How To Change A Thermostatic Radiator Valve

Tue 6th May 2014 - 9:23am DIY Troubleshooting How To Change A Thermostatic Radiator Valve

Benjamin Clarke

This article is going to look at how to change over an old thermostatic radiator valve for a new one. But these instructions can also be followed if you’ve got a radiator with two lockshields on it and you want to change one of those lockshields for a TRV.

The first thing we do is shut off the water supply to the heating system and we turn all electricals off. Make sure your programmers are calling for no heat, your room thermostats are turned down and ensure that the system is not going to kick in at all. If we have a dry system when the pump turns on, it could burn the pump out and the boiler could also overheat.

Once you’ve isolated the water supply and the electrical supply, find the drain off nearest to the radiator you want to do the work to and make sure it’s below the radiator! Pop your hose onto the drain off and remember to put a towel underneath because sometimes they have a habit of leaking, then take the hose outside. Once the hose is outside, use a star key to undo the drain-off. You could also use a pair of grips or an adjustable spanner instead.

Next, take a radiator bleed key and go around all the radiators in the house opening up the bleeds. Use the bleed key to let air into the system. The main reason for doing this is, not only does it prevent the system from allowing more water in, but it also stops the system from creating a vacuum (a problem whereby water being held in radiators above you is suddenly released while you’re working on the rad, causing a flood or a mess.) Once you’ve opened all the bleeds and your hose is emptying water outside, wait around 10-15mins for the system to drain down fully.

When you get your new TRV, make sure you remember that the numbers on it don’t correlate to the temperature of the radiator, they actually correlate to the temperature of the room. That is a common misconception about thermostatic radiator valves.

To remove and change the TRV, the first thing to do is place a towel under the valve in case of any leaks. On the old TRV, get a pair of grips and fully undo both nuts - one that is connected to the rad, and the other that is connected to the pipe. You may see that your old TRV has an arrow indicating the flow of the water. It is highly likely that your new one is a universal TRV, as shown by the arrows pointing both ways on the body of the valve, which means it can be fitted at either end of the radiator.

Slacken off both nuts as much as you can and you should be able to wiggle the old valve and remove it completely. Use an old cloth to clean the two joints off and spread some jointing compound around the two olives that will still be there. Also run some of the compound around the inside of your new valve. You should then be able to add the body of the new TRV onto the two joints and, again, use a grip or pair of adjustable pliers to tighten both nuts back up. You can then take the head of the new valve, add it on top of the body and set it to 4.

Next, you should go back around all the other radiators in the house and shut all the airbleeds that you had open earlier on. (It’s very important that you don’t forget one, so always double check. This is because you need to fill the system back up and if you’ve missed an airbleed, it is likely to cause a puddle by the forgotten radiator!) Go to your drain-off and shut that, then remove your hose and take it outside. Now you are ready to refill the system.

Filling up the system is quite easy, so don’t worry if you haven’t done it before. If you’ve got a gravity-fed system, go to your airing cupboard and open the valve up that you shut off earlier, and water will automatically start to go into the system. Then you need to go round each one of the radiators that you shut a minute ago, open each one individually and stay with it until water comes out. You go round the whole house doing that and filling the system up gradually and once you’re happy, you can test your new valve for leaks. The most important part of this process is to then add inhibitor.

If you have a pressurised system, it’s a little more complicated. You need to open up the valve on your filling loop, which could either be in your boiler or inside your airing cupboard, then pressurise the system to 1 bar. Then go to the radiator that you want to bleed first and bleed it. Once the pressure drops, you’ll find that no more air comes out, so you’ll have to shut that radiator again, go back to your valve and re-pressurise to 1 bar. Keep venting the radiators out and keep re-pressurising until you’ve filled up all the radiators in the system. Only then can you test your new valve for leaks and be sure that you can add inhibitor. (Adding inhibitor should always be the last step of this process, because if you add inhibitor, then discover that you’ve got a leak, you’ll have to drain down the system again and will lose all of the inhibitor you’ve just added.)

You can view a video of the entire process of changing a TRV here.

If you do need to change or upgrade your TRVs, please view our fantastic thermostatic radiator valve range

Click here to buy inhibitor

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