Radiator installation glossary
Plain-English explanations of the terms you will come across.
- Balancing
- Adjusting the lockshield valves so that hot water is shared evenly between all the radiators, rather than the nearest ones taking most of the flow and the furthest ones staying cool.
- BTU
- British Thermal Unit, a measure of heat. Radiator output is often quoted in BTU per hour. One watt equals about 3.41 BTU per hour, so a 1,000 W radiator gives roughly 3,400 BTU per hour.
- Delta T
- The difference between the average water temperature in a radiator and the room temperature. Radiator outputs are usually quoted at Delta T50, and the real output falls when the system runs cooler.
- Heat loss calculation
- A room-by-room calculation of how much heat escapes through walls, windows, floors and ceilings, used to work out the radiator output each room needs at a given temperature.
- Inhibitor
- A chemical treatment added to central heating water to slow internal corrosion and limescale. It protects radiators, pipes and the boiler and should be topped up whenever the system is drained.
- Lockshield valve
- The valve at the opposite end of a radiator to the TRV, usually with a plain cap. It is set by the installer to control flow through the radiator and is used to balance the system.
- Magnetic filter
- A device fitted on the heating pipework, usually near the boiler, that captures magnetic debris such as iron oxide as water passes through it. It is emptied during servicing.
- Microbore
- Small-diameter heating pipe, usually 8 or 10 mm, used in some homes from the 1970s onwards. It limits flow and may need upsizing for heat pumps or larger radiators.
- Open-vented system
- An older type of central heating system fed by a small feed and expansion tank, usually in the loft, which keeps the system topped up and allows for water expansion.
- Pipe centres
- The distance between the centres of the two pipes or valve tails feeding a radiator. Matching pipe centres is what allows a like-for-like radiator swap without changing the pipework.
- Power flush
- A deep clean of a central heating system using a pump that circulates water and cleaning chemicals at high flow to loosen and remove sludge and debris from radiators and pipework.
- Sealed system
- A pressurised central heating system with no tank in the loft, common with combi and system boilers. It is filled through a filling loop and normally sits around 1 to 1.5 bar when cold.
- Sludge
- Black sediment, mostly iron oxide, that forms when steel radiators and pipes corrode from inside. It collects at the bottom of radiators, causing cold spots and reducing system efficiency.
- TRV
- Thermostatic radiator valve. A valve with a numbered head that senses the room temperature and adjusts the flow of hot water into the radiator to keep that room at a steady temperature.
- Type 22
- A steel panel radiator with two water panels and two sets of convector fins, also called K2 or double convector. It is the most common choice where good output is needed from limited wall space.
Last updated 6 November 2026