Do I need new radiators when I get a new boiler?

In most homes, no. When you replace your boiler, radiators that heat up evenly, aren't rusting or leaking and are big enough for their rooms can usually stay. What often does need changing is the valves on them, and the water inside the system, which should be cleaned before a new boiler goes on.

This post explains what decides it, the signs a radiator is near the end of its life, what the rules ask for when a boiler is changed, and the checks you can safely make before your quote. Feeling radiators and looking them over is fine to do yourself. Draining the system, taking a radiator off the wall or undoing valves is a job for a heating engineer.

Why most radiators can stay

A radiator is a steel panel that hot water flows through. The boiler heats the water and the pump pushes it round. If the panel is sound and clean inside, its age matters little, and a well-kept radiator can see out more than one boiler.

Swapping radiators that work adds cost and mess without making the house warmer. The question isn't "how old are my radiators?" but "are they still doing their job?"

Signs a radiator may need replacing

These are the signs that point towards a new radiator rather than just a clean or new valves:

• Cold patches that come back soon after the radiator has been bled or the system cleaned.

• Rust, bubbling paint or damp marks along the seams or the bottom edge.

• Slow weeping leaks, often showing as stains on the floor or carpet underneath.

• Valves that are seized solid or have broken.

• A room that never gets warm, even when its radiator is fully hot. That usually means the radiator is too small for the room.

One rusty or leaking radiator doesn't mean the rest are the same. It's normal to replace one or two and keep the others.

Moving from a tank in the loft to a combi boiler

Older systems with a water tank in the loft run at very low pressure. A combi or system boiler runs a sealed system, which is topped up to a set pressure, usually around 1 to 1.5 bar on the boiler's gauge when cold.

Radiators and joints that were thin with rust but held at low pressure can start to weep once the system is sealed and pressurised. That's why a conversion is when replacing tired radiators makes most sense, and why some of our customers have new radiators fitted at the same time as the boiler, while others keep every radiator they have.

Sludge, and why a clean matters more than new radiators

If a radiator is warm at the top but cold at the bottom, the usual cause is sludge. This is mostly magnetite, a black iron oxide that forms as steel radiators and pipes slowly rust from the inside. It settles in the bottom of radiators and can block the narrow passages in a new boiler.

A new boiler should go onto a clean system, finished with an inhibitor, which is a chemical that slows that rusting inside. Cleaning often brings cold radiators back to life, so do it before deciding they need replacing. You can read how we clean a heating system when we fit a new boiler for more on what that involves.

Radiator valves and the rules for new boilers

In England, the Building Regulations now say that when a boiler is replaced, each heated room should have its own thermostatic control. For most homes that means a room thermostat in one room and a thermostatic radiator valve on the radiators in the other rooms. A thermostatic radiator valve, or TRV, is the valve with a numbered head that senses the room's temperature and turns that radiator down when the room is warm enough.

So a new boiler often means new valves, not new radiators. The exception is a radiator whose old valves are seized or whose connections are badly corroded, where fitting new valves can cost nearly as much as a new radiator.

Is each radiator big enough for its room?

Modern boilers are condensing boilers. They recover extra heat from their own exhaust gases, but they can only do that well when the water coming back from the radiators is fairly cool.

A radiator that is big enough for its room can heat it with cooler water, so the boiler runs more efficiently. A radiator that is too small forces the boiler to run hotter, which wastes some of the gain from the new boiler. This matters most in rooms that have changed, such as an extension or a loft conversion. An engineer can compare each radiator's heat output with how much heat the room loses through its walls, windows and floor.

Safe checks you can make before your quote

With the heating back on for winter, now is a good time to look. None of these need tools:

1. Put the heating on and wait about half an hour, then carefully feel each radiator at the top and the bottom. They can be hot enough to burn, so touch lightly.

2. Note any that are cold at the top, which usually means air, and any cold at the bottom, which usually means sludge.

3. Look for rust, bubbling paint, damp marks, or stains on the floor below the valves.

4. Note any room that never gets comfortable.

5. Check which radiators already have a TRV with a numbered head.

Don't drain the system, loosen valves or take radiators off the wall. If you bleed a radiator, check the pressure on the boiler afterwards, as bleeding lets some out.

How we look at radiators on a new boiler quote

When we quote for a new boiler, we look at the whole heating system, not just the boiler on the wall. One customer described being "really impressed with their thorough assessment of my whole heating system and full explanation of what was required."

We'll tell you which radiators are fine as they are, which only need valves, and which are worth replacing and why, so you can decide what to do now and what can wait.

If your boiler is due for replacement and you're not sure about your radiators, B.Brill Plumbing & Heating LTD can look over the whole system on the quote visit in Milton Keynes and nearby towns. Book a visit through our boiler installation and replacement service. If it's one radiator that has let you down, our radiator installation and service covers single replacements too.