Radiators cold after your new boiler was fitted? Here's why, and what to do

Last checked: November 2026 · Combi and system boilers · 6 minute read

Your new boiler is under warranty. Never take its cover off or touch anything inside it. The checks below are the normal things a householder is expected to do, like bleeding radiators and checking the pressure. Anything beyond that is the installer's job, and they should come back for free if their work caused the problem.

It's frustrating: you've paid for a new boiler, and now some radiators are lukewarm or stone cold. It happens a lot, and it's usually caused by the installation work rather than a faulty boiler. Most causes are quick to fix.

Quick version

Check the settings: thermostat, radiator valves and the boiler's radiator temperature.
Check the boiler pressure is between 1 and 1.5 bar.
Bleed the cold radiators, then check the pressure again.
Make sure the valves at both ends of each cold radiator are open.
Work out the pattern: which radiators, and which part is cold.
Still cold? Ask your installer to come back.

Each step has important details below. Tap a step to jump to it.

Why it happens after a new boiler

To fit a new boiler, the installer usually drains some or all of the water out of your heating system, then refills it. That can cause problems that weren't there before:

Air: refilling lets air into the pipes, and it collects in the radiators.
Sludge: black sludge (rust from inside old radiators) can get stirred up and block radiators or pipes.
Closed valves: radiator valves sometimes get turned off during the work and aren't fully opened again.
Balance: a new boiler pushes water round differently, so the radiators furthest away may not get their share.
Settings: new boilers are often set to send cooler water to the radiators, because they run more efficiently that way.

Step by step

1Check the settings

Make sure the heating is switched on at the boiler and on the timer, and turn the room thermostat up well above the room temperature.
Turn the valve with numbers on each cold radiator (the thermostatic valve) up to its highest setting.
Most boilers have a control for the radiator temperature, usually marked with a radiator symbol. If it's set low, radiators will feel warm rather than hot. That's often deliberate, to save gas, and it's fine if your rooms still get warm. If they don't, turn it up a little, or ask your installer what it should be set to.

2Check the boiler pressure

Look at the pressure gauge on the front or underneath the boiler. When the heating is off and the system is cool, it should usually read between 1 and 1.5 bar. Check your boiler's manual for the exact figure.

If it's below 1 bar, the boiler may not run properly, or at all. The manual explains how to top it up using the filling loop. If you're not confident doing this, ask your installer to show you. It's a normal thing to ask.

If the pressure keeps dropping again within a day or two, there may be a leak. Tell your installer.

3Bleed the cold radiators

A radiator that's cold at the top and warm at the bottom usually has air in it.

Turn the heating off and let the radiators cool, so you don't get scalded.
Hold a cloth under the small square valve at the top corner of the radiator, and turn it slowly anticlockwise with a radiator key.
You'll hear hissing as the air comes out. Close the valve as soon as water starts to come out.
Do the same on any other cold radiators. Start with the ones downstairs, then do upstairs.
Check the boiler pressure again. Bleeding lets water out, so you'll often need to top it back up to 1–1.5 bar.

Air can keep coming out for a few days after a new boiler is fitted, so you may need to bleed more than once.

4Check the valves at both ends are open

Every radiator has a valve at each end at the bottom:

The thermostatic valve (with numbers) should be turned up fully. If the radiator still stays cold, the small pin inside may be stuck. With the head removed, a pin that's stuck down won't move when you press it. Your installer can free it.
The other valve usually has a plastic cap. It's set by the installer to balance the system. If a radiator is completely cold from top to bottom while the others are hot, this valve may have been left shut. Ask your installer before changing it, because it affects how heat is shared between all your radiators.

5Work out the pattern

Where the cold is tells you a lot. It's also useful to tell your installer.

What you notice	Most likely cause	What to do
Cold at the top, warm at the bottom	Air	Bleed it (step 3)
Cold at the bottom or in the middle, warm at the top	Sludge settled in the radiator	Installer: the system may need cleaning
One radiator completely cold	Valve shut or stuck	Check the valves (step 4)
Radiators furthest from the boiler, or upstairs, are coolest	System needs balancing	Installer can balance it
All radiators only lukewarm	Radiator temperature set low, or low pressure	Check settings and pressure (steps 1 and 2)

6Still cold? Ask your installer to come back

If the problem started with the new boiler, it's the installer's job to sort it out, and it should be covered by their work. When you call, tell them which radiators are cold, which part is cold, and what you've already tried.

It's also fair to ask:

Was the system cleaned before the new boiler was fitted? The industry standard for heating systems (BS 7593) expects the system to be cleaned and treated with a chemical called inhibitor when a boiler is replaced, and usually a filter fitted to catch sludge.
Has the system been balanced?
Can they fill in the Benchmark checklist in the back of your boiler's manual? It records how the boiler was set up and how the water was treated, and manufacturers often need it for the warranty.

Questions people ask

Is it normal for radiators to be cooler with a new boiler?

A bit, yes. Modern boilers often run the radiators cooler than old ones, because it uses less gas. Radiators that feel warm rather than hot are fine as long as your rooms reach the temperature you want.

Do I need a power flush?

Not always. If the radiators are cold at the bottom from sludge, the system needs cleaning, but there are different ways to do it. Ask the installer what they did when fitting the boiler before paying for anything extra.

Will bleeding radiators affect my warranty?

No. Bleeding radiators and topping up the pressure are normal household jobs. Your boiler manual explains how. Just don't open the boiler itself.

Related guides

This guide is general information, not professional advice. It was checked against guidance from boiler manufacturers, energy companies and heating industry sources in November 2026. If in doubt, contact your installer or a Gas Safe registered engineer.