Gas central heating boilers
by Chris Woodford. Last updated: November 2, 2021.
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 gas jets and heat exchangers are hidden behind the white front cover,
but you can see what they look like in the photos 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
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
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: 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.
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
the boiler has to keep firing: to keep the water at a high enough
temperature to heat your home. An electric pump inside the
very near to it) keeps the water flowing around the circuit of pipework
Photo: This is a very old Chaffoteaux combination gas boiler 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 main heat exchanger at the top.
How do thermostats help?
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.
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.
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?
Photo: Thermostatic valves fitted to radiators can help you heat your home more efficiently, saving energy and money.
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.
How do combi 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? Typically, they have two independent heat exchangers. One of them carries a pipe
through to the radiators, while the other carries a similar pipe through to the hot water supply.
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 faucets, even if they're already lit to power the
Artwork: How a typical combi boiler works—using two separate heat exchangers.
Gas flows in from the supply pipe (1) to the burners inside the boiler (2),
which power the primary heat exchanger (3). Normally, when only the central heating is operating,
this heats water circulating around the heating loop (4), following the yellow dotted path through the radiators (5), before returning to the boiler as much cooler water (6). Hot water is made from a separate cold-water supply flowing into the boiler (7). When you turn on a hot faucet, a valve (8) diverts the hot water coming from the primary heat exchanger (3) through
a secondary heat exchanger (9), which heats the cold water coming in from the outer supply (7), and feeds it out to the faucet (10), following the orange dotted path. The water from the secondary heat exchanger returns through the brown pipe to the primary heat exchanger to pick up more heat from the boiler (11), following the white dotted path.
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. They also have at least one notable design flaw. Condensing the flue gases produces moisture, which usually drains
away harmlessly through a thin pipe. In cold weather, however, the moisture can freeze inside the pipe
and cause the entire boiler to shut down, prompting an expensive callout for a repair and restart.
Artwork: How a modern condensing boiler works.
Left: In a traditional (non-condensing) boiler, hot exhaust gases leave the combustion chamber through
a flue (1), helped on their way by an electric fan (not shown); they take heat with them, reducing the boiler's efficiency by as much as a third. Right: In a modern condensing boiler, the exhaust gases pass through an extra heat exchanger (2),
which soaks up most of their heat energy and passes it across to the cold water returning from
the radiators. This is what makes a condensing boiler so efficient. The colder exhaust gases exit through a flue in the usual way (3) and moisture produced by condensing them drains away through a separate pipe (4).
How is boiler efficiency measured?
Some parts of the world have color-coded scales to help consumers compare the efficiency of different makes
and models of boilers—and also to get a sense of how much better off they'd be with a new boiler than the
one they have already. In the UK, the SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) scale rates boilers from A (best)
to G (worst). A modern condensing boiler would score an A; an old pilot-light boiler would typically be a G.
- A: >90%
- B: 86–90%
- C: 82–86%
- D: 78–82%
- E: 74–78%
- F: 70–74%
- G: <70%
Although SEDBUK ratings are still used in Britain, that scale was replaced by another European one,
the ErP (the Energy-Related Products Directive) from 2009 onward. A new condensing boiler would score
an A or A+ on this scale; a pilot-light boiler would be more like a C or D.
- A+++: ≥150%
- A++: ≥125%
- A+: ≥98%
- A: ≥90%
- B: ≥82%
- C: ≥75%
- D: ≥36%
- E: ≥34%
- F: ≥30%
- G: <30%
In the United States, gas boilers need to have an annual efficiency of 90 percent or more
to be ENERGY STAR certified. Legally, US boilers now have to be rated on a measurement called AFUE (annual fuel utilization efficiency), which
measures the amount of useful heat a boiler pumps into your home compared to the amount of fuel it consumes.
New high-efficiency boilers come in at 90–99 percent efficient, while old pilot-light boilers
would creep in at more like 56–70 percent.
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—typically
about 100 liters of it.
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.
But what about the environment?
Gas boilers are great, in many ways: compared to how we used to heat our homes and hot water half a century or a century ago
(mostly by burning coal), they're relatively efficient, relatively clean, and very convenient.
But they're machines from the past, not the future: together, the world's gas boilers burn vast
amounts of fossil fuels and contribute significantly to climate change;
they also give off gases like nitrogen dioxide, which is a major component of urban
One recent British study found that the country's gas boilers produce twice as much
of the greenhouse gas carbon dioxide as all the country's gas-fired power stations put together—and eight times more air pollution.
Photo: Many of today's gas boilers will eventually be replaced by heat pumps.
Air-source heat pumps work like refrigerators running backward. One part (left) goes inside your home and blows warm air in. The other part (right) sits outside and extracts heat from the air, even on cold days.
Photo by Dennis Schroeder courtesy of NREL (National Renewable Energy Laboratory) (photo ref: #25167).)
What to do? If the best (A-rated) gas boilers are over 90 percent efficient, there's not that much scope for making them
any better; and however efficient they are, they're still going to be burning fossil fuel. Energy experts believe our
future lies in better insulation and in using technologies like
heat pumps, which suck heat from the outside air (or
the ground beneath our buildings) and pump it inside—a bit like refrigerators working backward.
But they're very expensive and not "exact replacements" for gas technology: so far, they're relatively expensive,
less well understood, and not always compatible with people's existing central heating.
Another possibility is a future where we use "greener gas" made from biomass, syngas (produced from something like gas plasma recycling), or other biofuels. (The British renewable energy company Ecotricity, for example, is currently investigating whether it can commercially produce "greener gas" from anaerobically digested silage.) All these things have major environmental drawbacks, as well as benefits, and it remains to be seen whether they're better overall than the natural gas they're seeking to replace.