After trawling the net trying to find an answer I found this site which cam the closest with a previous post so I thought i'd try and get a bespoke answer for myself!
I have an old old boiler in my kitchen which heats the radiators and at the same time heats the water in a tank upstairs (there are no upstairs radiators) I recently had a problem with an old downstairs radiator which had rusted through at the bottom, after trying to turn off the valves and realising that they were both cream crackered I removed the radiator and blocked the pipe ends.
The hot water has been fine for a couple of weeks, then I noticed that one of the other radiators was only half hot, so bled it.........now no hot water..
I have bled all other radiators, but still nothing.....
I am guessing from other posts that there is an air lock in the coil. BUT I don't know which is which pipe on the tank? nor how to remove the air lock? Do I just crack one of the pipes open until water flows out? Which one?!!
Incidently, the boiler is located directly underneath the tank, neither of the 2 "large" pipes which go upward from the boiler are hot when the system is running.
Any help is appreciated so that I am armed with some info when I go home tonight with my spanner!
Now that i'm back home and had another look I can see which pipes are for the coil, but there are no bleed valves on them and all connections are soldered not compression fittings. What can I do to bleed it?
OK, at the moment it seems to be fixed as the water is now warm but I will have to wait a while to for it to heat up......
Basically I couldn't find a bleed valve or compression fitting anywhere so a combination of opening one of the radiator bleed valves and banging the pipes leading to the coil on the tank seems to have worked, when banging the pipes there was a kind of gurgling sound which I assume was some or all of the air passing through the pipes.
I have a system very similar to yours. If the hot water is OK from now on, all well and good but if the coil should airlock again, have another look around the pipes to and from the coil. On my system the vent pipe to the smaller cold water tank (the CH top up or Feed & Expansion tank) comes off the higher of the two pipes to the coil and the water feed from this same tank goes into the lower pipe from the coil. If yours is like this, you could try fixing a hosepipe over the open end of the vent pipe with the other end attached to a cold water tap. Then get an assistant to slowly open the tap whilst you look into the F&E tank. if any bubbles come up through the bottom of the tank, leave the tap running until they stop. This should then have cleared the airlock. Be careful not to open the tap too much otherwise water may flow into the small tank too fast for its overflow to cope.
Have heard they can but I don't know anyone it's happened to - and it's never happened to me.
The reason I advise you need an RGI - even though they're not working on the gas part of the boiler - is that they should know the limits and signs it's starting to go wrong, running the boiler until it goes off at max setting on stat then removing the stat, sometimes less than a minute sometimes 2 or 3 minutes and you'll normally hear it clear with no problems and no damage.
Whereas a DIYer may pull the stat for 10-15-20 mins or more - where they're very likely to damage the boiler, it'll start venting over and possibly overflow the tank.
Although with older boilers it's always in the back of your mind - sit well back finger on spur switch, I also always make sure there's no-one in bed directly under the tank etc - don't want any injuries if the heat-ex does go ;-)