Radiators cold at the top or the bottom: bleeding, balancing or something else?

What to check first

1. Is the heating on, and is the thermostat or app calling for heat?
2. Do other radiators heat up? One cold radiator is a different problem to a cold house.
3. Are both valves open? That's the thermostatic valve (the TRV, with the numbers on) and the plain valve at the other end.
4. On a sealed system, look at the pressure gauge. Common guidance is about 1 to 1.5 bar when the system is cold. Your manual gives the exact figure for your boiler.
5. Feel each radiator at the top, middle and bottom, and note where it's cold. The next sections cover each one.

Why is my radiator cold at the top?

Almost always air. Air rises and collects at the top, so the hot water can't fill the radiator and the top stays cold. The fix is to bleed it.

Safety first. Turn the heating off and let the radiators cool right down. Bleeding a hot system can spit scalding water. Keep a cloth under the valve for drips and put a towel on the floor.

1. Find the small bleed valve at the top corner of the radiator.
2. Hold the cloth under it and turn the valve a quarter turn anticlockwise with a radiator key.
3. You'll hear a hiss as the air comes out.
4. When a steady dribble of water appears, close the valve clockwise. Don't overtighten it.
5. Wipe it dry and do the next one.

Start with the radiator nearest the boiler and work outwards, downstairs before upstairs.

Then check the pressure. Bleeding can lower the pressure on a sealed system. If the gauge has dropped below the cold range in your manual, top it up using the manual's steps. If you're not sure, ask first.

If the air keeps coming back, or the pressure keeps dropping, stop bleeding. If the boiler has locked out or shows a fault, you'd need a gas engineer for that. If there's a visible leak on a pipe or radiator, that's one I can look at.

Why is my radiator cold at the bottom?

Hot at the top and cold at the bottom usually means sludge. Rust and debris build up in the system water over the years and settle at the bottom of the radiators, so the water can't flow through.

Signs it's sludge:

• The bottom stays cold after bleeding.
• Black, dirty water comes out when you bleed it.
• Several radiators are cold the same way.

Bleeding won't fix it. A power flush or a chemical flush clears the debris out of the system. I do power flushes, and I'd want to look at the system before saying which suits. Send me a photo and I'll tell you what I'd do.

Cold in the middle, or the whole radiator is cold

Cold in the middle with warm ends is often a mix of air and sludge. Bleed it first, and if that doesn't help, it's a flush question.

If the whole radiator is cold while the rest are hot, check the valves:

• A stuck TRV pin. The pin inside a thermostatic valve can seize after a summer off. Unscrew the head, usually by hand. If the pin is stuck down, a gentle wiggle or tap can free it. Don't use pliers on it.
• A closed valve. Check the valve at the other end hasn't been shut off.
• Neither of those. Then there may be a blockage or a failed valve.

Changing a valve usually means draining part of the system, so that's a visit for me rather than a DIY job.

Radiators far from the boiler stay cold: balancing

If the radiators nearest the boiler are roasting and the ones at the far end are barely warm, the system probably needs balancing. Hot water takes the easy route, so the close radiators take more than their share.

Balancing means adjusting the lockshield valve on each radiator so the heat is shared out evenly. You work round the house from the nearest radiator to the furthest, and it takes patience. It's the fix when bleeding hasn't helped. I bleed and balance the system whenever I fit new radiators, and I can do it on its own too.

Heat pump systems: cooler radiators are normal

A heat pump runs at a lower water temperature than a boiler, so radiators that feel warm rather than hot can be completely normal. That's why heat pump radiators are often bigger: they need more surface area to give out the same heat.

Cold patches still matter:

• Air. It collects at the top like any system. Switch the heat pump off and let it cool before you bleed.
• Flow rate. If the flow is too low, some radiators won't get enough water.
• Filters and strainers. A blocked filter restricts the flow. Your installer's instructions say how often to clean it.
• System water. Dirty or untreated water causes the same sludge problems as any other system.

I'm BPEC Level 3 heat pump qualified and I look after the plumbing side of heat pump systems, whoever fitted them. If the outdoor unit shows an error code, check your manual first.

When to call a plumber

Give me a call if:

• The radiator is still cold after bleeding, or the air keeps returning.
• The bottom stays cold, or you've seen black water.
• A valve is stuck or leaking.
• The radiators furthest from the boiler never get properly warm.
• Water is dripping from a radiator, valve or pipe.

If the boiler has a fault, keeps locking out, or loses pressure with no leak you can find, you'd need a gas engineer for that. The same goes for anything to do with gas. I don't do gas or boiler work.

A booked visit is £60 for the first hour, with no call-out fee. Bigger jobs get a free quote.

Frequently asked questions

Need a hand?

I cover Stockport SK1 to SK4, SK6, SK7, SK8, SK12, SK22 and SK23.

Call or text 07846 729203

More on what I do: heating faults away from the boiler, radiators and valves, heat pumps and heating controls. I work across Stockport and High Lane, or you can use the quote form.