Fit a Thermostatic Valve With Drayton TRV4 in 6 Steps for 20% Less Heat Loss

Why the TRV4 body matters before you touch the head

The Drayton TRV4 sold in two visible parts: a chrome or white sensing head and a valve body with a 15mm compression connection at the radiator tail. The head carries a liquid-filled capsule that expands as room air warms, pushing a pin down onto the valve seat and throttling flow. When you buy the TRV4 as a pair you get both; when you replace only a seized head you reuse the existing body, and the thread you are checking is M30x1.5, the de facto standard across Drayton, Honeywell and Danfoss heads. A head from one brand will physically screw onto another brand body if both use that thread, though the pin travel differs and calibration shifts.

Flow direction is the part that catches people. The TRV4 body is bidirectional, so it tolerates either flow direction on the radiator, which is unusual. Many older valves are unidirectional and will chatter or fail to close if fitted backwards. Confirm the body says TRV4 cast into the brass, not TRV3 or an unbranded copy, because the TRV3 head will not seat correctly on a TRV4 body. Check the radiator tail thread is 1/2 inch BSP before assuming the 15mm compression olive will mate, since some panel radiators ship with a 3/4 inch tail and a reducing bush.

Step one: drain below the valve

Close the lockshield valve at the opposite end of the radiator and count the turns so you can reopen it to the same position. Closing the existing TRV by hand or with the manual head as far as it goes is not enough on its own, because a seized valve may not seat. Run a hose from the boiler drain cock if you want to empty the whole circuit, or isolate just the one radiator by closing both ends and catching the water in a shallow tray. A single 1000mm by 600mm double-panel radiator holds roughly four to six litres, so a washing-up bowl and a few old towels handle a single-radiator job.

Crack the bleed screw at the top with a clock key to let air in once both valves are shut. Without that air path the water column holds by vacuum and dribbles unpredictably when you loosen the union nut. Open the bleed slowly and keep the tray under the valve tail. If you are draining the full system, attach the hose to the drain cock at the lowest point, run it to an outside gully, and open the cock with an adjustable spanner. Watch the pressure gauge on a sealed combi system drop toward zero before you undo anything.

Step two through four: the union, the body, the tape

Undo the union nut that joins the valve body to the radiator tail using a 24mm or adjustable spanner, holding the valve body still with a second spanner so you do not twist the copper pipe. The pipe will move and work-harden if you let it rotate, and a hairline crack at the next solder joint is the classic delayed leak. Lift the old valve away and inspect the radiator tail. If the tail itself is sound you keep it; if the thread is corroded you wind out the tail with a hexagonal radiator spanner that fits inside the tail bore.

Wrap the male tail thread with PTFE tape, eight to ten turns in the direction the nut tightens, so the tape beds in rather than unwinding. The TRV4 supplies a fibre or rubber washer for the union face, and that washer makes the actual seal; the PTFE on the tail thread is belt and braces. Slide the union nut and olive onto the pipe, present the valve body, and start the nut by hand for two full turns before any spanner touches it. Cross-threading brass at this point ruins the nut and the body.

Tighten the radiator-tail union until the washer compresses and the joint feels solid, then a further quarter turn. On the 15mm compression side, tighten the olive nut one and a quarter turns past finger-tight as a starting figure, then check for weeps under pressure and nip further only if needed. Overtightening a compression olive deforms it and guarantees a leak you cannot cure without cutting the olive off. Set the valve body so the head will end up roughly horizontal, because a TRV4 head mounted pointing straight up sits in the rising warm air off the radiator and reads high, closing the valve before the room is warm.

The orientation rule is why the body has a union that lets you rotate it. A head fitted vertically above a radiator can read three to five degrees warmer than the room because of the convection plume, which makes the room run cold while the valve thinks it has hit target. Horizontal mounting, with the numbers facing into the room and clear of curtains and shelving, puts the capsule in still air at genuine room temperature.

Step five: refill and check the joints

Close the bleed screw, reopen the lockshield to the counted number of turns, and refill. On a sealed system top up through the filling loop until the gauge reads 1.0 to 1.5 bar cold. Reopen the radiator valves and bleed the top of the radiator again until water runs without spitting. Watch both union joints and the compression nut for the first five minutes under full pressure, because a weep shows as a bright bead before it becomes a drip. Wipe each joint dry with kitchen roll and look again after the radiator has reached full temperature, since brass and copper expand and a marginal joint opens up hot.

Step six: the calibration nobody does

The TRV4 head has a numbered collar from 0 to 5 plus a frost setting marked with a snowflake. Those numbers are not temperatures. Setting 3 corresponds to roughly 20°C on a factory-calibrated head, and each number step is approximately 3 to 4 degrees, but the capsule responds to the air immediately around it, so the true room setpoint depends on where the head sits. Set the collar to 3, let the room stabilise for a few hours with internal doors as you normally keep them, and read a separate thermometer placed at seating height in the middle of the room. If the thermometer reads 22°C with the collar on 3, drop to between 2 and 3 to reach 20°C.

The frost setting holds the valve open enough to keep the radiator from freezing, around 5 to 7°C, which matters for a room left unheated through winter. The TRV4 also has a maximum-temperature limiting feature on some versions, set by lifting and repositioning the collar against internal stops, useful in a child’s bedroom where you want a hard ceiling. None of this works if the head is jammed against a windowsill or buried behind a long curtain, both of which trap the convection layer and feed the capsule false air.

What a thermostatic valve does not fix

A TRV4 on every radiator regulates each room independently, but it cannot lower the flow temperature the boiler produces, and it does not replace the room thermostat that tells the boiler to fire at all. If every TRV closes because every room is warm, the boiler keeps circulating against closed valves unless an automatic bypass or a wired room stat shuts it down. That dead-heading wastes pump energy and short-cycles the boiler.

The heat-loss reduction from fitting TRVs comes from stopping individual rooms overheating past the point you actually want, particularly south-facing rooms that gain solar heat by afternoon while north rooms stay cold. A manual valve heats both to the same flow and the warm room overshoots. Where a room already runs at the right temperature from solar gain or a wood burner, the TRV4 throttles that radiator to near nothing and the saved heat stays in the water for rooms that need it. The open question is what your lockshield balance was doing before the swap, because a system never balanced across its radiators will still send too much flow to the nearest radiator and starve the far one, and no thermostatic head corrects a balancing fault upstream of itself.