Boiler stopped working after bleeding the radiators? Here's how to fix it

Last checked: November 2026 · Combi and system boilers · 5 minute read

Never take the boiler's cover off. Everything in this guide is done from the outside: reading a gauge, turning a tap and pressing a reset button. If you smell gas, leave the property and call the National Gas Emergency number on 0800 111 999.

You bled your radiators to get rid of cold spots, and now the boiler won't fire up. Perhaps it shows a fault code or a flashing light, or there's no heating and no hot water. This is very common, and in most cases it's a quick fix you can do yourself in a few minutes.

The usual cause is simple: bleeding radiators lets water out of the system, as well as air, so the pressure drops. Most modern boilers switch themselves off when the pressure gets too low, to protect themselves.

Quick version
  1. Check every bleed valve is closed.
  2. Look at the pressure gauge on the boiler.
  3. If it's below 1 bar, top it up with the filling loop.
  4. Reset the boiler.
  5. If it still won't work, try these other checks.
  6. Call an engineer if the pressure keeps dropping or the fault comes back.

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

Why it happens

Most homes in the UK have a sealed (pressurised) heating system. This is always the case with a combi boiler, and with most system boilers too. The water in the pipes and radiators is kept under a little pressure, usually shown on a gauge on or under the boiler.

When you open a bleed valve, air hisses out first. Then a bit of water comes out before you close it. Do that on several radiators and the system can lose enough water for the pressure to fall below the boiler's safe minimum. The boiler then locks out and shows a fault, even though nothing is broken.

Less often, the problem is air that has moved into the boiler or pump, or a bleed valve that wasn't closed fully.

Got a tank in the loft and a hot water cylinder in the airing cupboard? You may have an older open vented system with no pressure gauge. It refills itself from a small tank in the loft, so skip to step 5.

Step by step

1Check every bleed valve is closed

Go round each radiator you bled. Gently turn the bleed valve clockwise with the radiator key until it's snug. Don't force it, as they're small and easy to snap. Check for drips. A valve left even slightly open will keep letting water out and pressure drop.

2Look at the pressure gauge

On a combi or system boiler, there's a pressure gauge. It's either a dial (often marked in "bar", with a green zone) or a number on the boiler's display. It's usually on the front, or underneath behind a flap.

  • About 1 to 1.5 bar when the heating is cold is normal for most boilers.
  • Below 1 bar (or near 0) is too low and is almost certainly why your boiler has stopped.

Check the pressure when the heating has been off for a while. It rises a little when the system is hot, which is normal. Your boiler's manual will give the exact figures for your model. Most manuals are also free online if you search the boiler's make and model.

3Top up the pressure with the filling loop

The filling loop connects your heating system to the mains water so you can add water. It's usually under the boiler. It looks like either:

  • a silver braided hose between two pipes, with a small tap or valve at each end, or
  • a built-in lever, knob or key underneath the boiler (common on Worcester Bosch, Vaillant and some others).

Before you start, turn the heating off and let it cool. Then:

  1. Open the valves slowly. With a hose, open one end fully, then slowly open the other. You'll hear water flowing.
  2. Watch the gauge the whole time. Close the valves when it reaches about 1 to 1.5 bar, or what your manual says.
  3. Make sure both valves are fully closed afterwards. If you have a hose, leave it connected unless your manual tells you to remove it.

Don't overfill. If the pressure goes too high (around 3 bar), a safety valve releases water through a small pipe, usually to outside. If you overshoot, you can bring it down by bleeding a radiator a little, catching the water with a cloth.

Some combi boilers don't have a filling loop fitted. If you can't find one, check your manual, or ask a heating engineer to top it up. Filling loops are a simple part to have fitted.

4Reset the boiler

Once the pressure is back up, press the boiler's reset button. It's often marked with a flame or a circular arrow, or is a "reset" option on the display. The boiler should start up within a minute or two.

If it locks out again straight away, don't keep pressing reset over and over. Try once or twice, then move on to step 5.

5If it still won't work

  • Gurgling or rushing noises from the boiler? There may be air in the boiler or pump. Let it run for 10–15 minutes with the heating on, then bleed the radiators again (top first if you have two floors). Afterwards, check the pressure again and top up if needed.
  • Pressure looks fine, but still a fault code? Look the code up in your manual. The meaning varies by brand. It may be unrelated to bleeding the radiators.
  • Open vented system (tank in the loft)? Look in the small tank (the "feed and expansion" tank, smaller than your cold water tank). It should have a few inches of water in it. If it's empty, the float valve may have stuck. A heating engineer or plumber can sort this out.
  • Thermostat and timer: check they're still calling for heat. It's easy to knock them while moving round the house.

6When to call an engineer

Call a heating engineer if:

  • the pressure keeps dropping over a few days after you've topped it up. There's probably a leak, often a small one under the floor, or in the boiler itself;
  • water is dripping from the small pipe outside;
  • the boiler keeps locking out even though the pressure is correct.

Tell them you bled the radiators, what the pressure was before and after topping up, and any fault codes. If anyone needs to work on the boiler itself, they must be Gas Safe registered. You can check at gassaferegister.co.uk.

Questions people ask

Is it safe to top up my boiler pressure myself?

Yes. Manufacturers expect homeowners to do it, and the steps are in your boiler's manual. Just go slowly, watch the gauge and close the valves fully afterwards.

How often should I need to top it up?

After bleeding radiators, once is normal. If you're having to top up every week or two without bleeding anything, there's a leak somewhere and it needs looking at.

Should I bleed radiators with the heating on or off?

Off, and wait until the radiators are cool. It's safer, because the water can be very hot, and the air settles at the top, so it bleeds out better.

Related guides

This guide is general information, not professional advice. It was checked against boiler manufacturer and heating industry guidance in November 2026. Always follow the manual for your boiler. If in doubt, contact a heating engineer or a Gas Safe registered engineer.