TRV Sizing Calculator — Kv & Flow Limiting

Kv is the flow coefficient representing the flow rate of water in m³/hr through the valve at a pressure drop of 1 bar. A higher Kv means less flow restriction. TRV Kv values typically range from 0.1 to 2.0 depending on size and setting. Kv = Q (m³/hr) ÷ √ΔP (bar).

A pre-settable TRV has an adjustable flow limiter (usually behind the head or in the body) that restricts maximum flow rate regardless of the temperature setting. This allows system balancing without needing a separate lockshield valve. Settings typically range from 1 (most restricted) to 6 or 8 (fully open). The Kvs does NOT rise in equal steps — the characteristic is non-linear (equal-percentage): on a Drayton TRV4 15mm body the Kvs(max) per setting 1–6 is 0.10, 0.14, 0.22, 0.38, 0.66 and 1.01 m³/hr. Always pick the lowest setting whose Kvs is at least your required Kv, and confirm against your chosen valve’s own pre-set chart.

Match the TRV body to the pipe size: 8/10mm for microbore, 15mm for standard domestic, 22mm for larger radiators. Check the required Kv against the valve body’s published Kvs(max): the standard Drayton TRV4 body (used for the 8mm, 10mm and 15mm sizes — the smaller sizes are the 15mm body fitted with reducers) has a Kvs(max) of 1.01 m³/hr. If your required Kv approaches that figure the valve will be fully open with no control authority, so upsize to a 22mm body. Most domestic radiators use 15mm TRV bodies on 15mm copper pipework.

Building Regulations Part L (Approved Document L) requires room-by-room temperature control on new and replacement wet heating systems — in practice a TRV on every radiator except in the room containing the main room thermostat (so the two controls do not fight each other). Where a thermostat room does have a radiator, leave it fully open. A separate automatic bypass valve is normally also required to protect the boiler/pump when TRVs close — this is not the same as omitting a TRV. Existing, fully working TRVs do not have to be replaced. Use our Heating Balancing Calculator to ensure the system is balanced correctly once TRVs are fitted.

The available differential pressure (ΔP) across the TRV depends on the system design. For domestic systems with a standard circulating pump, typical ΔP across radiator valves is 0.05-0.15 bar. Higher ΔP gives more control authority but increases pump energy consumption.