Dave From Leeds wrote:Fergusondj, it seems from your description that you have an indirect hot water system. There should be two pipes one directly above the other on one side of the hot water cylinder one near the bottom and the other about halfway up. These are likely to be 22mm copper pipes. The higher one of these should be the hot water feed from the boiler and the lower one the gravity return to the boiler. Have a look to see if there are any other pipes teed off them going upwards, possibly 15mm copper pipes. If so the one from the lower point will be the feed from your central heating top-up tank and the higher one will be the vent to that tank. If you can access this top-up tank, try fitting a hosepipe onto the open end of the vent pipe with the other end attached to a cold tap. Get an assistant to slowly turn on the tap then check for any air coming up through the bottom of the top-up tank. If it does, keep the tap running until the air stops then turn off the tap. If your problem is an airlock in the cylinder heating coil, this should clear it. Be careful not to have the tap turned on too fast or the top-up tank may fill up too quickly for its overflow to cope. This work should be done with your central heating and hot water systems turned off.
Hi Just joined and probably doing this all wrong so sorry. While searching on-line saw the above posted by Dave from Leeds and it describes my system and problem, Except the only bleed vents I can find are on CH pipes in lower loft & open vent above expansion tank top loft. My problem with no hot water was directly after I had drained system to install a new radiator. Thinking air-lock I have tried lots of ways to purge the primary /return flow through cylinder coil with no success as only connection points are on CH lines and do not want to disconnect coil pipes at boiler . Thinking would this solve this problem and any in the future would it be ok to install a circulating pump on the coil loop only to purge ( not to keep running ). Note Central heating system & pump working fine.