Combi boiler issues. Possible airlock

Should be an automatic vent fitted somewhere on the boiler. Normally a small brass vessel about 2cm diameter with a removable top. It works on the same principle as a cistern fill valve IE the internal float lifts when water appears; it can choke and need cleaning. Just make sure you isolate the water before removing it.

Would that be on the cylinder?
If so, should I let all the air out?

My understanding is there is an expansion vessel somewhere that should have some air.

My old boiler did this. I think there was a bleed screw on the pump itself. No idea about newer ones

My bleed valve is on the heat store...
There will be a bleed connection on the expansion vessel but this is for topping up the air in the vessel.

....the auto bleed valve should let all the air out itself but can get choked so may need cleaning.

Anything else I can do before looking for a bleed valve on the boiler?

If there's more than 1 bleed valve I want to make sure I get the correct one.

Don’t think so…..if you Google boiler auto vent valve you’ll know what you’re looking for. There should be a plastic cap like a tyre valve cap that you can remove and then press the centre with a small Allen key or screwdriver… Good luck.

Try a 2p peice on the front of the pump first. Unscrew maybe 3 turns to ensure the impeller housing is full of ch fluid.
Then try the schrader bleed valve on the top of the boiler expansion vessels.
Some have an extra bleed on the h.ex. as well.

Thanks. I'll give this a go this evening.

No luck locating a valve. I've set the central heating to on and can hear the pump working but it's still going shlurp...shlurp...shlurp.
The heating supply pipe gets hot but only the first 40cm of it. Water runs hot for a few seconds, then ice cold.

I think its trapped air. Radiators have no air at all. Any ideas?

Have you bled the pump ?

I am no plumber but did you check the pressure is right for your boiler - something like 1-1.5 bar I expect.

Have you got a magna filter thing near the boiler- that often needs bleeding too

Have you bled the pump ?

How do I do that?

Pressure is at 1.5.

I'll check the magna filter next.

OK, so bled the pump. No air.

Checked the magna filter. Some air.

Still the same issue.

Are you sure all the valves are open on the boiler?
I made that mistake last week.
Also I wonder if. draining the boiler to zero pressure and refilling may clear the air blockage.

Do you have hot tap water? Could be a diverter valve issue.
You could try turning it off then on again several times to see if the pump will self prime. Let it go through the start up process each time.
Auto bleeds look like a sp2 battery with a splined knurled screw top, in brass.

Try turning all the rads off except one. Then the full pump force will be concentrated and May get things flowing in the circuit Repeat for each rad. If that works Then remember to balance the lock shields.

No hot water from the taps.
Not touched any valves so can be them.

I think I've found the manual vent on the top of the cylinder. It's a plastic wing nut on a sort of cam. Looks like this: link

I'm guessing I should use that to let any air out? It has a short plastic pipe coming out of it but not connected to anything.

I'll try draining and refilling again tomorrow.

i think combi boilers heat water for hot taps directly rather than indirectly via circulating system water, that’s used for radiators or hot water cylinders. So maybe something else wrong

What type of boiler do you have?

If the pump comes on then I doubt its an air lock in the boiler, it 'shouldn't' run as the presure would be too high. When you say the first 40cm of pipework gets hot, does it stay hot if you leave it running or does it go cold?

I'd guess it's the burner itself tripped out,  but the (cold) water is still circulating in the system.