TRV vs Manual Radiator Valves: The Plumber's Complete Guide

If you're fitting, replacing or upgrading radiators on any job, understanding thermostatic radiator valves (TRVs) is non-negotiable. Building regs demand them on most new or replacement radiators, and specifying the right type — straight, angled or corner — avoids call-backs. Here's everything you need to know.

What Does a TRV Actually Do?

A thermostatic radiator valve senses the air temperature in a room and automatically adjusts the flow of hot water into the radiator to maintain a set level of warmth. The head contains a wax or liquid-filled capsule that expands when warm, pushing a pin to restrict flow, and contracts when cold to open it back up.

That self-regulating action is the key difference from a manual lockshield or wheelhead valve, which stays at whatever position you set it. A TRV responds to the room — a sunny south-facing room gets less heat, a cold north bedroom gets more.

The result: lower energy bills, less CO₂, and a more comfortable building. That's why Part L of the Building Regulations (conservation of fuel and power) requires TRVs on radiators in new build and most replacement work. If you're installing radiators without TRVs in 2026, you need a good reason.

TRV vs Manual Valve: When Does Manual Still Apply?

Manual radiator valves aren't dead. There are legitimate situations where you skip a TRV:

• The room with the room thermostat — if a thermostat already controls the zone, adding a TRV to that radiator can cause hunting (the boiler cycling on and off). Leave that radiator on a manual valve so the thermostat has full authority.
• Bathrooms — not required where there's a separate heat source (towel rail), though many plumbers fit one anyway for comfort control.
• Where TRVs would be inaccessible — under-sink radiators built into furniture may be fitted with lockshields only if the installation genuinely prevents access.

Outside those exceptions: fit TRVs. It's good practice, it's regs-compliant, and your client will thank you at the next energy bill.

Straight, Angled, or Corner — Which Do You Need?

This is where most mistakes happen on first fix. The geometry of the pipe entry determines the valve type:

• Straight valve — pipe enters horizontally and connects straight into the radiator valve. Used when the pipe runs parallel to the floor along the skirting.
• Angled valve — pipe enters from below (vertical floor pipe) and turns 90° into the radiator. The most common type on modern installs.
• Corner valve — pipe enters from below and the valve body turns 90° in the horizontal plane to enter the radiator from the end. Used where the radiator sits in a corner or the pipe exits the floor behind the radiator end.

Getting this wrong costs time. Measure the pipe position relative to the radiator inlet before ordering. Most domestic installs on replacement jobs are angled or corner.

TRV Temperature Settings Explained

Most TRV heads are numbered 1–5 (or 1–6), not in degrees Celsius. The actual room temperature varies by brand, but the rough guide is:

• * (frost protect) — ~7°C, pipes protected but room stays cold
• 1 — ~12°C (unheated storage room)
• 2 — ~16°C (hallways, utility rooms)
• 3 — ~20°C (most living rooms)
• 4 — ~22°C (bathrooms)
• 5 — max, fully open (not thermostatic at this setting)

Advise your client to set living rooms to 3 and bedrooms slightly lower. Setting everything to 5 defeats the entire purpose.

Pipe Size: 15mm Is the Standard

Almost all domestic radiator valves in the UK are 15mm — matching standard central heating pipework. You'll occasionally encounter 10mm on older systems or 22mm on commercial radiators, but 15mm is what you'll order 99% of the time.

Connection to the radiator is typically 1/2" BSP — the thread on the radiator tail. Check before fitting: some continental radiators use a different thread and need an adaptor.

Products We Stock

APM carries a solid range of 15mm TRVs for domestic installs:

• Embrass Peerless EDEN TRV Angled 15mm — White Head — a reliable workhorse angled TRV with a clean white finish, ideal for modern radiators
• Taurus 15mm Corner TRV with Lockshield — corner configuration with matching lockshield included, chrome finish
• Taurus Corner TRV with Lockshield — White/Chrome — same corner geometry in a two-tone white and chrome finish

Browse the full Heating & Controls collection or the Radiators collection for the complete range. Same-day collection from our Acton trade counter — 24 Western Avenue, W3 7TZ. Call ahead on 020 8702 8080 if you're after something specific.

Installation Tips

A few things worth checking on any TRV installation:

• Fit the valve in the flow, lockshield on the return — TRVs should be on the flow side. The lockshield is balanced on the return. Getting them the wrong way round doesn't break anything, but it's better practice on the flow.
• Remove the TRV head before soldering — the wax element in the head won't survive soldering heat. Fit the body first, let it cool, then fit the head.
• Don't overtighten the head — finger tight plus a quarter turn. Overtightening damages the pin and the head won't operate correctly.
• Commission with head removed — when filling and bleeding, remove TRV heads so valves stay fully open. Refit when the system is running at temperature.
• Balance the system after fitting — TRVs do some of the balancing work, but a properly balanced system gets to temperature faster and runs more efficiently.

Summary

TRVs are the default for domestic radiator work in 2026 — Part L, energy efficiency, and client expectation all point the same way. Know your geometry (straight/angled/corner), confirm your pipe sizes, and leave the manual valve for the room with the thermostat. Get them from the counter in Acton and you're on the job same day.