Rads have all been bled, bottom to top, but the pump still cannot get water through. At the moment all the rads have just been opened on the in valve. Do I need to open the out flow too to get a weight of water in the system?

Yes you need both to be open,Google how to balance your rads and all should be well! 🙂

Yep all valves fully open. Make a note of the number of turns for each lockshield valve as they should have been balanced when it was installed and you’ll need to put them back.

Still nothing. Drain and start again…

presumably you havent a pressurised system.. open all the rads make sure water in them and pump ( take the cap off end).. wack boiler to max let it try for an hour cycling in and out.. no joy.. take the internal thermostat out of the boiler and let it boil.. make sure you know what your doing and ut it back in once your all cosy

Judging from the OP I don’t think allowing the boiler to boil is a very clever suggestion; that’s just showing off. In any case how the heck would the OP know where to find the boiler thermostat?

A clear understanding of the physics is all that’s needed. Allow the water to fill the system from the bottom up by opening all the valves and venting off the air. If that doesn’t get the airlock cleared, maybe it’s time to call a plumber.

Yep. I’ll get all the water out and start again. Failing that pockets will have to be plundered!

First is this open vented if so make sure you remove any sludge gathered in the F&E tank in your loft! if you don’t have any circulation to radiators this will probably be due to an air lock in the feed circuit to the rads make sure there aren’t any AAV manual or auto that have been missed I have seen these hiding in cupboards the other side of the house, check the circulator (pump) is running to do this remove the bled screw on the front and check it is revolving be careful as this may be very hot, if still no luck then close down all rads except one if possible the furthest from the circulator and try to force the air to move, make sure that if you have a differential by pass that this is set at its highest or the water will flow over this, cheers hope this helps good luck.

Our system always air locks for a few hours after a refill. It cuts off downstairs and after a few hours it just works it’s way through and is fine. I’ve got used to it now and just wait. I flush the system maybe once every 1-2 years when I remember.

had a similar thing with my crap setup at home. Basically after replacing a rad the system cant be filled from the tank in the loft. the reason is several. there are bends in the feed so we get air locks and we also have magnetite in the system which can block it up.

we had a plumber here for 4 hours last time and he gave up trying to get it going!

I have 2 methods now. one is to fill an old fire extingisher with water. pressurise it to 50psi and force water into the headertank outlet. you could do this with a hose. secondly i reverse fill it from the bottom rad.

before this however open all valves on the rads fully (both sides) and give it some time (an hour to settle) then try switching it on and off a few times (pump) to see if you can shift any airlocks.

it can be a time consuming thing. also dont do it when it snows like i did 🙁

Some sound advice right there. All except defeating the boilers safety device (thermostat) and letting it boil?
Most helpful comment is isolating one rad and forcing air to it. I always check every rad on initial start up and there’s usually at least one getting hot first. If others are struggling, I shut off the trv or valve on the hot one and check around again. Isolating each one that’s getting hot until all have had hot water through them. Bleeding air as I go. Finally opening all valves and letting it run. As previously stated, this can be a bit of an art and quite time consuming because it needs to have the rads balanced once all the air has been purged . Another reason we plumbers get paid ridiculous amounts of money.