Heating not working after the system was drained and refilled? Here's what to check

1Work out which type of system you have

• Sealed system: you have a combi boiler, or a boiler with a pressure gauge and no water tanks in the loft for the heating. Most modern homes have this. Follow step 2 and skip step 3.
• Open vented system: there's a small tank in the loft (the "feed and expansion" tank) as well as the larger cold water tank, and usually a hot water cylinder in the airing cupboard. Skip step 2 and follow step 3.

2Sealed system: check the pressure

Look at the pressure gauge on or under the boiler. With the heating cold, it should usually read about 1 to 1.5 bar. Check your boiler's manual for the exact figure.

After a refill, the pressure often drops as air escapes from radiators. So check it again every time you bleed radiators in step 5, and top up with the filling loop when it falls below 1 bar. Our guide Boiler stopped working after bleeding the radiators shows how to use the filling loop safely.

3Open vented system: check the tank in the loft

Look in the small feed and expansion tank. It should have a few inches of water in the bottom, and the float (the ball on an arm) should be floating.

• If it's empty or very low: the water supply to it may still be turned off. Look for a small valve on the pipe feeding it. Often it was turned off for the work, or the float arm was tied up. Turn it back on or untie it, and let it refill.
• If water is spurting out of the pipe above the tank when the heating runs, switch the heating off and call the plumber back. This is called "pumping over", and it needs sorting by an engineer.

4Make sure all the valves are open again

• Every radiator valve: thermostatic valves turned to the highest number for now, and lockshield valves (the plain capped ones) not fully closed.
• Any small valves under the boiler on the heating pipes, often with a slot for a screwdriver or a little lever. The slot or lever should usually line up along the pipe when open. If you're not sure, leave them and ask the plumber.
• Valves either side of the pump, if you have a separate pump (often in the airing cupboard).

5Bleed every radiator, then do it again

1. With the heating off, bleed each radiator using a radiator key. Start downstairs, then go upstairs. Close each bleed valve as soon as water appears.
2. On a sealed system, check the pressure and top it up if it's dropped.
3. Run the heating for 20–30 minutes so the pump pushes more air round.
4. Switch off, let it settle for a few minutes, then bleed all the radiators again.

It's normal to need two or three rounds of this over a day or two after a full drain-down. You'll notice the gurgling getting quieter each time.

6Reset the boiler and let it run

If the boiler is showing a fault, press its reset button once the pressure is right and you've bled the radiators. Many boilers also have a small automatic air vent that lets air out of the boiler by itself as it runs. Gurgling from the boiler itself often settles down after a few hours of use.

If it locks out again straight away, don't keep resetting it. Try once or twice, then call the plumber.

7When to call the plumber back

Call the person who did the work, or a heating engineer, if:

• the boiler keeps locking out after you've bled the radiators and checked the pressure;
• you can hear the pump running but nothing gets warm. The pump itself may have an airlock or have stuck;
• the pressure keeps falling over the following days, which suggests a joint is leaking somewhere;
• water is pumping over into the loft tank.

Tell them when it was drained, what you've tried and any fault codes. It's also worth asking whether they added inhibitor when refilling. This is a chemical that protects the inside of the system from rust and sludge, and it's lost when the water is drained. Anyone working on the boiler itself must be Gas Safe registered. You can check at gassaferegister.co.uk.