Baxi’s jargon buster
Heating terms explained simply
Helping you see clearly
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understand the language your heating engineer uses.
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Baxi’s jargon buster – heating terms explained simply
Acronym, word or phrase
Description
Annual service
Just like your car needs to have a service to make sure the brakes are safe and to change the oil, a boiler needs to
be serviced annually by a Gas Safe registered engineer, to make sure it is running safely and efficiently. It is also a
requirement of the warranty to have an annual service.
ASHP/air source
heat pump
An air source heat pump extracts heat from the outside air in the same way that a fridge extracts heat from its
inside, and uses the heat for radiators, underfloor heating or warm air convectors in the home.
BAR
A unit of pressure relating to water, gas or air.
BBU/
Back boiler unit
A back boiler is a small, compact boiler fitted at the back of an open fireplace hearth. It sits behind an electric fire
(in the chimney breast) and provides hot water and central heating to the property.
Benchmark
Benchmark is a nationally recognised scheme that makes boiler manufacturers and installers responsible for best
practice during installation, commissioning and servicing of domestic heating and hot water products in line with
Building Regulations in England and Wales. It is managed and promoted by the Heating and Hotwater Industry
Council (HHIC). Benchmark requires installers to complete a Benchmark checklist when commissioning a new
boiler. Failure to install and commission according to the manufacturer’s instructions and complete the Benchmark
commissioning checklist will invalidate the warranty for gas boiler installations.
Baxi’s jargon buster – heating terms explained simply
Acronym, word or phrase
Description
Biomass
The name given to fuel that comes from biological matter, such as wood pellets. It is often known as carbon neutral,
because as much energy is absorbed when the plant was growing as is released when it is burned. Biomass boilers
use biomass fuels rather than natural gas or LPG.
Bleeding
the radiators
If there is air trapped in your central heating system, and the tops of your radiators are cooler than the rest, you
may have to ‘bleed’ the radiator. This is easily done by using a radiator key to open the small valve at the top of the
radiator to let the air out. When water starts to come out, the air is gone and you need to tighten the valve again.
Check the pressure gauge on the boiler when you have bled the radiators, to see if you need to repressurise the
system.
BP/burner pressure
CHP/combined heat
and power
In a boiler, gas is supplied through the gas valve then mixed with air. The mixture in the burner is at a specific
pressure and air/gas ratio to make sure that it burns consistently, and at the best possible efficiency.
When your installer commissions the boiler, he must record the burner operating pressure in the Benchmark checklist,
as part of the general combustion measurements he has to do.
Combined heat and power (CHP) is the simultaneous production of heat and electricity from a single source, close to
where they are to be used. This means that the heat and electricity generated can be used in the building extremely
efficiently because there is no energy loss during transportation to another site.
Baxi’s jargon buster – heating terms explained simply
Acronym, word
or phrase
Description
CO/carbon
monoxide
A colourless, odourless, poisonous flammable gas
formed when gas or LPG does not burn properly. If your
boiler is not working correctly or your flue is blocked or
badly fitted, carbon monoxide can accumulate in your
home and it can poison or kill.
DOMESTIC HOT WATER
SUPPLY TO ALL OUTLETS
CO2/carbon
dioxide
A colourless, odourless gas produced by burning
carbon and organic compounds and by respiration. It
is naturally present in air (about 0.03 per cent) and is
absorbed by plants during photosynthesis. It is a major
component of greenhouse gas, responsible for global
warming and climate change.
RADIATOR
RADIATOR
MAINS PRESSURE
COLD WATER SUPPLY
TO ALL OUTLETS
ROOM
THERMOSTAT
BOILER
PROGRAMMER
Combi/
combination
boiler
A boiler that heats water instantly when you turn on
a hot tap. It does not need a water storage cylinder
or water tanks in the loft. The water it heats comes
straight from the mains, so the flow rate at the tap is
usually very good. See diagram to the right.
RADIATOR
MAINS WATER
Combi boiler
RADIATOR
Baxi’s jargon buster – heating terms explained simply
Acronym, word or phrase
Description
Condensate pipe
Condensing boilers remove so much of the heat from the flue gases that sometimes the water left in the gas
condenses. This ‘condensate’ is discharged through a narrow plastic pipe called the condensate pipe. Sometimes, if
it has been fitted outside and is not insulated, the water can freeze in the pipe in very cold weather. This makes the
boiler shut down to protect itself, and it can’t be restarted until the condensate pipe has been thawed. We have made
videos to show you how to thaw the condensate pipe and also what you can do to ensure it doesn’t freeze again.
Controls
Controls for your boiler or heating system include room thermostats, room programmers and digital timer or
mechanical timers. Controls help you get the best out of your heating system and make it work efficiently and cost
effectively.
CWM/cold water
mains
The water that comes from the water mains, directly into your home, under mains pressure.
Cylinder
The hot water storage cylinder supplies hot water that has been heated by a system or heat only boiler to the taps.
It can also have an immersion heater, so it can be heated by electricity if the boiler is not switched on.
DHW/domestic
hot water
The water that comes from your hot tap or is stored in a hot water cylinder.
Baxi’s jargon buster – heating terms explained simply
Acronym, word or phrase
Description
Expansion vessel
As the water in your system heats up, it expands and the expansion vessel is a small, pressurised container that
accommodates the extra volume.
F&E tank/feed and
expansion tank
Usually two tanks found in the loft; the larger one feeds fresh water to the hot water storage cylinder and the other,
smaller tank maintains the correct level of water in the heating system. The tanks also allow for the expansion of
water when the system gets hot.
Flue
The flue is a bit like an exhaust pipe, taking away the waste gases from the boiler’s burner. It also supplies air into
the burner so the boiler can burn the best mix of gas and air, making it as efficient as possible. Flues are either
concentric – they have one pipe inside the other – or twin pipe – one pipe takes the exhaust away and the other
supplies air to the boiler.
FR/flow rate
The rate at which water comes out of your tap – usually measured in litres per minute (LPM). If you have a poor flow
rate, it will take a long time for your bath or sink to fill. You heating engineer can recommend ways to increase your
flow rate. If you have a combi boiler, the flow rate refers to how much hot water can be heated as it flows through
the boiler, at a given rate, usually litres per minute. If you need large quantities of hot water, a system or heat only
boiler may be better for you than a combi.
Baxi’s jargon buster – heating terms explained simply
Acronym, word
or phrase
Description
Fuel poverty
A household is said to be in fuel poverty when it
spends more than 10% of its income on energy to heat
the home.
GSHP/ground
source heat
pump
A ground source heat pump uses pipes buried under
the ground to extract stored thermal energy. This
energy is converted to provide clean, efficient and
renewable heating and hot water for the home.
Heat only
boiler
Also known as a ‘conventional’ or ‘regular’ boiler.
This type of boiler is often found in older homes and
needs a feed and expansion tank in the loft to store
cold water. It also needs a hot water storage cylinder,
and a pump to pump hot water around the system.
See diagram to the right.
DOMESTIC HOT
WATER SUPPLY
TO ALL OUTLETS
MAINS PRESSURE
COLD WATER
SUPPLY TO ALL
OUTLETS
RADIATOR
DOMESTIC
HOT WATER
STORAGE
TANK
RADIATOR
PUMP
ROOM
THERMOSTAT
BOILER
PROGRAMMER
RADIATOR
Heat only boiler
RADIATOR
Baxi’s jargon buster – heating terms explained simply
Acronym, word or phrase
Description
High efficiency
or condensing
High efficiency or condensing boilers are much more energy efficient than standard efficiency boilers. Condensing
boilers have bigger heat exchangers and recover more heat out of the flue gases before they escape to the outside.
Boilers can be made even more energy efficient by adding a passive flue gas heat recovery device like a GasSaver.
LPM/litres per minute
The rate at which water comes out of your tap, also known as the flow rate (FR).
Micro-combined heat
and power
Micro-combined heat and power (CHP) is the simultaneous production of heat and electricity from a single source,
close to where they are to be used, on a small scale, usually a single home.
Open-vented system
A heating system that has a feed and expansion tank in the loft.
Output
The output of boilers is usually measured in kilowatts (kW). Your installer can determine the right output for your
heating system by carrying out a survey of your home and your personal requirements. If you are having a combi
boiler and require a lot of hot water, you will need a high output, like the Baxi Duo-tec Combi 40 HE A. However, if
you have a system or heat only boiler, you will need the lowest output required to heat your radiators, because hot
water will be stored in the cylinder.
PCB/printed
circuit board
The PCB controls the boiler electronically. Sometimes, it will develop a fault or stop working properly, or sometimes it
may shut down the boiler because another component is not working properly. Your heating engineer should check
all the components before changing the PCB.
Baxi’s jargon buster – heating terms explained simply
Acronym, word or phrase
Description
Pilot light
Old, standard efficiency boilers had a small flame, burning constantly, to ignite the burner when heating or hot water
was required. Modern boilers don’t have pilot lights because they are lit by electronic ignition.
Power flush
A cleaning process that removes all the sludge and debris from your heating system. If you have cold spots on your
radiators, they could be filled with sludge that has built up inside the system over many years. If you put a brand new
boiler onto a dirty system, it could damage or block the heat exchanger. Your installer should always perform a power
flush before installing a new boiler, and add a suitable inhibitor to stop the sludge building up again. This will make
your heating system much more energy efficient and reliable.
PPI/pounds
per square inch
A unit of pressure. On boilers there is usually a pressure gauge to tell you your boiler is working at the correct
pressure. If the pressure drops, it could be due to a leak in the system. We have a useful video to show you how to
top up the pressure in your boiler. However, you should ask your Gas Safe registered engineer to check for leaks in
the system.
PV/photovoltaic
solar panels
Solar panels that generate electricity.
RF/radio frequency
controls
Wireless controls for your heating system that don’t need to be wired into the boiler.
SEDBUK
Stands for Seasonal Efficiency of Domestic Boilers in the UK, the rating given to a boiler’s efficiency.
Baxi’s jargon buster – heating terms explained simply
Acronym, word
or phrase
Description
Smart meter
An electricity or gas meter that records how much
electricity is used, and if you have micro-CHP or
Solar PV, how much electricity you generate. By 2020,
all UK households will have their standard meters
replaced with smart meters.
DOMESTIC HOT
WATER SUPPLY
TO ALL OUTLETS
Solar
thermal
Solar panels that use energy from the sun to generate
hot water (rather than electricity).
Standard
efficiency
Old, non-condensing, or standard efficiency boilers
can be as little as 60% efficient. Modern condensing
boilers are over 90% efficient. So replacing an old boiler
can help you make substantial savings on your gas bills.
RADIATOR
DOMESTIC HOT
WATER STORAGE
TANK
PUMP
BOILER
RADIATOR
System
boiler
A boiler that works with a hot water storage cylinder
but does not generally need a tank in the loft to fill
the system. See diagram to the right.
MAINS PRESSURE
COLD WATER SUPPLY
TO ALL OUTLETS
System boiler
MAINS WATER
Baxi’s jargon buster – heating terms explained simply
Acronym, word or phrase
Description
System filter
A filter can be fitted to your central heating system to protect it from sludge and rust that builds up over time. This
debris can be harmful to your boiler and make your heating system less efficient. Filters come in different designs,
including magnetic filters. Your installer will recommend the right filter for your system.
TRVs/thermostatic
radiator valves
Thermostatic radiator valves can be fitted to individual radiators so you can vary the temperature in each room, to
suit your needs.
Unvented/
sealed system
A sealed heating system does not need feed and expansion tanks in the loft. Instead, the system is filled with
mains water through a filling loop and there is an expansion vessel in the boiler to accommodate the hot water as it
expands in the system. The flow rate on a sealed system is usually very good.
Weather
compensation
An outdoor mounted weather compensation sensor works with the boiler to give greater fuel savings and improved
comfort levels in the home.