What are the different types of hot water systems?

There are three main types of water systems used in our homes.

Mains Pressure Combination Boiler:

Mains pressure combination boilers have become more popular in the UK. This type of system will have a 'combination' or 'combi' boiler which is normally hung on an external wall, they are a high efficiency water heater and central heating boiler in a single compact unit. Combi boilers rely on mains pressure to transport water around your home, heating water directly from the mains when you turn on a tap so there’s no need for a hot water storage cylinder or a cold water storage tank in the roof space.

The output of a combination boiler is typically around 20 psi = 1.8 bar = 18m head (refer to your boiler instructions manual for full details). The available pressure means you should have a good flow rate with this type of water system from taps and showers.

Here is a video from plumberparts.co.uk, which gives a great explanation on how this works.

https://www.youtube.com/watch?v=Ixh814fICEQ

Mains Pressure Unvented System:

An mains pressure unvented system will have a boiler and a separate hot water storage tank and are commonly found in properties that do not have the space for a cold water storage tank which is normally in the loft space.

This type of system stores mains pressure water in a large strengthened hot water tank (cylinder), usually found in the airing cupboard. The hot water is heated either by immersion heaters fitted into the side of the hot water cylinder or by a wall mounted central heating boiler. It is then carried by an outlet pipe around the property and as a tap or shower is turned on, the pressure of the incoming cold mains water to the cylinder disperses the hot water to the tap, shower or outlet.

As the system is kept under pressure from the incoming supply of water from the mains cold feed, it provides hot water at a similar pressure to the mains pressure coming in. The system is called unvented as the cylinder has no open vents to keep the water under pressure.

Mains pressure unvented systems normally have around 1.5 - 1.8 bar of pressure (refer to system manual for full details). The system provides a high water flow rate and good water pressure so typically you wouldn't have difficulties with taps or showers.

Low Pressure Gravity System:

Low pressure gravity systems are one of the most common domestic water systems installed in homes in the UK and are most often found in older properties. A low pressure gravity-fed system generally consist of a cold water tank (cistern) usually situated in the loft and a hot water tank (or cylinder) on the first floor - normally in the airing cupboard.

The low pressure gravity system is named as such because the water is kept under pressure via the gravity-fed water delivered from the cistern. This type of system relies on a combination of factors, commonly the mains pressure to your home and the distance between the cold water tank and your water outlet, both of which can both affect the pressure and flow rate you receive.

The pressure your system offers is determined by the height at which the base of the cold water cylinder (the bottom of the tank) is situated above the outlet (shower head/tap). The amount of pressure is calculated in bar and is worked out to be roughly 1m fall = 0.1 bar of pressure, so the greater the height of the cistern above the water outlet the more pressure is created.

Low pressure gravity systems affect the flow at any outlet so showers and bath fillers would need to be suitable for a low pressure system. For example, if the vertical distance between the base of your cylinder and an outlet such as a shower was 5m this would roughly equate to 0.5 bar. This is the maximum available pressure, if the route the pipes take is not direct though or has lots of bends or long horizontal runs the available water pressure will reduce.

If you live on the higher floors of a tall building or draw your cold water from a private supply, you should also check the available cold water pressure and use the stated minimum water pressure in these cases for both the hot and cold supplies to receive the maximum available flow.

There are some ways that the pressure or flow rate can be improved on a low pressure gravity fed system, such as fitting a water pump. Click here to read out blog guide to water pressure and how you may be able to improve your water flow rate.