• Conventional gravity fed system
• Combination Boiler System
• Mains or High Pressure System

As a brief guide only, these systems are described in the following sections.

Water may be supplied to the boiler's heating circuit from a small Feed and Expansion tank often situated in the loft. This tank ensures the system is kept topped up and allows for any expansion or evaporation that may occur.

Alternatively, a System Boiler may be used which has no need of a separate expansion tank.

The hot water cylinder is supplied from another larger tank often situated in the loft. As you open a hot tap, water drawn from the cylinder is replenished by this tank.

Water is supplied to the loft tank/s by the rising main, usually through a ball cock valve and are themselves replenished when levels drop.

Because the tank/s in the loft are isolated from the main supply by a valve, they use only their height to force water down by means of gravity. The pressure at your hot tap is proportional to the distance of the tank above the outlet.

Such a system is therefore known as a Gravity System.

Positives: Suitable for addition of power shower pump to provide exceptional flow rates. 

Negatives: Uneconomical for space. An unpumped system may offer low pressure at taps. 

Economy: Water must be heated and stored whether or not it's used

Typical System Requirements: Boiler, Hot Water Cylinder, Loft Tank/s, Rads.

Offering the advantage of space saving, Combi Boilers are popular.

They can however have the disadvantage of lower hot water flow rates - more often noticed when filling a bath or when simultaneous supply is called for at more than one outlet.

Not particularly suitable for properties with more than one bathroom or shower.

Boilers with bigger and better flow rates are being achieved but performance is still a little lacking.

Positives: Compact unit requiring no external tanks - good for space saving. Perform well with showers. Endless amounts of hot water on demand.

Negatives: Typically lower flow rates than other systems. Baths can take a long time to fill. Will not support simultaneous demand from several outlets. You cannot install a shower pump.

Economy: You only heat what you need.

Typical System Requirements: Combination Boiler Only - (No external tanks) - Rads.

The term typically refers to the pressure you achieve at your hot taps. Mains pressure systems provide mains pressure hot water.

This is only an advantage if you enjoy good mains pressure within your property. If your supply pressure and flow rate are poor there is little point.

There are different system types that can offer this facility and you are recommended to examine them carefully before you decide which is best for you.

Remember, hot water at high pressure can present a risk. Some systems require notification to building control, certified installation and commissioning, and annual servicing to ensure they continue to function safely. A system storing hot water under pressure that is incorrectly installed or poorly serviced can be dangerous.

Positives: Hot water at mains pressure throughout your property. Potentially high flow rates. No feed or expansion tanks in loft.

Negatives: Slightly more costly to install. Some HP Systems require certification and annual maintenance for reasons of safety. Pressure and flow only as good as your incoming main. You may not install a shower pump on a mains-fed system.

Economy: Choosing a system type requiring annual maintenance will lock you into long term cost. Water must be heated and stored regardless of use.

Typical System Requirements: Boiler, Pressure Vessel or Thermal Store, Rads.

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