Back to basics - cylinders

Ever wanted to know about installing cylinders? Here are Graham’s top tips.

What’s the difference between an unvented and a vented cylinder?
Unvented systems work by drawing water direct from the cold-water mains, which means that there is no need for a cold-water tank in the loft. The cylinder is pressurised, so is supplied with a number of components to control pressure and expansion. Because the cylinder uses mains pressure, there’s a more even flow of hot water, and because the system is sealed, there is less risk of contamination.

Vented cylinders are connected to a cold-water cistern via a cold feed pipe, with pressure relieved through a vent pipe that is open to the atmosphere that discharges into the cold cistern.

What is the difference between direct and indirect heating?
Indirect systems (the most common type) get heat from an external source (gas boiler, solar panels etc). The heated water then circulates through a coil in the cylinder, which heats the surrounding water. The coil is usually made from the same material as the cylinder – for example, a stainless-steel cylinder, such as a Megaflo will have a stainless-steel coil. 

A direct heating system uses an internal heat source, such as an electric immersion heater, in the cylinder. These types of system are most common when the property has no access to mains gas.

Do I need to be qualified to fit a hot water cylinder?

You must be G3 certified to fit and maintain an unvented cylinder. To get on to a G3 course you will need an NVQ Level 2 or equivalent.  Find out more about our Unvented water storage systems (G3) course here

Does the size of the cylinder matter?
It is not true that a bigger the cylinder performs better. It is crucial that the size is in proportion to demand. When sizing a cylinder, consider how much hot water will be required and how often, as the recovery time will depend on the heat source. Our technical helpline will help you calculate the correct size of cylinder you need.

Does a hot water cylinder need to be serviced?
We advise servicing the cylinder every 12 months to keep it performing at its best.
Here are the five key checks you need to carry out: 
Check that the electrical components and wiring are safe
Check whether the temperature and pressure relief valve and expansion valves are working 
Check that there are no blockages in the tundish drain 
If the cylinder has an anode, this should be checked and replaced if necessary
Check the condition of the seals

How do I regenerate the air gap in a Megaflo?
With systems that use an internal expansion volume, such as the Megaflo, the air gap is regenerated by following these steps: 
1. Turn off the mains cold water supply 
2. Open the lowest hot tap in the property 
3. Hold open the Temperature & Pressure Relief Valve (TPRV) until the gurgling noise stops and water stops running from the tap and valve
4. Close the TPRV and open the mains cold water supply . When water flows from the hot tap close the tap.
5. If water continues to drip or flow from the safety valves, the  cylinder requires attention. Switch off heat sources (immersion heaters and boiler), DO NOT turn off water supply.
We have a handy video to help here.

We hope your cylinder questions have been answered.  For more information, visit