Gas central heating boilers
by Chris Woodford. Last updated: April 17, 2015.
A few hundred years ago, people would have given an arm and a leg to have central heating in their homes. Just imagine the inconvenience of having to light a wood and coal fire in every separate room to keep your house warm. The basic idea of central heating is really simple: you have a boiler (an easily controllable furnace, fueled by gas) in a handy place like your kitchen or bathroom and it uses water, moved by an electrically powered pump, to carry heat into radiators in all the other rooms. It's simple, convenient, efficient, and it makes even winter days a pleasure to endure!
Photo: A typical gas central heating boiler. The big pipe coming out of the top is the flue. The hot and cold water and gas pipes enter the boiler underneath. The little black rectangle in the white front cover is an inspection window through which you can watch the gas jets firing up and heating water (see photo below).
What does the boiler do?
The boiler is the most important part of a central heating system. It's like a big fire that has a continuous supply of natural gas streaming into it from a pipe that goes out to a gas main in the street. When you want to heat your home, you switch on the boiler with an electric switch. A valve opens, gas enters a sealed combustion chamber in the boiler through lots of small jets, and an electric ignition system sets them alight. The gas jets play onto a heat exchanger connected to a pipe carrying cold water. The heat exchanger takes the heat energy from the gas jets and heats the water to something like 60°C (140°F).
Photo: Left: The gas jets inside a boiler fire up to heat the water. Natural gas burns blue when it has the right amount of oxygen. If it burns yellow, there's not enough oxygen and your boiler may be creating a dangerous, toxic gas called carbon monoxide. That's why you should always have a carbon monoxide detector somewhere near a gas boiler.
Right: This is the gas boiler from our top photo with its cover removed. The inset closeup photo shows where the gas jets are (at the bottom) and how they transfer heat energy to the heat exchanger at the top.
The water pipe is actually one small section of a large, continuous circuit of pipe that travels right around your home. It passes through each hot-water radiator in turn and then returns to the boiler again. As the water flows through the radiators, it gives off some of its heat and warms your rooms in turn. By the time it gets back to the boiler again, it's cooled down quite a bit. That's why the boiler has to keep firing: to keep the water at a high enough temperature to heat your home. An electric pump inside the boiler (or very near to it) keeps the water flowing around the circuit of pipework and radiators.
How do thermostats help?
A basic system like this 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 boiler 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 in your living room. 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.
Photo: An electric thermostat. Simply set the temperature you want (in degrees centigrade) and the thermostat switches the boiler on and off to keep the room temperature roughly constant.
How do radiators work?
Photo: A hot water radiator is simply a copper pipe repeatedly bent at right angles to produce a heating surface with the maximum area. The heat pipes follow 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 copper pipe
bent back and forth 10-20 times or so to create a large surface area
through which heat can enter a room. 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?
Most gas boilers also double up as hot-water heaters. Some (open-vented boilers) heat water that's stored in a tank; others (combi boilers) heat water on demand. How do combi boilers work? When you turn on a hot water faucet (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 faucet, 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.
Can you run a boiler on fuels other than gas?
Think of central heating systems as being in two parts—the boiler and the radiators—and you can see that it's relatively easy to switch from one type of boiler to another. For example, you could get rid of your gas boiler and replace it with an electric or oil-fired one, should you decide you prefer that idea. Replacing the radiators is a trickier operation, not least because they're full of water! When you hear plumbers talking about "draining the system", they mean they'll have to empty the water out of the radiators and the heating pipes so they can open up the heating circuit to work on it.
Do you always need a pump?
Most modern central heating systems use an electric pump to power hot water to the radiators and back to the boiler; they're referred to as fully pumped. A simpler and older design, called a gravity-fed system, uses the force of gravity and convection to move water round the circuit (hot water has lower density than cold so tends to rise up the pipes, just like hot air rises above a radiator). Typically gravity-fed systems have a tank of cold water on an upper floor of a house (or in the attic), a boiler on the ground floor, and a hot water cylinder positioned in between them that supplies hot water to the faucets (taps). As their name suggests, semi-pumped systems use a mixture of gravity and electric pumping.