Installation and Connection of Domestic Hot Water Cylinder

Cylinders differ mainly in what heats the water inside them. That decides how the whole installation is done.

• Direct electric cylinder: an electric heating element inside the tank heats the water. It is the simplest to install and does not depend on a boiler. It suits flats and homes with no other heat source
• Indirect cylinder: inside is a coil, a metal spiral that carries hot water from the boiler or heat pump. The water you use never mixes with the heating water. It only takes heat from it. If you already have a boiler, this is usually cheaper to run
• Combined cylinder: it has both a coil and an electric element. It makes sense if you only run a solid fuel boiler in winter and want electric heating in summer, if you want a backup when the boiler breaks down, or if you want to use surplus power from solar panels
• Wall-hung or floor-standing: smaller cylinders hang on the wall and are common in flats. Larger ones stand on the floor, usually in a utility or plant room of a house

The size should match how many people live in the home and how they use hot water. A cylinder that is too small runs out quickly. One that is too large keeps water hot that nobody needs. If you are unsure, talk to the installer before you buy.

A good installation starts with a look at the site. The installer checks whether the wall can carry a full cylinder. Water weighs roughly a kilogram per litre, so a wall-hung cylinder full of water is very heavy. The installer also looks at where the water, electrical and heating pipes run and where the discharge from the safety valve can go. The work then usually follows these steps:

1. Turn off the water supply and isolate the electricity. When replacing a cylinder, drain and remove the old one.
2. Fit the wall brackets, or prepare a level, solid base for a floor-standing cylinder.
3. Fix the cylinder in place and check that it is vertical and secure.
4. Fit an isolating valve and a safety valve with a non-return valve on the cold supply, as the manufacturer specifies.
5. Run the discharge from the safety valve through a visible funnel (tundish) into a drain.
6. Where needed, fit an expansion vessel designed for drinking water.
7. Connect the hot water outlet to the pipework, and the secondary return pipe if there is one.
8. For an indirect cylinder, connect the coil to the boiler circuit.
9. Fill the cylinder, bleed out the air by opening a hot tap, and check every joint for leaks.
10. Only once the cylinder is full, switch on the heating, set the thermostat and confirm the water is getting hot.

The electrical connection of the element must be done by a qualified electrician, with its own circuit protection and proper protection against electric shock under local wiring regulations. Switching on the element while the cylinder is empty is a common mistake, and it ruins the element within moments.

In some countries, unvented cylinders that run at mains pressure may only be fitted by a specially certified installer, and the installation has to be registered. Check what applies where you live.

With an indirect cylinder, connecting two pipes is not enough. The boiler has to know when to heat the cylinder and when to heat the house. That is why the following points matter:

• Pipe connections: the flow and return of the coil are connected as the manufacturer marks them. If they are swapped, the cylinder heats slowly and unevenly
• Circulation pump or diverter valve: these send hot water from the boiler into the coil exactly when it is needed
• Temperature sensor and controls: a sensor in the cylinder tells the boiler the water temperature, and the boiler switches on to heat it when needed
• Hot water priority: on most boilers, heating the cylinder takes priority over heating the house, so hot water is ready quickly
• Bleeding and pressure: after connection, the heating circuit is bled and topped up to the pressure the boiler manufacturer specifies

Check that the cylinder and boiler actually work together. Not every boiler has a connection for a cylinder sensor, and not every coil is large enough for a heat pump, which runs at lower water temperatures. Heating systems are designed according to EN 12828.

If the job involves any work on a gas boiler or gas pipework, only a qualified, registered gas engineer may do it. If you smell gas at home, do not use light switches or naked flames, open the windows, leave the room and call your local gas emergency number or 112.

The amount of work can vary a lot, even with the same cylinder. The job is usually simpler when:

• You replace like for like with the same connection sizes
• Water, electrical and drain connections are right next to the installation spot
• It is an electric cylinder with no boiler connection

These situations make the job take longer:

• Switching from electric heating to heating via the boiler, where new pipework and controls are needed
• A combined cylinder fed by several heat sources
• A weak or hollow wall that needs different fixings
• Tight spaces, lofts or cellars with narrow stairs that a large cylinder barely fits through
• Old corroded pipes that snap when the old cylinder comes out

In a block of flats, talk to the building management or owners' association first, especially if the work affects shared pipework. Simply replacing a cylinder does not usually need building approval. If it is part of a larger renovation or a change of heating system, check with your local building authority what you need.

In practice, the same mistakes come up again and again:

• The safety valve is missing, or there is a shut-off valve between it and the cylinder, which is dangerous
• The safety valve discharge is not piped to a drain, so water drips on the floor or someone "fixes" the valve by blocking it
• The cylinder is switched on before it is full
• The coil flow and return are swapped, or badly set controls make the boiler switch on and off constantly
• Weak fixings that work loose over time
• No earth connection or no separate circuit protection for the electric element
• Installation that ignores the manufacturer's instructions, which can void the warranty

If the water is kept at a low temperature for long periods, bacteria such as Legionella can grow in the cylinder. Manufacturers therefore usually recommend heating the water to a higher temperature from time to time, and many controls do this automatically.

At handover, ask the installer to show you how the cylinder works, and go through these points:

• The cylinder is firmly fixed, does not move and is vertical
• No joint drips, even after the water has heated up
• The safety valve is fitted, there is no shut-off valve between it and the cylinder, and the discharge runs through a funnel to a drain
• The isolating valve on the cold supply is easy to reach
• The water reaches the set temperature and the thermostat holds it
• With boiler heating, the boiler responds when the cylinder calls for heat and the heating circuit has been bled
• With electric heating, you have a certificate for the electrical work from a qualified electrician
• If gas appliances were worked on, you have a record from the registered gas engineer
• You receive the manual, the warranty card and an explanation of how to drain the cylinder and when to check the anode