Plumbing and heating glossary
The words that come up in quotes, service reports and gas safety records, each explained in a few sentences. 39 terms, with a link to the page that goes into more detail.
B
- Back boiler
- A back boiler is a boiler fitted in the fireplace opening behind a gas fire, usually with the fire and boiler sharing one flue. It is an old, non-condensing arrangement that still turns up in many London houses, and replacing it normally means moving the boiler and making good the chimney breast.More: Removing a back boiler
- Balancing radiators
- Balancing radiators means adjusting the lockshield valve on each radiator so that hot water is shared out evenly and the last radiator on the circuit heats as well as the first. It is done when a system is commissioned or after a flush, and it is measured by the temperature difference across each radiator rather than guessed.More: Full central heating
- Bleeding a radiator
- Bleeding a radiator means letting trapped air out through the small bleed valve at its top corner. A radiator that is cold at the top and warm at the bottom usually has air in it; on a sealed system the boiler pressure drops a little afterwards, so the gauge is worth a look.More: Low boiler pressure
- Boiler pressure
- Boiler pressure is the water pressure in a sealed heating system, shown in bar on the gauge on or under the boiler. With the heating off and the system cold most domestic boilers want between about one and one and a half bar, but the manufacturer's figure for your appliance takes precedence; below about half a bar most boilers lock out.More: Low boiler pressure
- Building Regulations compliance certificate
- A Building Regulations compliance certificate confirms that notifiable work, such as fitting a new boiler or an unvented cylinder in England, was reported to building control and meets the Building Regulations. A Gas Safe registered installer notifies a boiler installation through the Gas Safe Register scheme and the certificate is sent to the householder, who will want it when the property is sold.More: New boiler installation
C
- Carbon monoxide alarm
- A carbon monoxide (CO) alarm sounds when it detects carbon monoxide, a poisonous gas with no smell that a faulty boiler, fire or flue can produce. In England, landlords must fit one in every room used as living accommodation that contains a fixed combustion appliance such as a boiler or gas fire (not a gas cooker), under the alarm regulations amended in October 2022.More: Landlord gas safety in England
- Chemical flush
- A chemical flush cleans a central heating system by adding a cleaning chemical, running it through the system on the system's own pump for a period and then draining and refilling. It suits light contamination; a heavily sludged system usually needs a power flush, which drives debris out with a separate high-flow pump.More: Power flush or chemical flush
- Combi boiler
- A combi (combination) boiler heats the central heating and heats hot water on demand straight from the mains, so there is no hot water cylinder and no tank in the loft. Its hot water output is shared between whatever taps and showers are open at the same time, which is why it suits homes that use one outlet at a time.More: Combi, system or regular boiler
- Condensate pipe
- The condensate pipe is the plastic pipe that carries the slightly acidic water a condensing boiler produces away to a drain. Where it runs outside it can freeze in a cold snap, and a frozen condensate pipe is one of the most common reasons a boiler stops working in winter.More: Your boiler has stopped
- Condensing boiler
- A condensing boiler recovers extra heat from its flue gases by cooling them until the water vapour in them condenses, which is where its efficiency comes from. It only condenses when the water returning to it is cool enough, so correct sizing and lower flow temperatures matter; with few exceptions, new gas boilers fitted in England have had to be condensing since 2005.More: New boiler installation
- CP12 (landlord gas safety record)
- CP12 is the name still commonly used for a landlord gas safety record, the document a Gas Safe registered engineer issues after the annual gas safety check of a rented property. Under the Gas Safety (Installation and Use) Regulations 1998, landlords must have gas appliances and flues they provide checked at intervals of no more than 12 months and give tenants a copy of the record.More: Landlord gas safety in England
E
- Expansion vessel
- An expansion vessel is a small tank with a pre-charged air cushion behind a rubber diaphragm that absorbs the extra volume of water as it heats up in a sealed system or an unvented cylinder. When it loses its air charge the pressure rises and falls sharply, which can make a relief valve discharge; checking the charge is part of a service.More: Megaflo and unvented cylinders
F
- Filling loop
- A filling loop is the connection used to top up a sealed heating system with water from the mains, usually a braided hose with a valve at each end under the boiler, or a built-in key or lever on newer appliances. A system that needs topping up regularly has a leak or a faulty component and should be looked at.More: Low boiler pressure
- Flow rate
- Flow rate is how much water your incoming main delivers, measured in litres per minute. It sets the ceiling for a combi boiler or an unvented cylinder, because no appliance can deliver more hot water than the supply feeding it, and it is different from mains pressure.More: Combi, system or regular boiler
- Flue
- A flue is the pipe or duct that carries the combustion products from a boiler, fire or other gas appliance safely to the outside air. Where a flue may run and terminate is governed by the manufacturer's instructions and the regulations, and any work on it is gas work for a Gas Safe registered engineer.More: New boiler installation
G
- G3 qualification
- A G3 qualification shows that an installer is competent to fit unvented hot water storage systems under requirement G3 of Part G of the Building Regulations in England. Installing an unvented cylinder is notifiable work, and the safety devices and discharge pipework have to be fitted to G3.More: Megaflo and unvented cylinders
- Gas Safe Register
- Gas Safe Register is the official list of businesses and engineers legally allowed to carry out gas work in Great Britain, required by the Gas Safety (Installation and Use) Regulations 1998. Every registered engineer carries an ID card that shows which types of gas work they are qualified for, and the register can be checked online.More: Landlord gas safety certificates
H
- Heat exchanger
- The heat exchanger is the part of a boiler where heat from the burner passes into the water of the heating system. In modern boilers its waterways are narrow, which is why sludge from an old system can block it and why most manufacturers want the system cleaned before a new boiler goes on.More: Power flushing
- Heat loss calculation
- A heat loss calculation works out how much heat each room loses on a cold day, from its size, construction, insulation and glazing. It is the basis for sizing radiators and the boiler, instead of copying the output of the appliance being replaced.More: Full central heating
- HMO (house in multiple occupation)
- An HMO is a rented property where people from more than one household share facilities such as a kitchen or bathroom; in England the definition starts at three occupants forming more than one household. The landlord gas safety duties apply to an HMO as to any let property, larger HMOs need a licence from the council, and depending on the borough smaller ones can too, under additional licensing schemes that are common in London.More: Landlord gas safety in England
I
- Immersion heater
- An immersion heater is an electric heating element fitted inside a hot water cylinder. It can be the main way of heating the water or a backup when the boiler is off; combi boilers have no cylinder and so no immersion heater.More: Megaflo and unvented cylinders
- Inhibitor
- Inhibitor is a chemical added to the water in a central heating system to slow down corrosion and scale inside radiators, pipes and the boiler. It is added after a system is cleaned or refilled, because without it the sludge that a flush removed starts forming again.More: Power flushing
L
- Loft tanks
- Loft tanks are the cold water storage tank that feeds a vented hot water cylinder, and often a smaller feed and expansion tank that keeps an open-vented heating system topped up. A regular boiler system normally has them; changing to a combi or an unvented cylinder usually means they can be removed.More: System to combi conversion
M
- Magnetic filter
- A magnetic filter is a canister fitted on the heating pipework near the boiler with a magnet inside that catches magnetite, the black iron-oxide sludge circulating in the water. It protects the boiler's heat exchanger and is emptied as part of a service; many boiler manufacturers expect one to be fitted.More: Power flushing
- Mains pressure
- Mains pressure is the pressure of the water supply coming into a property, measured in bar. Together with the flow rate it decides how well a combi boiler or an unvented cylinder can perform, and 'mains pressure hot water' means hot water delivered at that pressure without a pump or a loft tank.More: Megaflo and unvented cylinders
- Megaflo
- Megaflo is a brand of unvented hot water cylinder made by Heatrae Sadia, and its name is widely used for any unvented cylinder. What applies is the same whatever the brand: G3 installation and safety devices, and the annual service the manufacturer asks for.More: Megaflo and unvented cylinders
- Microbore pipework
- Microbore pipework is heating pipe of a small diameter, typically 8 or 10 mm, used widely in systems installed in the 1960s and 1970s. Its narrow bore silts up with sludge far sooner than standard 15 or 22 mm pipe, which matters when a new boiler is connected to it.More: Power flushing
P
- Power flush
- A power flush cleans a central heating system by connecting a dedicated high-flow, low-pressure pump and cleaning chemicals, flushing each radiator in turn and reversing the flow to lift settled sludge. It is followed by inhibitor and usually a magnetic filter, so the system stays clean.More: Power flush or chemical flush
- Pressure relief valve
- A pressure relief valve opens to release water if the pressure in a sealed heating system rises too high. An unvented cylinder has two such valves, an expansion relief valve on the cold feed and a temperature and pressure (T&P) relief valve on the cylinder, both discharging through the tundish; a valve that keeps discharging points to a fault such as a flat expansion vessel.More: Low boiler pressure
R
- Regular boiler
- A regular boiler, also called a heat-only or conventional boiler, heats the central heating and a separate hot water cylinder that is fed from tanks in the loft. It is the traditional arrangement in older houses and can still suit a large property with poor mains supply.More: Combi, system or regular boiler
S
- Screed
- Screed is the layer of sand and cement, or a similar flowing compound, laid over underfloor heating pipes to form the floor and spread the heat. The pipe loops are pressure-tested and kept under pressure while it goes down, and the heating is brought up gradually once it has cured.More: Underfloor heating
- Sludge (magnetite)
- Sludge in a heating system is mostly magnetite, black iron oxide formed as steel radiators corrode, which settles wherever the water moves slowest. It causes radiators that are cold at the bottom, blocked pipes and a strained boiler, and it is removed by flushing the system.More: Power flush or chemical flush
- System boiler
- A system boiler heats the central heating and a stored hot water cylinder, with the pump and expansion vessel built into the boiler. Paired with an unvented cylinder it needs no loft tanks and lets several showers and taps run at once.More: Combi, system or regular boiler
T
- TRV (thermostatic radiator valve)
- A TRV is a radiator valve with a head that senses the room temperature and reduces the flow of hot water as the room warms up. The numbers on the head are settings rather than degrees, and TRVs let rooms be kept at different temperatures on the same system.More: Full central heating
- Tundish
- A tundish is a small open funnel in the discharge pipe from an unvented cylinder's safety valves, fitted where it can be seen. It creates an air gap so discharged water cannot be drawn back, and water dripping through it is a sign that a valve or the expansion vessel needs attention.More: Megaflo and unvented cylinders
U
- Underfloor heating manifold
- The manifold is the distribution point of a wet underfloor heating system, where each pipe loop connects and its flow is set and switched on or off by zone. It is usually fitted with a blending valve and pump so the floor receives water at a lower temperature than radiators would.More: Underfloor heating
- Unvented cylinder
- An unvented cylinder stores hot water fed directly from the cold mains, so every tap and shower runs at mains pressure without a loft tank or pump. Because it is a pressure vessel it has an expansion vessel, safety valves and a tundish, must be fitted by a G3 qualified installer and should be checked every year.More: Megaflo and unvented cylinders
V
- Vented cylinder
- A vented cylinder is a hot water cylinder fed by a cold water storage tank in the loft and open to the air through a vent pipe. Its pressure comes only from the height of the tank above the outlets, so showers on upper floors are often weak without a pump.More: System to combi conversion
Z
- Zone valve
- A zone valve, or motorised valve, is an electrically operated valve that opens and closes the flow of hot water to one part of a heating system, such as a floor or the hot water cylinder. Each zone has its own thermostat, so one area can be heated without the rest.More: Full central heating