There are barely any instructions in the manual. Do I need more than a spanner to do replacement?
Is it necessary to shut off the water supply to the radiator in order to replace the radiator valve?
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You need to drain the water from the heating system, effect the remove and replace, then refill the heating system.
Some circulating-water heating systems are filled with tap water from the home's water supply, while others are isolated from the water supply plumbing and are filled with some other liquid. My experience is entirely with the most common tap water type, so I cannot tell you how to empty and refill an isolated system.
If you have a tap water connected system, you need to find three connections on your heating system, one where the water is admitted, one where it can be drained, and one where air can bled from the system.
The water will be admitted into your heating system from your home's cold water pipes, at a connection with a valve, pressure regulator, and probably a backflow preventer. Look for a pipe from the house plumbing into the furnace or into the pipes near the furnace. Look for a pipe interrupted by a valve and one or two other fittings.
The water can be drained from your heating system via a faucet or similar valve near the bottom of your furnace. It will have threads on its spout to accept a garden hose. Attach a hose to this faucet and place the other end of the hose outside your house, at a point lower than the bottom of the furnace.
The air bleed valve is usually found at the top of each radiator. These rarely have a handle attached; you are supposed to open it with a small spanner or a special key.
To empty the heating system: turn the furnace off, close the admitting valve, open the drain faucet, and open the highest air bleed valve in the house. Wait until water stops flowing from the end of the hose.
Use two spanners to remove the broken thermostat valve, and to install the new one.
To refill the heating system: close the drain faucet, open the admitting valve, and wait by the air bleed valve with a pan or bucket until water comes out, then close the air bleed valve. Now visit every radiator in the house, open its air bleed valve, wait with your bucket to catch the water that comes out, and close the valve.
If you have trouble confidently locating these places on your heating system, take some photos and post them here. Include a picture of one of your radiators, so you can get specific pointers from someone who has experience with your make and model.
Heating systems can vary by region. I have answered this from a UK perspective as you appear to have bought from B&Q.
If the broken part is the white thermostatic cap, then it would be much easier to find a compatible replacement to avoid having to change the valve body. The cap can be removed by undoing the knurled ring directly below it, finger force only is required. On all the ones I have seen, the cap can be removed without water leaking.
If the valve stem has broken then you will need to replace with a complete new assembly.
This isn't a difficult job if you are comfortable with plumbing connections and heating circuits, but there can be complications and there are real risks of leaks, minor floods or system damage if you make a mistake. The water in the bottom of radiators can sometimes be filthy and contain sludge. If it gets onto your floor it will make a mess. If you are unsure, then ask a heating engineer or plumber to do this.
Rather than me explain in detail how to do this, I recommend watching some videos and reading some guides. For example: https://youtu.be/oMXgKXVZw_0
In summary though, you will need to turn off the boiler, turn off the fresh water supply to the boiler and heating circuit, drain the water from this radiator and possibly the other radiators in your house, remove the old valve and possibly the tail pipe between the valve and radiator, replace with the new parts and refill the heating circuit, adding corrosion inhibitor. As the system refills, check for leaks and bleed off the air. Remember not to leave any bleed valves open. Only turn on the boiler when you are sure the system is full of water. It might take hours or weeks for any remaining air to gradually find the high points and allow you bleed it all out.
You will need a spanner for the valve nuts, an adjustable spanner to hold the valve body when you loosen or tighten the nuts, a spanner or key appropriate for the existing radiator tail and also for your new one, a drain off hose and key, a bleed key, some PTFE tape for the tail to radiator joint and some corrosion inhibitor fluid. Also have plenty of rags or old sheets on hand just in case.
Be mindful of:
It is possible to buy special 'freezer packs' that are designed to fit round pipework, with the intention of locally freezing the contents of the pipe, causing a temporary blockage.
Having said that, I would be rather wary of using them, (and have never done so) as the result if you get it wrong would be some catastrophic and messy flooding.