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How to Fit a Thermostatic Radiator Valve

Skill Level: Intermediate


Thermostatic radiator valves (or TRV’s) are fittings attached to a central heating radiator that allows you to control the temperature to individual rooms and, if turned down when not needed can also help reduce heating costs.

Thermostatic valves, known commonly as “thermostats” can be fitted to both gas and oil fired central heating systems, but aren’t always compatible for solid fuel systems – check with an expert or your boiler manufacturer.


Older Properties

Many older, period types of property have up-graded central heating systems installed, so there would be no problem in fitting a thermostat, but some may not, and may have imperial size pipes – in which case you will need to check that compatible fittings are still available.
Before fitting a thermostatic radiator valve you must first drain part, or all of the central heating system. If in doubt, consult your manufacturer or supplier.

Single and two pipe layout heating systems

Thermostatic valves can only be fitted to heating systems with a two pipe layout. On this system radiators have an inlet and outlet connected to separate flow and return pipes that carry the hot water around your home.
For safety purposes, when fitting a thermostatic valve to this type of system you must always leave a least one radiator with a manually operated wheel-head valve left permanently in the “open” position, to allow pumped water to move freely throughout the system, in order to help prevent a build up of pressure.

Lightweight boilers

Be aware that some lightweight boilers demand a high water flow rate and could require two or more radiators to be left open. If in doubt, check with your supplier or manufacturer.

Choosing the right thermostatic radiator valve

There are many manufacturers of thermostatic radiator valves from which to choose. Some are designed to be mounted vertically, others horizontally. It’s just a matter of preference. All are designed to do basically the same job, but there are a few features you should look out for.

Thermostat sensors

Although most thermostats have temperature sensors, often inside the cap of the valve body; some valves have remote sensors which can be positioned some distance from the radiator, and can give a more accurate estimate of the “room” temperature.

Some thermostat valves are fitted with ceramic discs and although they are a bit more expensive than others, they are less likely to be affected by “hard water” and scale build up, and therefore less prone to jamming or seizing up.
Consider also, a thermostat valve incorporating a frost setting, ideal for when you are away from home and temperatures plummet.


Draining the heating system

This is the first thing to do before fitting the new thermostatic valve. This is done by firstly;
Turn off the entire heating system, including the pump, programmer, and the boiler.
Next, tie up the ball valve in the feed and expansion tanks by laying a length of rod or timber across the top of the tank. Then drain the system using a hose pipe connected to the drain-cock, located at the base of the system.

Removing the old valve

When you have completely drained the heating system, loosen the compression joint that connects the old wheel-head valve to the pipe. Then, unscrew the coupling nut to the radiator tail pipe.
If you find the old valve has an external nut, simply loosen off with an adjustable spanner or wrench.

Fitting the new radiator thermostatic valve

The first you need to do is un-screw the head and tail of the new valve and wrap the thread of the tail piece with PTFE tape. This will ensure a watertight seal.
Next, connect, and screw tighten up the valve tail piece into the water inlet pipe using the adjustable spanner. Then, re-connect the valve body to the fixed tailpiece, and open the valve using the manual hand cap.
When you are satisfied there are no leaks, remove the manual cap and fit and secure the valve sensor head. Tighten the securing nut, making sure the temperature value numbers are facing forward.

Refilling the system

Firstly, shut closed the drain cock and release the suspended tank ball-cocks.
As the system is filling, bleed the system of air loosening off the venting nipple, situated at the top of the radiator. As the system fills, air will escape through this vent, and when fully excluded (water spurting out) screw down tight again. Check the other radiators in turn, and check again for any leaks to the new valve. Job done.

Tools you will need:
Bahco (80 Series) 100mm Chrome Adjustable Wrench
Bahco (80 Series) 100mm Chrome Adjustable Wrench
- Rolson 350mm Stillson Type Pipe Wrench
Rolson 350mm Stillson Type Pipe Wrench

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