How to Bleed a Radiator and Fix Common Heating Faults

A radiator is a simple thing. Hot water goes in one end, gives up its heat to the room, and comes out cooler at the other. When one stops working properly, the fault is almost always in that flow - air where water should be, gunge where the water should run, or a valve that has stopped opening. You don't need to understand a boiler to fix most of it.

What follows are the faults a handyman gets called out for again and again, in plain terms, with the honest DIY-versus-pay line for each. One reassurance before you start: none of the jobs on this page touch the gas side of your system, so none of them legally need a Gas Safe engineer. The single hard limit - anything on the boiler itself - is spelled out at the end.

First, Read Where the Radiator Is Cold

Before you reach for a tool, put your hand on the radiator and work out the pattern. A cold radiator is not one problem, it is at least five, and where the cold patch sits usually tells you which one you've got. This one habit saves you bleeding a radiator that never had air in it.

Notice the last row. When the whole house is cold, the radiators are innocent - the problem is upstream, and that is where DIY ends and a heating engineer begins. Everything above it is fair game.

How to Bleed a Radiator

Bleeding is the fix for the classic complaint: hot at the bottom, cold across the top. Air has collected in the highest part of the radiator and is sitting where hot water should be. Letting that air out is the easiest job in the house, and it is where most people start.

You need a radiator bleed key (a small square key, about £1 to £2 from any DIY shop) or, on many newer radiators, just a flat-blade screwdriver. Have an old cloth and a small container ready, because a little water follows the air out. Then, following the method British Gas and Worcester Bosch both set out:

• Turn the heating off and let the radiators cool. Bleeding a hot system means scalding water under pressure, so wait until the radiators are cold to the touch.
• Find the bleed valve. It is the small square peg at the top corner of the radiator, on one side - the bit your key or screwdriver slots onto.
• Open it a quarter to half a turn anti-clockwise. You'll hear a hiss - that's the air escaping. Hold the cloth under it.
• Close it as soon as water dribbles out steadily. The hiss stops and a thin trickle of water appears; that means the air is gone. Turn the valve back the other way to close it - firm, but don't force it.
• Work through the house in order. Do the downstairs radiators first, then upstairs, and within each floor start with the one furthest from the boiler. Air rises, so this bottom-to-top, far-to-near order clears the whole system properly.

One step people forget: check the boiler pressure afterwards. Letting water out drops the pressure, and if the gauge reads below about 1 bar when the system is cold, top it back up to between 1.0 and 1.5 bar using the filling loop (the little braided hose under the boiler). If you bleed a radiator and it fills with air again within a few weeks, stop bleeding it on repeat - air getting in that fast points to a fault worth a proper look.

How to Balance Radiators

Here's a problem bleeding won't touch: the radiators near the boiler roast, and the ones at the far end of the house never really warm up, even though every one of them has been bled and none is cold at the bottom. That is a system that needs balancing - sharing the hot water out evenly instead of letting the nearest radiators grab it all.

Balancing is done at the lockshield valve, the plain valve at the opposite end of the radiator from the thermostatic one, usually hidden under a plastic cap. Throttling the lockshields on the greedy near radiators forces more flow to the starved far ones. As Ageas and Vaillant describe it, the method runs like this:

• Bleed every radiator first, then turn the heating off and let it all go cold.
• Open every valve fully, fire the heating up, and note the order in which the radiators warm - the ones nearest the boiler go first.
• Let it cool again, then start with that fastest radiator: close its lockshield fully, then open it about a quarter to half a turn.
• Using a thermometer on the pipe each side of the radiator, aim for roughly a 12 degree difference between the flow pipe going in and the return pipe coming out.
• Work outwards to the furthest radiator, opening each one's lockshield a little more than the last.

It is not hard, but it is fiddly and slow, and it wants a thermometer and some patience. Plenty of people do it themselves over an afternoon; plenty of others hand it to a handyman or heating engineer to sort in an hour or two. Either way it is worth doing, because a balanced system heats the whole house evenly and the boiler runs less to get there.

Radiator Cold at the Bottom: Sludge

A cold strip along the bottom of a radiator with a hot top is the mirror image of the air problem, and it has a different cause: sludge. Over years, the steel inside radiators corrodes, and the resulting rust and debris turns the system water black and settles as a heavy silt in the bottom of the radiators and pipework. It blocks the flow along the base, so the bottom stays stubbornly cold. Bleeding does nothing here, because there is no air to let out.

For a single affected radiator, it can sometimes be taken off the wall and flushed through outside with a hose - a messy job, but doable. When several radiators are cold at the bottom, the whole system is silted up and needs cleaning out. That usually means a power flush, where a pump pushes cleaning chemicals through every radiator and pipe to shift the sludge. According to Checkatrade and MyJobQuote, a power flush typically costs £300 to £600, and more for a big system, usually taking most of a day. To stop it coming back, a good engineer adds a corrosion inhibitor to the water and often fits a magnetic filter to catch debris before it settles.

When One Radiator Won't Heat Up at All

Different symptom, different fix: one radiator dead cold from top to bottom while every other radiator in the house is toasty. Nine times out of ten this is a stuck thermostatic radiator valve, or TRV - the numbered dial you turn to set a room's temperature. The pin inside it can seize in the closed position, most often after a summer of sitting unused, so the radiator never gets the signal to open.

There's a quick way to confirm it. Feel the pipe running into the radiator: if that pipe is hot but the radiator stays cold, hot water is arriving and the valve is refusing to let it in. To free it, as BestHeating and Trade Radiators explain, unscrew and lift off the TRV head to expose the small spring-loaded pin underneath, then gently work it up and down with your fingers or the back of a screwdriver until it moves freely. A squirt of penetrating oil (WD-40 or similar) helps a stubborn one. The trick to never doing this again: leave your TRVs turned fully open over the summer so the pins can't seize shut.

If it is not one radiator but all of them staying cold, the radiators are fine and the fault is with the boiler, the pump, the thermostat or low system pressure. That is where you stop and call a heating engineer rather than working through the rooms one by one.

Dripping Valves and Small Leaks

A small drip or a rusty damp patch under a valve is common and usually minor. The frequent culprit is the gland nut - the nut around the valve spindle where the head meets the body - working loose, or the washer inside it wearing out. As Radiators Direct notes, a gentle quarter-turn of the gland nut with an adjustable spanner stops many of these leaks, and if it doesn't, wrapping a little PTFE plumber's tape around the spindle thread usually seals it.

Know when to stop, though. If the valve body itself is cracked, the pipe is corroded, or the drip won't settle after you've tightened things, the valve needs replacing - and that means draining down that part of the system, which is the point to bring in a handyman or plumber rather than making a small leak into a soaked carpet. A slow weep can wait a day; a steady drip onto a wooden floor cannot.

What You Can Do Yourself, and When to Call a Pro

The reassuring bit first. Almost everything on this page is what the trade calls "wet work" - water, not gas - so it does not require a Gas Safe registered engineer. The Gas Safe Register is clear that gas registration covers gas appliances and pipework, and a non-registered person may carry out wet work such as fitting pipes and radiators. So a competent handyman can bleed, balance, free a stuck valve, cure a dripping gland and even swap a radiator like-for-like. The one hard line: any work on the gas boiler itself, and the final connection of the pipework to it, must be done by a Gas Safe engineer. Our guide to what a handyman can legally do sets out where that line sits across the whole home.

So when is it worth paying someone? When the job needs the system drained, when it's a whole-house issue rather than one radiator, or when the diagnosis is the hard part. Rough UK figures to budget against:

A heating engineer's labour tends to run around £40 to £75 an hour depending on where you live, with a like-for-like radiator replacement usually an hour or two of work. For the small stuff - a balance, a stuck valve, a new radiator on existing pipework - a local handyman is often the cheaper and quicker call than a specialist, and can knock several of these jobs off in one visit. Our handyman cost guide breaks down hourly and day rates in more detail.