Heating and plumbing glossary

A

Air vent

Air in a heating system causes gurgling, cold radiator tops and poor pump performance. Manual vents on radiators are opened with a bleed key, while automatic float vents fitted at high points, on manifolds or at the boiler release air on their own. After venting, check the pressure gauge and top up the system if needed. View products: Air vent valves →

B

Boiler return temperature protection

Arrangement that keeps the water returning to a solid fuel boiler above the point at which flue gases would condense on the cold heat exchanger, producing tar and acid corrosion. Boiler manufacturers usually require at least 60–65 °C. It is achieved with a thermostatic mixing valve and pump in the boiler circuit or with a ready-made loading unit. View products: for the correct connection of the boiler →

Buffer tank

Insulated steel tank of heating water that stores heat and releases it when the building needs it. With a buffer, a wood boiler can fire at full output with clean combustion even when the house is not drawing heat; a common sizing guide is 50 to 100 litres per kilowatt of boiler output. The water stratifies by temperature, and tanks with coils can also take solar input. View products: Buffer tanks →

Building heat loss

Heat output a building loses through walls, windows, roof, floor and ventilation at the design outdoor temperature. It is calculated to EN 12831 for the specific location and is the basis for sizing both the heat source and the individual emitters. Simply copying the output of the old boiler without a calculation often leads to oversized equipment. View products →

C

Castellated system panel

Expanded polystyrene insulation board with raised studs that hold underfloor heating pipe without clips – the pipe is simply pressed in between them. The insulation limits heat loss downwards, while the top film stops screed from running into the joints between boards. Pipe spacing follows the stud grid, usually in steps of 5 cm. View products: Polystyrene, system boards →

Circulating pump (circulator)

Pump that drives heating water round a closed circuit from the heat source to radiators or underfloor loops and back. It does not have to lift water to the top of the building – it only overcomes the resistance of pipes, valves and emitters. Electronically controlled models adjust their speed to the actual demand and use far less electricity than older pumps with fixed speed settings. View products: Circulating pumps - all →

Compression fitting

Detachable joint in which a nut squeezes a split clamping ring onto the pipe, which sits on an insert with O-rings. Only spanners are needed, but the pipe must be cut square and the end calibrated so the seals are not damaged when it is pushed on. Manufacturers generally recommend leaving compression joints accessible for inspection and re-tightening. View products: Screw connections PEX/AL/PEX →

Cooling loop (thermal discharge safety)

Safety heat exchanger built into a solid fuel boiler: if the water overheats (typically around 95 °C), a thermal discharge valve lets cold mains water through it and sends it to drain. This removes surplus heat, for example when the pump stops during a power cut. The water supply must work without electricity, so a domestic booster pump is not suitable. View products: for the correct connection of the boiler →

D

DN sizes and pipe threads

DN (nominal size) indicates the approximate internal diameter of a pipe or valve in millimetres. Inch thread sizes are historical and do not match the real dimension: a 1/2" thread measures about 21 mm outside and belongs to DN 15, 3/4" to DN 20 and 1" to DN 25. Parallel G threads seal with a flat washer or O-ring, tapered R threads seal in the thread with hemp or PTFE tape. View products: Brass threaded fittings →

Domestic hot water (DHW)

Hot water for washing, showering and cooking – unlike the heating water in a closed circuit, it is drinking water. It is produced in an indirect cylinder, an electric or instantaneous water heater, or a combi boiler. To prevent Legionella, the cylinder should regularly reach at least 60 °C, with a mixing valve limiting the temperature at the outlets. View products: storage tanks and hot water heaters →

E

Electric heating element

Electric element screwed into a towel rail or a tank to heat the water inside; versions with a thermostat hold the set temperature. Towel rail elements usually have a 1/2" thread, while buffer tanks and cylinders take larger sizes such as 6/4". In dual-fuel operation never close both radiator valves, otherwise the heated water has no room to expand. View products: Heating rods and ladders →

Electric underfloor heating

Heating in which an electric resistance cable or a thin mat laid in tile adhesive or screed produces the heat; heating foil is used under floating floors. Mats typically deliver about 100–200 W/m² and are controlled by a thermostat with a floor sensor. The connection must be made by a qualified electrician and the circuit protected by a residual current device (RCD). View products: Electric underfloor heating →

Expansion vessel

Sealed vessel with a membrane separating the water from a gas cushion; it absorbs the volume increase of heated water – about 3.5 % between 10 and 90 °C – and keeps system pressure within safe limits. The gas pre-charge is checked with the water side drained, at least once a year, and set according to the height of the system. Vessels for drinking water have a suitable membrane. View products: Expansion vessels →

F

Flow and return

The flow pipe carries heated water from the heat source to the emitters; the return brings the cooled water back. Swapping them on a manifold, valve or pump group causes poor operation or noise, so always follow the flow direction arrows. Return temperature affects efficiency: a condensing boiler needs it low, while a solid fuel boiler needs it high enough.

Flow meter

Small sight glass with a float, mounted on the flow bar of a manifold, that shows the flow through one circuit, usually in l/min. Most flow meters double as regulating valves, so turning them sets the required flow directly. This makes it possible to balance underfloor loops of different lengths so that every room heats evenly. View products: underfloor heating manifolds →

Flow–return temperature difference (ΔT)

Difference between flow and return water temperature; 55/45 °C means a ΔT of 10 K, and a third figure, as in 75/65/20 °C, is the room temperature. For the same output, a larger ΔT means less flow is needed. Catalogue radiator outputs apply to the stated temperatures; with cooler water the output is lower and has to be recalculated. View products →

H

Heated towel rail

Bathroom radiator made of horizontal tubes between two vertical headers that heats the room and dries towels at the same time. It connects from below – at the ends or centrally at 50 mm centres – or from the side; output is stated at 75/65/20 °C and is lower with cooler water. A T-piece allows an electric element to be added for use outside the heating season. View products: Heated Towel Rails →

Hot water circulation (secondary return)

A loop in which a small pump keeps hot water moving between the cylinder and the outlets, so hot water reaches the tap almost immediately. Because it handles oxygen-rich drinking water, the pump has a bronze, brass or stainless-steel body rather than cast iron. A timer or thermostat limits its running time to the hours when hot water is actually drawn. View products: Pumps for heating and water →

HT and KG drainage pipes

HT pipes made of polypropylene are used for indoor drainage – waste from basins, showers or washing machines – and withstand hot waste water. They join by push-fit sockets with rubber seals, and horizontal runs are laid with a steady fall, usually 1–3 %. Buried drainage outside the building uses stiffer orange-brown KG pipes made of PVC. View products: HT waste system →

Hydraulic balancing

Setting the flow through every radiator, underfloor loop or riser to its design value using presettable valves, balancing valves or flow meters. Without it, water takes the easiest path: nearby radiators overheat while distant ones stay lukewarm. A balanced system heats evenly, runs more quietly and lets the pump and heat source work more efficiently. View products: balancing valves →

Hydronic underfloor heating

Pipes embedded in the screed turn the whole floor into a large, gently warm emitter, so the system runs on low water temperatures of roughly 30–45 °C. Floor surface temperature in occupied areas should not exceed 29 °C. A single loop of 16 mm pipe is typically limited to about 100 m, and each loop is connected to the manifold. View products: surface heating and cooling →

I

Inspection chamber

Chamber on a sewer connection that gives access for inspecting and cleaning the pipe; it is placed at changes of direction or gradient, at junctions and near the property boundary. Plastic chambers consist of a base with inlets, a riser pipe and a cover. The cover is chosen by load class – A15 for gardens and pedestrian-only areas, B125 for footways and car driveways, D400 for roads. View products: Inspection shafts →

L

Low-loss header (hydraulic separator)

Vertical vessel or large-diameter pipe that hydraulically separates the boiler circuit from the heating circuits, so pumps on each side do not interfere with one another. It is used with cascaded boilers, several pumped circuits or boilers that need a minimum flow. Thanks to the slow flow inside, it also separates air and sludge.

M

Magnetic dirt separator

Device fitted in the return before the boiler, where the flow slows down, dirt settles and a strong magnet captures magnetite – the fine black sludge from corroding steel parts. It protects the pump, the boiler heat exchanger and valves from clogging. It is cleaned through a drain valve without dismantling, which makes it especially useful when a boiler is replaced on an older system. View products: dirt separators with a magnet →

Manifold (flow and return)

The flow manifold splits water from one supply into several circuits – usually underfloor heating loops or radiators – and the return manifold collects it back to the heat source. Flow meters normally sit on the flow bar, valves for thermal actuators on the return bar. Manifolds are made of stainless steel or brass; outlets are most often 3/4" with a Eurocone connection. View products: Stainless steel and brass manifolds →

Manifold cabinet

Sheet-steel cabinet that houses the manifold with its ball valves and, if required, a mixing unit with pump. Flush-mounted versions are recessed into the wall, surface-mounted ones are fixed to a finished wall. When choosing the size, allow for the full width of the manifold including all fittings, plus space for actuators and the wiring centre. View products: Manifold cabinets →

Mixing valve (3-way, 4-way)

Valve that blends hot water from the heat source with cooler return water to supply a heating circuit at a lower temperature, for example underfloor heating. It is set by hand or driven by an actuator from a controller. A 4-way valve also raises the temperature of water returning to the boiler; a 3-way valve can also act as a diverter, for example between space heating and hot water. View products: mixing valves →

Multilayer pipe PEX-AL-PEX

Composite pipe with inner and outer layers of cross-linked polyethylene (PEX) or PE-RT bonded to an aluminium core. The aluminium blocks oxygen, keeps the pipe in shape after bending and greatly reduces thermal expansion compared with all-plastic pipe. Sizes are given as outside diameter × wall thickness, for example 16×2 mm. View products: Pipes for water, heating →

O

Oxygen barrier (EVOH)

Thin layer, usually EVOH, that stops oxygen in the air from diffusing through the wall of a plastic pipe into the heating water. Dissolved oxygen speeds up corrosion of steel and cast-iron components and leads to sludge. Pipes for heating should therefore have a barrier complying with DIN 4726; in PEX-AL-PEX the aluminium layer does the same job. View products: pipe PEX, PE-RT →

P

Pellet boiler

Automatic boiler burning wood pellets, which an auger feeds from the hopper to the burner. Ignition, fuel dosing and output are electronically controlled, so the user only refills the hopper and empties the ash. Pellets are usually 6 mm in diameter and should meet EN ISO 17225-2 class A1; damp or crumbly fuel burns poorly. View products: for pellets →

Press fitting

Permanent joint made by crimping the fitting's stainless-steel sleeve onto the pipe with a press tool. The jaw profile (for example TH, U or B) must match the fitting system, otherwise the joint will not seal. Correctly pressed fittings may be buried in walls and floors; some are designed to leak visibly during the pressure test if someone forgot to press them. View products: Press fittings PEX/AL/PEX →

Pressure rating PN

PN is the pressure class of a component: the number gives the maximum permissible operating pressure in bar at 20 °C, so PN 10 means 10 bar. At higher medium temperatures the permissible pressure is lower, markedly so for plastics. A sealed heating system in a house usually runs at 1–2 bar, while a mains water supply may reach 6 bar or more. View products: Valves and actuators →

Pressure reducing valve

Valve that lowers mains water pressure to a set value and keeps it steady however much the inlet pressure fluctuates. It protects taps, appliances and water heaters, whose safety valve would otherwise drip at high pressure. It is fitted after the water meter and filter; in homes the outlet pressure is most often set to 3–4 bar. View products: Pressure reducing valves →

Pump head and flow rate

Head is the hydraulic resistance a pump can overcome, stated in metres of water column (1 m is roughly 0.1 bar). Flow rate is given in m³/h or l/min, and head falls as flow rises – the real duty point lies where the pump curve crosses the system curve. A designation such as 25/6 usually means a DN 25 connection and a maximum head of about 6 m. View products: Pumps for heating and water →

R

Radiator valve and lockshield

The thermostatic valve on the radiator flow takes the thermostatic head, while the lockshield on the return is used to isolate the radiator and preset it. Valves with presetting limit the flow through each radiator and are the basis of hydraulic balancing. Bottom-connected radiators are fed through a straight or angled H-block valve with 50 mm centres. View products: Radiator valves →

Room thermostat

Device that measures the temperature in a reference room and switches the boiler, pump or actuators accordingly; communicating models can modulate boiler output. Programmable and Wi-Fi versions lower the temperature at night or when nobody is home. Mount it on an inside wall about 1.5 m above the floor, away from radiators and direct sun, and leave the radiator heads in that room fully open. View products: Room thermostats and controllers →

S

Safety valve

Safety device that opens automatically when its set pressure is exceeded and releases water, preventing a boiler, cylinder or pipe from bursting. Heating systems typically use 2.5 or 3 bar valves, water heaters 6 bar. No isolating valve may be fitted between the safety valve and the protected appliance, and the discharge must run to a drain via a visible funnel (tundish). View products: safety valves →

Solar thermal collector

Panel whose absorber captures solar radiation and transfers the heat to an antifreeze fluid, which a pump station carries to the coil of a solar cylinder or buffer tank. Flat-plate collectors cost less, while evacuated-tube collectors make better use of weak winter sun. For hot water a rough guide is 1–1.5 m² of collector area per person, facing south. View products: Solar sets and collectors →

T

Thermostatic mixing valve (TMV)

Self-acting valve with a thermostatic element that blends hot and cold water to a set outlet temperature without power or a controller. On hot water systems it prevents scalding when the cylinder is stored hotter, for instance by solar collectors or a wood boiler. In heating circuits it limits the flow temperature for underfloor loops or protects the boiler return. View products: Thermostatic mixing valves →

Thermostatic radiator head

Head fitted on a thermostatic radiator valve; its filling expands as room temperature rises and throttles the valve, holding the temperature without electricity. Scale numbers correspond to temperatures defined by the manufacturer, with 3 usually around 20 °C. The head must fit the valve connection (often M30×1.5), and a remote-sensor version is used where the radiator is covered. View products: thermostatic heads →

U

Uninterruptible power supply (UPS) for boilers and pumps

Battery unit that keeps the boiler electronics and circulating pump running during a power cut. With solid fuel boilers it prevents overheating: without power the pump would stop while fuel is still burning in the firebox. For motors and boiler controls choose a UPS with a pure sine wave output; backup time depends on battery capacity and the connected load. View products: Backup power supplies →

V

Valve actuator

Device that opens and closes a valve in response to a thermostat or controller. Thermoelectric actuators on manifolds and radiator valves work on/off and slowly – a few minutes per stroke – and come as normally closed (NC) or normally open (NO) at 230 V or 24 V. Motorised actuators turn mixing and zone valves, often with 0–10 V proportional control. View products: Servo-drives →

W

Water filtration and treatment

Treatment from the point of entry to the tap: a mechanical filter with a mesh or cartridge traps sand and rust that would damage valves and appliances. Activated carbon cartridges improve taste and odour and remove chlorine, and an ion-exchange softener reduces hardness and limescale. The right combination is best chosen on the basis of a water analysis. View products: Water filtration and treatment →

Weather compensation

Control that sets the heating flow temperature from the outdoor temperature: the colder it is outside, the warmer the water sent to radiators or the floor. The relationship is described by the heating curve, whose slope and offset are adjusted to suit the building. It needs an outdoor sensor on a north-facing wall and controls either the boiler output or a mixing valve actuator. View products: regulation →

Wood gasification boiler

Boiler in which logs dry in the fuel chamber and gasify with a restricted air supply; the wood gas then passes through a nozzle into a lower ceramic chamber and burns with secondary air. Combustion is clean and efficient but needs dry wood with a moisture content below 20 %. The boiler is installed with a buffer tank and return temperature protection. View products: on wood →