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How to fit a TRV

Thermostatic Radiator Valve

What is a Thermostatic Radiator valve?

A thermostatic radiator valve (TRV) is a device fitted to your radiator pipework that will individually control the temperature in the room by setting the heat output of the radiator it’s attached to.

There are 2 main types of thermostatic radiator valve. One uses a spring that expands and contracts and the other uses a wax jacket that does exactly the same.

What’s the difference between a TRV and a normal radiator valve?

Firstly a normal radiator valve is just a valve that moves up and down and shuts off the flow of hot water from the boiler in to the radiator, just like the tap on a bath tap.

The main difference with a TRV is that it automatically controls the amount of hot water entering a radiator according to the current heat of the room thats its situated in.

How does a TRV work?

For this example we are going to look at the spring type TRV, but the wax jacket version is fitted in a similar manner.

So we have a spring that is made of metal that is very susceptible to the temperature around it. If the room is cold the metal spring will be contracted – because thats what happens when metal is cold. Therefore when the temperature in the room is cold and the spring contracts the valve jumper will be open and hot water will be allowed to flow into the radiator.

If the room is warm the spring will expand and the valve will be pushed down and the flow to the radiator will stop – thats basically how they work.

A common problem with customers is that when they come in and the room is cold. Although the radiator, when they feel it, is blazing hot they still go to the thermostatic radiator valve and open it up more. Thats completely pointless.The room is cold and the radiator is hot. Therefore the radiator valve knows that the room is cold and is trying to heat it up. If you go into a cold room and feel that the radiator is hot, do not touch the thermostatic radiator valve. Only touch it when you go into the room and its cold and you feel that the radiator is cold too.

Where should I fit a TRV?

Something you need to consider is where you can actually install TRV’s.

For example, pretend you have a house with 2 rooms. In one room you have a room thermostat, which controls the whole heating system and one radiator. In the other room you have another radiator with a TRV on it.

In this scenario you should refrain from fitting a TRV on the radiator that has the room thermostat. The reason for that is, if the TRV closes down and the room thermostat is not satisfied it will never knock the heating system off to the whole house, and you won’t get any saving.

The room thermostat needs to have an exact true reading of the maximum capacity that the heating system can give out. Therefore the radiator in the room with the room thermostat should have a lock shield on it that is fully open.

Disclaimer: The information contained in this page is for information purposes only. We cannot be held responsible for any problems that arise as a result of following the guidance contained on this page. We strongly recommend that plumbing work of this nature only be attempted by a qualified expert plumber. Performing this type of work as DIY can lead to other plumbing issues such as leaks, airlocks and central heating failure.

How to change fit a TRV?

So lets find out how you change a local shield over for a TRV. We assume you know how to drain down a heating system already. If you don’t then you can read our article on how to drain a heating system.

After draining the system the next step is to allow air into all the radiators where you want to install TRV’s. Generally if you are a bit worried about this then let air into every radiator in the house. Open the valve with your radiator key and you should hear the air sucking.

If you’ve followed our article you should have a hose from the radiator to either your back or front garden. As soon as you see your hose outside stop running that should indicate that the heating system is drained down and you’re ready to change the valve.

1. Removing the old lock shield valve.
To do this pull the top cover off to reveal the simple on/off valve. Using grips and an adjustable spanner, slacken off the nut that is coming out of the radiator pipework. Once this is loose, and if your pipes coming out of the floor have enough flex, you can gently move the pipework out to see if all the water has drained out of the system.

Next, loosen off the 15mm pressure fitting on the bottom of the valve so that you can then remove the whole valve from the radiator pipework.

2. Fitting the new thermostatic radiator valve.
Sometimes the new TRV will not fit on the current radiator insert. All new TRVs are supplied with a new insert so usually the one on the radiator will need removing. There are special tools to do this. We find a simple hex tool is best that you insert into the hole and then use adjustable spanner to loosen the insert.

Now PTFE the thread of the new insert (wrap it round about 10 times) and push the insert into the radiator and tighten it up. The old nut and olive should be fine to use but if you’re not happy just pull the nut off and use a pair of grips to pull the olive off.

Wrap a little bit of PTFE around the new olive to ensure a watertight seal. Push the fitting back on and tighten up the bottom joint. Tighten everything and remove the top plastic cap which should reveal the valve. You can test that it springs up and down by pushing the valve. Put the TRV head unit on the top and have it set to 5.

You should now have a working TRV.

If you need help from the professionals, then try our radiator services here in London.

How to fit a TRV

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