric Storage Water Heaters

Electric storage water heaters have a simple design, are mass produced so they are not expensive and they go on forever needing virtually no attention which is why they have become most common water heaters in the world. Here we look at their design, how they work, their important components, how large a heater you will need and their operating problems.


See the full Fixed Abode article "A Well Scrubbed Pig" here


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Electric storage water heater

Hot water is good for hygiene.

We all know that hygiene is important but even so, I was surprised recently to come across an old lady who, in spite of living in fairly dire poverty, was boiling water to wash the family's towels. She knew all too well of the importance of hygiene and that the most basic and effective form of hygiene is to use hot water for washing.

The vast majority of people around the world do not have running hot water. There are also quite a number of expatriates who have got used to hot climates and also don't have water heaters.

Times are changing and there are an increasing number of people across the world who have hot running water in their houses and by far the majority of these use electric storage water heaters.

Heaters of this type are not expensive to buy and are very convenient to use. They sit somewhere out of the way delivering hot water continuously for years without us giving them a second thought. One day our water heater will inevitably remind us of its existence by breaking down and we will have to either repair or place it.

A standard electric storage water heater.

A typical electric hot water heater consists of a cylindrical tank (which may come in either vertical or horizontal configuration) made of steel with an electric heating element to heat the water. The storage tank may be made of stainless steel or with a "glass" coating (stove enamel) to stop it rusting. The tank has a thick coating of expanded polyurethane insulation around it and a thin sheet steel cover over this.

Thermostat

A thermostat switches the heating element off when the water reaches its desired temperature which can be adjusted by means of a rotary knob usually on the bottom of the heater. The thermostat is usually a brass rod contained in a copper tube which extends into the tank with a temperature sensor attached to the exposed end.

Normal the temperature would be set at 60 to 65 degrees C.

Sacrificial anode

The water tank often has a sacrificial anode (a piece of zinc or magnesium) in side it which corrodes instead of the steel tank thereby protecting the steel. These anodes are usually designed to last between 5 and 8 years. It should be noted that heaters have a built in obsolescence in that once the sacrificial anode corrodes away the tank will start to rust. It is important that the heater is properly earthed to make sure the anode works correctly.

Electric water heaters are usually pressurised these days. This has a distinct advantage over the non pressurised systems of former times in that you can pipe them into your water supply pipework and the pressure in your hot and cold water taps will be virtually the same. This does mean that you need to be aware that the heater is a pressure vessel and should be well manufactured, handled with care and properly installed. The pressures found in your house water system, typically 20 to 50 pounds per square inch can be easily contained in a small diameter pipe but puts a considerable force onto the large internal surface of a water tank. Because of the risk of explosions most countries around the world have strict standards which govern the design and manufacture of pressure vessels.

Pressure relief valve

Pressurised tank water heaters should always be fitted with a pressure relief valve in the system which in many cases doubles as a bleed valve to allow air to be let out of the tank.

How big a water heater do I need?

I recently came upon a man who had done all the right calculations and had installed a fine 300 litre water heater (that is about the size the British army used to clean up the lads after the Battle of the Somme). When he switches it on the lights on the bypass go dim momentarily before his electrical circuits cut out.

You will probably find that, in fact, you don't need a lot of hot water in tropical climates. Bali is close to the equator and most people find they only need enough hot water in their shower to raise the temperature a small amount.

If you wish to fill a bath this is a different matter. Architects these days often have a tendency to believe that a luxurious house must have a very large bath without thinking about how much water it will take to fill it. Shortly after moving into their new residence proud new owners may try and fill the bath only to find that they run out of hot water just as the water level has covered their toenails. They may enquire about a larger water heater only to find that their electricity supply can't cope with it and the bath ends up never being used. If you are designing a house and you really want a bath it is probably a good idea to consider the amount of water your bath is going to need and make sure you buy a large enough water heater.

As a rule of thumb most houses use an 80 litre capacity water heater for a single bathroom and a 100 litre capacity heater for two bathrooms. For 3 bathrooms use an 80 litre and a 100 litre unit.

I recently came across a couple with a 50 litre tank for their bathroom and they say that they never have problems.

How much electricity do you have?

When deciding the size of water heater you will need to consider the amount of electricity it will use and whether you have enough power. Large heaters need large amounts of power to heat them up and cause palpitations for the staff down at PLN to say nothing of your electricity bills. Typically a 50 or 80 litre heater will have a heating element which uses 1,200 watts of power while a 100 litre uses 1,500 watts. Higher power means the water heats more quickly, useful if you have a well used bathroom. Some heaters use far more power than this particularly in Europe or America where there are not the same limitations on how much power is available and people don't expect to have to wait long for their water to heat up.

How hot will a water heater heat the water?

The temperature of water in water heaters is usually set around 60 to 65 degrees C. The temperature shouldn't be set much higher than this and, in fact, in many countries there are legal limits regarding allowable water temperatures to protect people from being scalded.

It is a good idea to keep pipework short between the tank and the bathroom or you'll need a good book to read waiting for the water to arrive.

Water heaters and typical maintenance problems

These heaters are low maintenance and will usually run for years without attention. Problems that may occur are:

  • Burned out heating elements which may be caused by chlorides in the water attacking the heating element, by running dry or simply element failure.
  • Thermostat failure which may mean the heating element will not switch on. Well designed systems are designed to fail in the off position to prevent overheating.
  • Build up of air in the heater lowering the water level below the water outlet pipe. This is easily solved by opening the pressure relief valve to release the air.
  • Rusting of the internal storage tank which means you have to buy a new heater.
  • Rusting of the outer casing - a common problem when mounted outdoors close to the sea.

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Copyright © Phil Wilson July 2010
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