Gas central heating boilers

Some time ago, people would have given an arm to have central heating in their homes. Imagine the inconvenience of having to light wood and coal fires in every room to keep your house warm. The idea of central heating is simple: you have an easily controllable boiler, fuelled by gas, in a handy place like your kitchen or bathroom and it uses water, moved by an electrical pump, to carry heat into radiators in all the other rooms. It’s simple, efficient, and it makes even winter days a pleasure to endure!

The main role of the boiler

The boiler is the most important piece of a central heating system. Imagine it to be like a big fire that has a continuous supply of natural gas, LPG, or oil streaming into it. When you want to heat your home, you switch on the boiler with an electric switch. A valve opens and gas enters a sealed combustion chamber in the boiler through lots of small injectors with an electric ignition system setting them alight. The water then gets pumped through a heat exchanger to absorb the heat from the flame.

The water pipe is actually one small section of a large, continuous circuit of pipe that travels in and around your home. It passes through each hot-water radiator in turn and then returns to the boiler. As the water flows through the radiator circuit, it gives off some of its heat through convection currents and warms your rooms in turn. By the time it gets back to the boiler again, it’s cooled down quite a bit ( up to 21 degree difference). An electric pump inside the boiler (or very near to it) keeps the water flowing around the circuit of pipe work and radiators.

How do thermostats help?

A basic system like the one described is entirely manually controlled—you have to keep switching it on and off when you feel cold. Most people have heating systems with electronic programmers attached to them that switch the boilers on automatically at certain times of day (typically, just before they get up in the morning and just before they get in from work). An alternative way of controlling your boiler is to have a thermostat on the wall. A thermostat is like a thermometer crossed with an electric switch: when the temperature falls too much, the thermostat activates and switches on an electric circuit; when the temperature rises, the thermostat switches the circuit off. So the thermostat switches the boiler on when the room gets too cold and switches it off again when things are warm enough.

How do radiators work?

A hot water radiator is simply a lump of hollow metal
with a maximum area. The heated water follows the ridged lines. Water enters and leaves through valves at the bottom.

Many people are confused by hot water radiators and think they can operate at different temperatures. A radiator is just a piece of metal. It’s either completely on or completely off: by its very nature, it can’t be set to different temperatures because hot water is either flowing through it or not. With a simple central heating system, each radiator has a basic screw valve at the bottom. If you turn the screw down, you switch the radiator off: the valve closes and hot water flows straight through the bottom pipe, bypassing the upper part of the radiator altogether. Turn the screw up and you turn the radiator on, allowing water to flow right around it. In this case, the radiator is on.

How do thermostatic radiator valves help?

Thermostatic valves (sometimes called TRVs) fitted to radiators give you more control over the temperature in individual rooms of your home and help to reduce the energy your boiler uses, saving you money. Instead of having all the radiators in your home working equally hard to try to reach the same temperature, you can have your living room and bathroom (say) set to be warmer than your bedrooms (or rooms you want to keep cool). How do radiator valves work? When the heating first comes on, the boiler fires continuously and any radiators with valves turned on heat rapidly to their maximum temperature. Then, depending on how high you’ve set the radiator valves, they begin to switch off so the boiler fires less often. That reduces the temperature of the hot water flowing through the radiators and makes them feel somewhat cooler. If the room cools down too much, the valves open up again, increasing the load on the boiler, making it fire up more often, and raising the room temperature once again.

There are two important points to note about radiator valves. First, it’s not a good idea to fit them in a room where you have your main wall thermostat, because the two will work to oppose one another: if the wall thermostat switches the boiler off, the radiator valve thermostat will try to switch it back on again, and vice-versa! Second, if you have adjoining rooms with thermostats set at different temperatures, keep your doors closed. If you have a cool room with the valve turned down connected to a warm room with the valve turned up, the radiator in the warm room will be working overtime to heat the cool room as well.
Photo: Thermostatic valves fitted to radiators can help you heat your home more efficiently, saving energy and money.

How do boilers make hot water?

Many gas boilers also double up as hot-water heaters. When you turn on a hot water tap, you open a valve that lets water escape. The water feeds through a network of pipes leading back to the boiler. When the boiler detects that you’ve opened the tap, it fires up and heats the water. If it’s a central heating boiler, it usually has to pause from heating the central heating water while it’s heating the hot water, because it can’t supply enough heat to do both jobs at the same time. That’s why you can hear some boilers switching on and off when you switch on the taps, even if they’re already lit to power the central heating.

What are condensing boilers?

Gas boilers work by combustion: they burn carbon-based fuel with oxygen to produce carbon dioxide and steam—exhaust gases that escape through a kind of chimney on the top or side called a flue. The trouble with this design is that lots of heat can escape with the exhaust gases. And escaping heat means wasted energy, which costs you money. In an alternative type of system known as a condensing boiler, the flue gases pass out through a heat exchanger that warms the cold water returning from the radiators, helping to heat it up and reducing the work that the boiler has to do. Condensing boilers like this can be over 90 percent efficient (over 90 percent of the energy originally in the gas is converted into energy to heat your rooms or your hot water), but they are a bit more complex and more expensive.