What is the Kv value of a TRV?

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).

What is a pre-settable TRV?

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).

How do I choose the right TRV body size?

Match the TRV body to the pipe size: 10mm for microbore, 15mm for standard domestic, 22mm for larger radiators. If the required Kv is near the maximum for a body size, consider upsizing. Most domestic radiators use 15mm TRV bodies on 15mm copper pipework.

Do I need TRVs on all radiators?

Building Regulations Part L requires TRVs on all radiators except one (which acts as a bypass). The room with the room thermostat should not have a TRV, or if it does, it should be set to maximum. Bathrooms may be exempted in some guidance.

What differential pressure should I use for TRV sizing?

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.

TradeCalculator

TRV Sizing Calculator — Kv & Flow Limiting

Calculate TRV Kv value from radiator flow rate and pressure drop. Size valve body and determine pre-settable flow limiting position.

Radiator Output

Radiator rated output

Valve Body Size

Match to pipe diameter

System ΔT

Flow-return temperature difference

Available ΔP

Pressure drop across valve

How We Calculate This

This calculator determines the required Kv value for a thermostatic radiator valve based on the radiator's heat output and the available pressure drop across the valve.

Formulas

Flow rate: kW ÷ (4.186 × ΔT) = L/s
Kv: Q (m³/hr) ÷ √ΔP (bar)
Body size: Selected from Kv range tables
Pre-set position: Kv ÷ (Kv per setting step)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.