Adding Radiators to an Existing System

Adding a radiator to an existing central heating system is one of the easiest ways to improve comfort in a cold room, extension, or home office. In many UK homes, it’s perfectly possible to install an extra radiator without replacing the boiler, as long as a few key checks are made first.

This guide explains everything you need to know before adding a radiator, from boiler capacity to choosing the right size and knowing when to call a professional.

Can You Add a Radiator to an Existing Central Heating System?

In most cases, yes. Modern central heating systems are designed to be adaptable, and adding one or two radiators is usually straightforward. Problems tend to arise only when the boiler is already working close to its maximum output or when the new radiator is significantly larger than the others in the home.

A quick check at the planning stage can prevent uneven heating, slow warm-up times and unnecessary energy use.

Check Your Boiler Capacity Before Adding a Radiator

Your boiler has a maximum heat output, measured in kilowatts. Every radiator in your home draws from this total output. When a new radiator is added, the demand increases, and if the boiler doesn’t have enough spare capacity, the system may struggle to heat all rooms effectively. For example, if your boiler produces 24 kW and your current radiators require 20 kW, adding a radiator that needs 6 kW would overload the system.

Most modern combi boilers can support an additional radiator or two without issue, particularly if the existing system is well balanced. If you’re unsure, a heating engineer can confirm whether your boiler output comfortably covers both your current radiators and the new one.

If you add a radiator without enough spare boiler capacity, you may notice:

• Radiators taking longer to heat up

• Some rooms not reaching the desired temperature

• Increased energy usage

  
  

Does Boiler Type Matter?

The type of heating system in your home can influence how simple the installation is.

• Combi boilers: Common in UK homes and usually well-suited to adding radiators, provided capacity allows.

• System and conventional boilers: These may involve additional considerations such as hot water cylinders and system pressure.

  
  

If you don’t know what boiler you have, checking the model number or speaking to a professional will clarify things quickly.

Choosing the Right Radiator Size

Selecting the right radiator isn’t just about style or wall space - heat output is key.

The amount of heat a room needs depends on:

• Room size and ceiling height

• Room type (Front room, bedroom, kitchen, etc)

• How exposed the rooms' external walls are to the elements

• Window type (single/double-glazed)

  
  

A radiator with too little output may leave the room feeling cold, while an oversized radiator can put unnecessary strain on the system.

Using our Heat Output (BTU) Calculator is one of the easiest ways to ensure the radiator you choose is suitable for the room. Ultimately, employing the services of a plumber will yield the most accurate results. That being said, our calculator below will provide you with a good guide as to what BTUs will be required to ensure your room is heated properly, which also serves to optimise your heating efficiency.

Where Should a New Radiator Be Installed?

Radiators are traditionally installed beneath windows to counteract cold air entering the room, but this is no longer a strict rule. Modern layouts often place radiators on other walls to suit furniture or interior design.

What matters most is access to existing pipework. If heating pipes are already run nearby, installation is usually simpler and less disruptive. If not, additional pipework may need to be routed through walls or floors, which can increase the project's complexity.

DIY or Professional Installation?

Installing a radiator involves draining the heating system, connecting to existing pipework and refilling the system correctly. While experienced DIYers may feel confident tackling this, many homeowners prefer to use a qualified heating engineer to ensure everything is installed safely and works as it should.

A professional installer will also check for leaks, ensure correct water flow and balance the system once the radiator is fitted, helping maintain even heat throughout the home.

Bleeding and Balancing After Installation

Once a radiator is installed, the system needs to be:

• Bled to remove trapped air - Radiator Bleeding Guide

• Balanced to ensure even heat distribution - Radiator Balancing Guide

Without balancing, radiators closer to the boiler can heat faster than those further away. Proper balancing improves comfort and energy efficiency throughout your home.

Do You Need to Upgrade Anything Else?

Sometimes adding a radiator highlights other areas for improvement, such as:

• Thermostatic radiator valves (TRVs) for better control

• Sludge build-up that reduces efficiency

• Older pipework restricting water flow

  
  

These upgrades aren’t always essential, but they can improve long-term performance and reduce heating costs.

Final Thoughts: Is Adding a Radiator Worth It?

For most homes, adding a radiator is a practical and cost-effective way to improve warmth and comfort. As long as the boiler has enough capacity and the radiator is correctly sized and positioned, the upgrade can make a noticeable difference without major disruption.

Taking a little time to plan and seek advice where needed helps ensure your new radiator integrates easily with your existing heating system. With the right preparation, your new radiator will heat efficiently, look great, and integrate seamlessly into your home.

FAQ

Will adding a radiator increase my energy bills?

Not necessarily. While you are heating a larger area, a well-insulated room with a correctly sized radiator reaches its target temperature quickly, allowing the boiler to cycle off. However, if your boiler is undersized, it may run longer and less efficiently to meet the new demand.

Can I add a radiator to a microbore (thin pipe) system?

Yes, but it is more complex. Microbore systems use very thin copper piping (usually 8mm or 10mm). If you add a large radiator to these small pipes, the water flow might not be sufficient to heat it properly. A professional can determine if you need to "tee off" from a larger manifold instead.

How long does it take to install an extra radiator?

If the pipework is easily accessible (e.g., under floorboards or behind a stud wall nearby), a professional can usually complete the job in 2 to 4 hours. If new pipes need to be routed through solid floors or across multiple rooms, it can take a full day or more.

Do I need to buy a specific type of radiator?

No, you can mix and match styles (e.g., adding a modern vertical designer radiator to a system with standard horizontal ones). The most important factor is the Delta T rating, which ensures the radiator's output matches the operating temperature of your specific boiler.