I have just installed a heated towel rail for an existing radiator linked into my central heating system. To do this, I drained the system, did the job including moving the rad valves and extra pipework, refilled the small header tank (with Fernox and water), bleed the rads starting from the top ones. I felt quite pleased with myself until the hot water did not heat up.
The hot water tank is in the loft below the 2 filler tanks and is heated indirectly via one of two motorised valves and and a pump. The other valve is for the central heating system. The valves and the pump appear to be working.
In draining the system have I generated air pockets that will not allow the flow of water around the pump, valves or THE BOILER. If so, how do I clear it?
sounds like an air lock in your primaries/cylinder coil.
easiest way is to pull the boiler stat and an boil it out.... or you could try cracking open nuts/vents near to the coil to see if air escapes.
If you touch your flow and return pipes before and after the cylinder coil you can diganose whether this is the case.... both should be hot if it is working correctly. If the return is cold but feed hot you are airlocked somewhere in between.
On my system, the feed line from the small F&E water tank in the loft goes into the primary system pipework where it leaves the hot water cylinder near the bottom and the vent pipe to the tank comes from a pipe going into the side of the cylinder about 3/4 of the way up, directly above the other one.
From your post it seems that there may be an airlock in the heating coil inside the cylinder. If you can, connect a hosepipe to the open end of the vent pipe with the other end attached to a mains-fed cold water tap. Then get an assistant to open the tap slowly whilst you check whether any air bubbles come up into the tank through the feed line from it. If so, keep the tap running until the bubbles stop. This should clear the airlock.
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