Bleeding radiators properly (and what most people get wrong)

Bleeding radiators is one of the most common DIY tasks, and one of the most commonly done wrong. If you’ve ever bled a radiator, found another one cold the next day, and bled that one too, only for the boiler pressure to drop again, you’ve experienced the classic mistake. Here’s the right way.

What you actually need

• A bleed key (the square one, not flat-head, for most modern radiators): £2 from any hardware shop. Some newer Stelrad / Quinn radiators use a flat-head; check yours.
• An old towel or cup: the water that comes out is dirty
• Phone with a torch: bleed valves are usually in awkward corners
• Knowledge of your boiler’s pressure gauge: the bit most articles skip

That’s it. No magnetic gadgets, no commercial bleed kits, no hose attachments.

Step 1: Turn the heating off and let it cool

You want the pump off so air pockets settle at the top of each radiator. Heating on full whack moves the air around. You’ll bleed the radiator and find another one cold an hour later because the air relocated.

Wait 30–60 minutes after turning the heating off before you start. Easier said in summer than winter.

Step 2: Find your boiler’s pressure gauge and note the cold pressure

This is the step half the internet skips. Your boiler should sit at 1.0–1.5 bar when cold. Note where it is right now. You’ll be back here in step 5.

If your pressure is already below 0.5 bar, you’ve got a leak somewhere, not just air. Bleeding alone won’t fix it. Stop and call us.

Step 3: Bleed in the right order

This is where most guides fall over. The correct order is:

• Downstairs first, then upstairs
• Closest radiator to the boiler first, working outward

Why? Because air rises through the system. If you bleed an upstairs radiator first, the air gap fills back up from below as soon as you turn the heating on again. Working from the bottom outwards forces air upward and out of the topmost radiators last, which is exactly what you want.

If you have a vertical column radiator or towel rail, those go after the regular rads on the same level (they trap air more easily).

Step 4: Bleed each radiator

For each one:

1. Hold the towel or cup under the bleed valve (top corner of the radiator, opposite the thermostatic valve)
2. Slowly turn the bleed key anti-clockwise until you hear a hiss of air
3. Wait until water starts to come out steadily, not in spurts
4. Close the valve clockwise: finger-tight, then a quarter turn more. Don’t over-tighten or you’ll round off the square.

If no air comes out, the radiator doesn’t need bleeding, so close it back up and move on.

Step 5: Top up the boiler pressure

Now go back to the boiler. The pressure will have dropped, possibly significantly if you bled several radiators. Top it back up to where it was before (1.0–1.5 bar typically).

Different boilers have different filling-loop arrangements:

• External filling loop (silver braided hose): open both valves slowly until pressure rises, close both valves
• Built-in filling key (Worcester Greenstar, Vaillant ecoTEC): refer to the boiler’s filling instructions

Top up slowly. The pressure can shoot up if you open the valves too fast. Stop at the top of your normal range and never go over 2 bar cold.

Step 6: Turn the heating back on and check

Run the heating for 30 minutes. Walk around all your radiators. If any are still cold at the top after 30 minutes, that’s a sign the bleeding didn’t work, or there’s a deeper issue:

• Cold at the top, hot at the bottom = trapped air still; re-bleed once
• Cold at the bottom, hot at the top = sludge / magnetite at the base: needs a power-flush, not bleeding
• Cold throughout despite hot pipes feeding it = TRV stuck closed, one of the cheapest fixes in plumbing at about £4

When NOT to bleed

If your boiler keeps losing pressure repeatedly after you’ve bled and topped up, stop bleeding. The system has a leak: usually the expansion vessel inside the boiler, sometimes a radiator valve, sometimes an underfloor pipe. Call us; bleeding more won’t help and you’ll just wear out the filling loop.

If you’ve never done it before and you have a system or regular boiler with a vented loft tank, the process is slightly different (the system isn’t pressurised; the tank just refills automatically). Same bleed order, no boiler pressure check needed.

When it actually justifies a call-out

Bleeding is genuinely a 10-minute DIY task in 95% of cases. Call a plumber if:

• You can’t find the bleed valves
• Your boiler is losing pressure repeatedly
• A radiator stays cold after correct bleeding
• You have black water coming out instead of clear (sign of magnetite: needs a flush)
• Bleeding hasn’t been done in 5+ years on an older system

We don’t charge for “talk me through it” phone calls. If you’re stuck halfway, ring us and we’ll walk you through it. Our customers do this regularly. It’s part of why repeat-business is so high.