Radiator cold at the bottom? What it means and what to do

Why is my radiator cold at the bottom?

In most cases the cause is sludge. Worcester Bosch says a radiator cold at the bottom is usually caused by a build-up of sludge or poor water circulation in the heating system.

Sludge is a mix of rust, dirt and debris that forms inside the system over time. Vaillant describes it this way, and Ideal Heating explains that iron compounds form on steel radiators, with limescale and other impurities adding to the build-up. Because it is heavy, it settles in the lower part of the radiator. Stelrad says sludge is generally found in the bottom and middle of a radiator.

Once the sludge blocks the channels the water flows through, hot water can't reach that part of the radiator, so the bottom stays cold while the top still warms up.

Other things that can stop a radiator heating properly include:

• A stuck or partly closed valve. Vaillant warns this can restrict water flow and lead to uneven heating.
• Low pressure in the system, which can let air in. A Worcester Bosch expert told Homebuilding this is often linked to small leaks, so look for brown marks or staining around radiator valves.
• Leftover materials from when the system was installed, if it was not properly flushed at the time, as a Worcester Bosch expert told Homebuilding.
• A pump that isn't working well. Vaillant lists a faulty pump alongside air and sludge as a cause of uneven heating.

Cold at the bottom or cold at the top: what's the difference?

Where the cold patch is tells you a lot about the cause.

• Cold at the top, warm at the bottom: this usually means trapped air. Air rises and stops hot water filling the top of the radiator. Bleeding the radiator normally fixes it.
• Cold at the bottom, warm at the top: this usually means sludge. Ideal Heating says it is not likely to be trapped air, so bleeding won't fix the problem.
• Cold all over: check the radiator's valve is open and the boiler is working. If no radiators are warm, the problem is more likely to be the boiler or its controls.

It also matters how many radiators are affected. Ideal Heating and Stelrad both say that if only one radiator is cold at the bottom, the problem is probably with that radiator rather than your whole heating system. If several are, the sludge is likely to be spread through the system.

What can you check yourself?

A few safe checks can rule out the simple causes before you call anyone.

1. Turn the heating on and give the radiators time to warm up, then carefully feel each one from top to bottom. Note which ones are cold and where.
2. Bleed any radiator that is cold at the top. Vaillant suggests bleeding and then feeling the radiator again. If the bottom is still cold afterwards, sludge is the likely cause.
3. Check the thermostatic valve isn't simply turned down. Turn it to its highest setting and see if the radiator warms up.
4. Check the pressure gauge on your boiler. If it has dropped below the level your boiler manual gives, top it up as the manual explains, or ask an engineer if you're unsure.
5. Make sure the boiler isn't set to a hot water only or summer mode, and that the heating temperature hasn't been turned right down.

Safety: Turn the heating off and let radiators cool before you open a bleed valve, as the water can scald. Keep water away from sockets, plugs and anything electrical. Never take the cover off your boiler or try to work on it. Any work on the boiler itself is a job for a Gas Safe registered engineer, and you should ask to see their Gas Safe ID card before they start.

How do you fix a radiator that's cold at the bottom?

Clearing sludge means getting it out of the radiator or the whole system. Vaillant says flushing out the sludge or power flushing the system can restore full heat, and that this needs to be done by a professional.

The right approach depends on how many radiators are affected:

• One radiator: a heating engineer or plumber can take the radiator off the wall, flush it through with water until it runs clear, and refit it. Vaillant suggests this if bleeding and checking the valves haven't worked.
• Several radiators, or the whole house: the system may need a chemical clean or a power flush, which treats every radiator at once.

You shouldn't normally need a new radiator. Vaillant says replacing one is a last resort, as it can usually be cleaned or the system flushed instead.

Power flush or chemical clean: which do you need?

Both clean the whole system rather than one radiator. The difference is how hard they work.

• Chemical clean: Ideal Heating explains that a cleaning chemical is added to the system to break up the sludge. It is run through the system and then drained and refilled. In bad cases the chemical may be left in for longer.
• Power flush: a machine forces high-pressure water and specialist chemicals through the system. Ideal Heating says this cleans every radiator, so it suits homes where several radiators are cold. It is not a DIY job.
• Glow-worm notes that installers can test the water afterwards with an electronic meter and compare it with tap water, to confirm the system is clean.

A heating engineer can look at how many radiators are affected, the age of the system and the condition of the water, and recommend which method makes sense. Ask what is included, and whether inhibitor will be added once the system is refilled.

How do you stop sludge coming back?

Sludge forms gradually, so prevention is mostly about slowing it down and catching it early.

• Inhibitor: a chemical added to the water in the system. Ideal Heating says it slows down the chemical reactions that cause iron oxides to form, and that using an inhibitor is required to comply with the industry's Benchmark best practice scheme. After any drain-down or flush, the system should be refilled with inhibitor at the right strength.
• Magnetic filter: a filter fitted to the heating pipework. Ideal Heating recommends one because it takes out grime before it can settle and build up. If you already have one, ask your engineer to check it at the boiler service.
• Annual boiler service: Vaillant recommends having the boiler serviced every year. It is a good time to ask the engineer about the condition of the system water.
• Bleed radiators once a year: Vaillant suggests doing this before winter, and booking a professional system flush every few years.

When should you call a heating engineer?

Get help if:

• A radiator is still cold at the bottom after you've bled it and checked the valve.
• Several radiators are cold at the bottom, or the problem keeps coming back.
• The boiler pressure keeps dropping after you top it up, which can point to a leak.
• You see brown staining or drips around radiator valves or pipes.
• The boiler shows a fault code, makes unusual noises, or your heating doesn't come on at all.

For radiators, valves and flushing, a heating engineer or plumber can help. For anything involving the boiler, use a Gas Safe registered engineer.