How to add a radiator to a combi boiler system

If you want to enhance your comfort levels by adding a new radiator, keep reading. Here, we look at when you might need to upgrade your heating system in this way and how to do it, and we offer tips on how to ensure your existing radiators are working as efficiently as possible.

Do I need new radiators for a combi boiler?

If you’re switching from a different type of boiler to a combi model, you might be wondering if you automatically need to switch to new radiators. In short, you don’t - unless there are problems with your current radiators. As long as they are functioning well, you can keep them in place. They will probably benefit from a powerflush though. This is cleansing process that removes deposits of debris and sludge from your central heating system.

Bear in mind that if you’re replacing a non-combi boiler, you will have to adjust your pipework to fit the new system. Your heating engineer will be able to advise you on what’s required and how much this will cost.

How many radiators can a combi boiler run?

If you wish to add a new radiator to your central heating system, you’ll need to make sure that your boiler is large enough to meet the extra demand placed on it. Boilers come in different sizes. This doesn’t refer to their physical dimensions, but instead to their output in kilowatts. 

For example, a typical three or four bedroom house with around 10 radiators usually needs a 24-30 kW combi boiler. Bigger properties with more radiators generally need more powerful combi boilers that could be anything from 30 to 42 kWs. Alternatively, rather than a standard combi model, these homes may benefit from a different type of boiler, such as a storage combi design.

Normally, adding one or two new radiators doesn’t cause any problems, but to be on the safe side, it’s a good idea to check your boiler’s output and make sure it’s powerful enough to accommodate this.

See our useful guide for more information about choosing the right boiler for your home.

How to add a new radiator to my central heating system

Unless you’re skilled and experienced in this type of task, it’s usually best to get a heating engineer to fit a new radiator for you. If you decide to go ahead and do this yourself, make sure you research the process first so that you have a clear idea of exactly what’s involved.

Firstly, you’ll need to establish where the new radiator will go and what size it will be. You can use an online British Thermal Unit (BTU) calculator to work out what size your new radiator should be depending on the room you’re installing it in. In terms of location, you may opt to place it under a window or in an area of the room that tends to feel particularly cold.

Once you know where it will go, you’ll have to work out the simplest way to connect your radiator to your central heating system. This means finding the nearest pair of flow and return pipes that you can connect it to. These may be attached to a wall or positioned under floorboards. You can work out which are the flow and return pipes by feeling them when your heating first comes on. The flow pipe will heat up more quickly. To avoid confusion later, it’s a good idea to mark them.

Before you start work to connect your radiator, make sure you turn your boiler off and drain the system. Next, wrap PTFE tape around the threaded parts of the valves five times and then fit them to your new radiator. This will help ensure they seal properly.

It’s then time to fix your radiator brackets to the wall. Note that when you’re measuring where these brackets need to go, you should leave around 125 mm of clearance between the bottom of the radiator and the floor. Once the brackets are up, you can fit your radiator to the wall.

Next, cut and fabricate the pipework to connect the radiator to the flow and return pipes. You could use copper or plastic piping to do this. Once you’re confident that the pipework is connected and everything is in place, you can refill your heating system. Because some air will be trapped, you should bleed the new radiator to ensure it works as efficiently as possible.

How to bleed radiators on a combi boiler

Bleeding a radiator is simply a way of getting rid of trapped air that can create cold spots. If your radiators sometimes make a gurgling or rattling sound, or they feel cooler at the top than the bottom, this is a good sign that they need bleeding. To do this, you’ll need a radiator bleed key (which you can get from most hardware stores) and a towel or cloth to catch any drips.

The first step is to turn your heating on to build up pressure in your radiators. Next, identify which radiators need bleeding by checking each one for cold spots. You should then turn your heating off to avoid scalding yourself and place towels down on the floor underneath the radiator bleed valve, which is usually found at the top of the radiator.

Then simply insert the radiator key into the bleed valve and slowly make a quarter to half turn anti-clockwise. If you don’t have a radiator bleed key and your radiators are modern, you may be able to use a flat-head screwdriver instead. You’ll notice a hissing sound as the air escapes. When water starts dripping from the valve, turn it clockwise to seal it.

If you’re bleeding all the radiators in your home, it’s best to start from the ground floor and work up. Once you’ve finished the process, check the pressure of your heating system. You might need to top it up.

How do you balance radiators for a combi boiler?

If some of the radiators in your home take much longer than others to heat up, you might benefit from balancing them. Also known as hydronic balancing, radiator balancing is a process of adjusting your radiators so that hot water is distributed evenly to each one. It can increase the efficiency of your heating system by as much as 15 per cent.

For best results, you may want to call your heating engineer. Alternatively, you can give this a go yourself. Start by closing the lockshield valves on each of your radiators. These are the valves that control water flow and they are located on the opposite side to the thermostatic radiator valves. Ensuring your central heating is turned on, start with the radiator nearest to your boiler and slowly open the lockshield valve until the radiator is warm and you can feel a difference between the temperature of the flow pipe and the pipe at the opposite side of the radiator. Repeat this with the remaining radiators, moving gradually further away from your boiler.

How to change a radiator valve with a combi boiler

As well as being able to balance and bleed your radiators, you might want to know how to replace radiator valves on a combi boiler system. This can be useful if you need to replace a faulty valve or you want to fit a new thermostatic valve. Be aware that as with replacing a radiator, you should only attempt this if you are confident and experienced in this type of DIY.

If you are doing this yourself, start by switching your heating off and draining the system. Next, place a towel or bowl underneath the valve you are replacing to catch any residual water. You can then undo the top and bottom nuts of the valve and disconnect it. Once it has been removed, you will need to use a plumber’s wrench or hex key to detach the chrome tail that connects the valve to the radiator.

After cleaning the area, you can fit the replacement valve. Start by attaching the new chrome radiator tail - using PTFE tape to help prevent any leaks. You can then secure the new valve in place using the nuts and other fixings provided.

At this point, you can refill the system and switch your heating on, keeping an eye out for any leaks. You should then bleed your radiators to get rid of any trapped air.

As mentioned previously, whether it’s adding a new radiator or replacing a valve, only attempt tasks like these if you are confident, experienced and have the equipment you need. If you don’t, it’s much better to get a heating engineer to do this work for you.