Should a TRV go on the flow or the return pipe?

The question only matters for valves that care, and telling which yours is takes ten seconds. But a single-direction valve fitted backwards produces a very characteristic and very annoying knocking, and it is worth recognising.

What you'll need

• A hand
• A torch to read the valve body

Step by step

1. Look for an arrow on the valve body.Cast or stamped into the brass, usually near the union nut. An arrow means it is single-direction and the water must flow the way the arrow points.
2. Look for a bidirectional marking instead.Many modern valves are marked with a double-headed arrow, or state bidirectional on the body or packaging. Those may go on either end.
3. Find the flow pipe.With the system stone cold, turn the heating on and feel both pipes to that radiator. The one that gets hot first is the flow.
4. Understand what conventionally goes where.On most installations the TRV goes on the flow and the lockshield on the return, but plenty of systems are the other way round and work perfectly with a bidirectional valve.
5. Recognise the symptom of a reversed valve.A single-direction valve on the wrong pipe knocks, chatters or hammers as it modulates, and can shut abruptly rather than gradually.
6. Do not simply reverse the pipework.If a valve is on the wrong end, the answer is to fit a bidirectional valve or move the valve, not to re-plumb the radiator.
7. Check the head orientation as well as the body.A head pointing straight up under a windowsill, or squashed against a wall, reads its own microclimate rather than the room. Most heads can be repositioned on the body.
8. Leave the lockshield on the other end.Whichever end the TRV takes, the other gets the lockshield, and that is the one used for balancing.
9. Fit a bidirectional valve when replacing.It removes the question entirely and costs no more. There is no reason to buy a single-direction valve now.

Tips

• Feel the pipes from stone cold. Once the system is hot, both pipes feel warm and the test tells you nothing.
• If you are replacing a valve body, buy bidirectional. It ends the issue permanently.
• A knocking radiator valve that has always knocked is very likely fitted the wrong way round.

Common mistakes

• Ignoring the arrow on an older valve — Single-direction valves rely on the flow pushing the plug closed. Reversed, the flow tries to push it open, so it oscillates and knocks and does not control the room properly.
• Fitting the head where it cannot sense the room — The head contains the sensor. Behind a curtain, inside a cabinet, or directly above a radiator, it reads a false temperature and the room ends up cold or roasting.

If it doesn't work

Questions people ask