Boilers are special purpose water heaters. Whereas conventional systems carry heat through warm air, boiler systems distribute heat in hot water, which in turn gives off heat as it passes through radiators or other appliances in rooms throughout the house. The cooled water then returns to the boiler to be heated again. Hot water systems are often called hydraulic systems. Residential boilers usually use natural gas or heating oil for fuel.

In steam boilers, which are much less common in homes today, water is boiled and the steam carries the heat through the house, condensing into water in radiators as it cools. Oil and natural gas are widely used.

Instead of a fan and duct system, a boiler uses a pump to circulate hot water through pipes to radiators. Some hot water systems circulate water through plastic pipes in the floor, a system called radiant floor heating (see “State-of-the-Art Heating”). Important boiler controls include thermostats, aqua stats and valves that regulate circulation and water temperature. Although the cost is not insignificant, it is often much easier to install “zone” thermostats and controls for individual rooms with hydraulic systems than with forced air. Some controls are standard features on new boilers, while others can be added to save energy.

As with furnaces, condensing gas-fired boilers are relatively common and are significantly more efficient than non-condensing boilers (unless very complex controls are used). Oil-fired condensing boilers are rarely used for a variety of reasons related to low latent heat potential and the potential for greater fouling with conventional fuel oil.

How do different boiler types work?

Although the basic process of burning fuel and sending water to radiators and taps is the same for most boilers, each boiler type works differently.

Below we summarise how each different boiler type works to provide heating and hot water:

Heat only/conventional boilers

Only heat boilers are the traditional form of home heating. They operate with an additional cold-water tank for water storage and a hot water cylinder.

These systems are also built with a feed and expansion tank that brings in cold water to regulate water levels. This tank helps to cope with the expansion that occurs when water is heated and also replaces water lost through evaporation or leakage.

The cold-water tank is filled with water from the mains. When the boiler fires, it sends hot water to the hot water cylinder or radiators. As long as there is enough hot water in the cylinder, you should be able to access water from more than one tap at the same time.

These traditional boilers can be fitted with a back-up submersible heater to give you access to hot water in case you encounter a boiler failure.

system boilers

System boilers provide heating and hot water using an additional hot water cylinder. Most of the main heating components are already built-in, so they do not require any storage tanks. They operate similarly to heat-only boilers, but require fewer additional components.

System boilers are sealed pressurised systems. Cold water is supplied directly from the mains using a filling mechanism. This water is heated and sent to the hot water cylinder or radiators where it is stored. When you need hot water, the system will send the water to your taps.

Find the right boiler for your home and your needs

Enthalpy can help you choose a suitable boiler for your home. We install a range of combi, heat only and system boilers from some of the industry’s leading brands. Our engineers can provide the help and advice you need to achieve quality heating and hot water in your home.