Open/Vented Boiler not firing after radiators bled

Hi there.

I'm having a bit of trouble, the radiators became quite noisy when the heating was on and needed bleeding, which was done, they were still a little noisy but settled down a bit after a while.

But now the two radiators upstairs are only half heated (the bottom half), indicating they need to be bled again, however, whenever you go to bleed them, you get nothing but air come out.

Then after this has happened, the boiler refuses to fire, so it seems like some sort of automatic pressure cut off, but Open/Vented systems don't seem to be pressurised?

If you leave it off for a couple of hours then it fires again, but the same thing will happen if you try and bleed the upstairs radiators again later, and you then have to go through this whole charade.

For whatever reason the radiators aren't pressurising/filling up properly, and you get about one attempt a day to try and bleed it, and if you're lucky to get water come out after the air, it still sounds as if it needs bleeding and doesn't heat up properly again later, but then the next time you bleed it to try and remedy this, you get nothing but air, and have to go through the whole tirade again.

So any advice on what may be happening would be appreciated.

If it was a closed system I could just re-pressurise it after bleeding the radiators and do them again if needed, but I don't know what to do with this open system.

Classic symptoms of a blocked cold feed pipe.Follow the 15mm outlet pipe from the small header tank to where it joins your system and there is where the blockage is.Needs to be cut out and cleared.

It might we worth checking the header tank (the smaller one in the loft) has water in it.

You are bleeding these rads with the system OFF arnt you - ie without the pump running.

Mr. W.

System most likely blocked with magnetite, as above, but could also be frozen tank inlet or duff ballvalve.

Radiators bled with the system/pump off and the header tank has water in it.

Would it immediately stop working after I bled the radiators as above if there was a blockage? as that's when the boiler stops firing.

The pump continues to run after the boiler fails to fire, and the pump seems to be very hot, so I tried to bleed it, but nothing came out, I also turned up the flow to 1800l/h from the 1600l/h it was on, but this made no difference.

But I suppose this could also be caused by a blockage?

The system was teated with 'Sentinel X100 inhibitor' 7 years ago if that has any effect on buildup etc.

Thanks for the input

With a blocked cold feed, every time you open an air vent, air goes in.

The pump getting very hot is a classic pump with no water in it. If you continue to run it like that it will die.

Putting some X100 in 7 years ago isn't a passport to perpetual happiness.

The cold feed gets blocked with scale deposits and sets as hard as iron. The only way to resolve it is to cut out the offending bits of pipework. Blocked cold feed sections can in the early stages cause pumping over by restricting the water flow and causing pressure drop around the 'H' section.

This is the most likely scenario and can happen to any open vent system with a few years under its belt.

Thanks for the help, doesn't look like it's a quick fix then.

Sometimes when I bled the radiators, nothing would come out, then after this fact, bleeding it again, air would come out followed by water as usual, would this be consistent with a blockage?

Also what would cause the boiler to fire up again and then run fine after leaving it for a bit? would it stop working all together if it was a blockage or not? as the problems only occur when trying to bleed the system to quieten it down.

Understand I'm not questioning the advice, it's most appreciated.

I'm just trying to understand the causes myself and try to eliminate any extra variables.

Thanks a lot.