Guide written by:
John, DIYer & IT developer, Brighton
Direct hot water cylinders are electric storage water heaters that range from 50 to 200 litres in size. When choosing your direct water heater, consider the number of people in your household, the element and anode rod type, and the type of water where you live. Read on for our top tips!
- Hot water cylinder size
- Anode rod and cathodic protection
- Immersion heater
- Vertical or horizontal cylinder
- Hot water tank features
4 steps for choosing your storage water heater
You don't have to know everything there is to know about water heaters in order to choose the right hot water cylinder.
The following four points will help you find the right direct water heater for your needs:
- Vertical or horizontalcylinder: installation type will depend on your space.
- Cylinder capacity: 75 litres for 1 person; 100 litres for 2 people + 50 litres for each person after that.
- Heating element: choose between single and dual element water heaters, and low- to high-density.
- Anode rods: entry-level sacrificial anode rods for medium hard water; impressed current rods for hard water;
- Thermostatic mixing valve: to limit water temperature to 50°C at the outlet.
- Temperature / pressure relief valve: regulates cylinder pressure.
Components of hot water cylinders
Parts of a hot water cylinder
Direct water heaters, or hot water cylinders, function using electricity. They are made up of:
- a lined tank; volume measured in litres (l) according to hot water demands of the household;
- a water inlet and outlet;
- single or dual immersion element(s) used to heat the water;
- a thermostat that allows you to set temperatures and trigger the immersion heater to heat up the water;
- an anode rod for anti-corrosion protection;
- electrical supply;
- an access panel;
- LED light (optional);
- a dip tube to ensure your water is circulated and heated evenly.
Choosing a hot water cylinder size
Hot water cylinder capacity: 50 to 250 litres
The capacity of your cylinder will really just depend on how much hot water you consume and the number of people in your household. Generally speaking, you should account for 50 litres per person. Nonetheless, it is worth keeping in mind that a shower will use up around 30 to 60 litres of water while a bath can consume up to 150 litres. Washing your hands will use up a further 5 litres. Running the dishwasher will use another 30 litres. Basically, if everyone in your home takes showers, washes their hands with cold water and you have a dishwasher, 50 litres per person should be largely sufficient. However, if everyone takes baths, washes their hands in hot water every five minutes and you wash your dishes under running water, calculating 50 litres per person may lead to a few cold showers! In this case, you should pick a cylinder on the larger side.
Calculating your hot water cylinder capacity
Number of people
1 adult + 1 child
2 adults + 1 child
2 adults + 3 children
2 adults + 3 children
2 adults + 4 children
*It's a good idea add an extra 25 litres to make up for any unexpected increases in hot water usage (guests, etc.).
Formula for hot water consumption
In order to make your calculations more precise, use the following formula: firstly, you will have to find out your overall daily water consumption using a water meter. The water you use will generally be limited to 40°C while this temperature is around 65°C in the hot water tank. You therefore need to divide your overall water consumption by 1.8 to find out the actual amount of hot water drawn from the tank. Why do you need to divide this number? Simply because you are always mixing your hot water with cold water so you will never use as much hot water as you think.
Vertical or horizontal hot water cylinder
Choosing where to install your hot water cylinder
Hot water cylinders take up a lot of space meaning it can be tricky to find room in your home. Fortunately, manufacturers offer a range of different shapes to meet all needs.
Vertical cylinders are the most common type of hot water cylinder.
However, if the layout of your home does not allow for an upright hot water cylinder, you can purchase a horizontal cylinder. These tanks can fitted to the ground in a roof space, to the ceiling (if your ceiling height allows) or even on a wall.
Please note: you will have to select a model with a capacity of 50 litres more than the equivalent vertical cylinder.
The capacity of these hot water cylinders is limited to 200 litres.
Hot water cylinder stands
Installation on a load-bearing wall
Storage water heaters store a lot of water and are therefore very heavy. Cylinders weighing up to 200 kg can be installed to load-bearing walls using fixing plugs.
Installation of cylinders weighing over 100 litres
Any cylinder with a capacity over 100 litres that cannot be installed on a load-bearing wall should be installed on feet or a cradle.
Installation of cylinders weighing over 200 litres
Any cylinder holding over 200 litres must be installed on a stand. Be sure to check that your floors are able to withstand the weight of your tank (this isn't always the case in older properties). If you have any doubts, contact a professional.
Extra-flat hot water cylinders
It is also possible to find extra-flat or space-saving hot water cylinders. These are designed to fit into smaller spaces.
While these tanks are attractive, they are more expensive.
Low-density vs. high-density density elements
There are two different types of elements: low density and high density.
High-density heating elements
These elements feature a smaller surface area and generate a lot of heat. High-density elements gather more limescale due to their smaller surface area. As such, they are only recommended for use in areas with soft to medium water as your electric consumption will increase as the element degrades.
Low-density heating elements
These elements are feature a greater surface area and use the same amount of energy as their high-density counterparts. As these elements feature a larger surface area, they do not wear down as quickly. They produce less heat per square inch than high-density elements meaning they attract limescale more slowly. This is an important factor to take into account if you live in an area with hard water.
Heating elements are controlled by a thermostat which controls the temperature of the water based on set temperatures.
Water heater anode rods
The tank of your water heater is covered with a glass lining to protect it from corrosion. However, this lining can feature tiny cracks where rust can set in. To prevent these micro cracks from corroding, all hot water cylinders are equipped with an anode rod which acts as an anti-corrosion device (known as cathodic protection).
Hard vs. soft water
Hard water has a high mineral content (a rate of 200 mg of calcium carbonate per litre), meaning it contains a lot of limescale. Soft water contains has a lower calcium and magnesium content, i.e. it contains less limescale (60-120 mg of calcium carbonate per litre). The hardness of water combined with its acidity determines how corrosive it is: soft water (that is more acidic) tends to be more corrosive.
Magnesium or sacrificial anode rods
The magnesium of the anode rod attracts the corossive particles to protect the tank. Sacrificial anode rods represent the first generation of anode rods and are used for low-end storage water heaters. These anode rods are recommended for use in areas with moderately hard water. Once the anode rod has worn out, you will need to replace it. You should keep an eye on your anode rod to ensure it has not worn out prematurely.
Impressed current anode rods
These electric-powered titanium anode rods generate a low voltage current which prevents corrosive particles contained in the water from attacking the metal of the tank. These anode rods are not sacrificial and are advised for use in areas with very hard water.
These anode rods are made of titanium with a magnesium coating, to combine the action of the two elements. The magnesium is sent out to weak points in the tank using an electrical current. Hybrid systems are considered high-end anode rods as they are more resistant to corrosive and hard water.
How does an anode rod work?
It's worth noting that some level of water hardness will help protect your hot water cylinder.
While hard water will make your water heaterless efficient, soft water tends to corrode your cylinder more quickly and shorten its service life. A sacrificial anode rod will break down fairly quickly and even impressed current or hybrid systems can have a shorter service life than promised.
If you have a water softener, adjust this according to settings provided by the manufacturer. Water containing 60 mg/l of calcium carbonate is generally considered as soft; 60–120 mg/l is moderately hard; 120–180 mg/l is hard; and more than 180 mg/l is very hard.
Shorter service life than promised
Installing a storage water heater: accessories
Various installation accessories and safety features are available for storage water heaters:
- Dielectric unions are used to avoid the risk of two different metals corroding.
- An expansion tank is used to store excess water caused by thermal expansion. This will allow you to make savings as the water is not lost but rather reintroduced into the circuit. You will need an 8 litre expansion tank for a cylinder capacity of 100 to 150 litres, an 11 litres expasion tank for a cylinder capacity of 150 to 200 litres and a 25 litre expansion tank for a cylinder of 200 to 300 litres.
- A temperature / pressure relief valve (or T&P valve) must be fitted to relieve excess pressure or temperature.
- A drain valve is used to release water from the cylinder.
- A shut-off valve is essential for managing incoming water when draining or inspecting the cylinder.
- A thermostatic valve is used to limit the temperature of the water to 50°C at the outlet of the water heater.
Safety features of a hot water cylinder
Your cylinder will be equipped with a number of features to avoid any accidents including a T&P valve, drain valve and thermostatic valve.
The drain valve is used to flush mineral deposits out of the tank. It can also be used to drain the water heater.
T&P valves are used to relieve your cylinder of excess pressure or temperatures that are too high. Once the pressure returns to a safe level, the valve automatically closes.
Thermostatic valves are used to maintain a safe water temperature. They do this by adding cold water to your hot water at the cylinder outlet to limit the temperature of the water to 50°C.
Water heaters must be fitted with a junction box, as for any major electric appliance. They must be protected by a 30-amp, double-pole circuit breaker.
3 tips for saving money
- Install a timer. A timer can be used to heat water over night in dual element water heaters. This can allow you to make save money as your hot water tank will consume electricity at night when it is less expensive. You can find yourself making fairly significant savings by the end of the year.
- Place the water heater in a warm room or take care to insulate the water heater properly if placed in a colder room.
- Limit the temperature of the water in the tank to 65°C. This temperature is sufficient to kill legionella bacteria, but will avoid consuming too much energy.
Guide written by:
John, DIYer & IT developer, Brighton, 70 guides
Since I was a child, I was always interested in manual and technical works. Always fascinated by woodworking, I took advantage of my first flat as a playground. On the cards: electricity (of course, safety first!) and some partition walls; but also decorating with the help of the missus, made-to-measure furniture and little tricks to optimise the space, all the while remaining as original as possible. When the little one arrived, I started building bits and pieces for him! Lacking space, I have not got a permanent workshop and certain tools I dream about but are not part of my collection. Not to worry, I already know a lot about DIY and I have a high-tech profile that I hope will guide you in your decisions!