Airlock in HW Cylinder Gravity Fed Open Vented HW/CH system

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I'm hoping someone can help me with my issues here...

In my flat I have a fairly old open vented gravity system that provides h/w and c/h. The system has a couple of 2-port zone valve for h/w and c/h. There is a pump on the return pipe from the h/w and c/h and the only drain cock on the system I can see is on the inlet pipe of the first radiator. On the return leg from the h/w cylinder there is also a gate valve before where the centre heating joins and the pump.

Above the floor standing boiler there are 2 tanks, one is a cold water tank and the other a feed and expansion tank. The feed and expansion tank has the over hanging vent pipe coming off from the h/w feed pipe.

I believe there is an airlock in the h/w cylinder coil after the system was partially drained by a plumber to fix an issue/replace the h/w zone valve. He didn't drain the system from the drain cock, but opted to shut off the water mains and then uncouple the return pipe where the gate valve is installed to remove the water from the top half of the system, just enough to carry out his work. He then filled the system back up after replacing the zone valve by closing the return pipe and opening the water mains so the system filled from the top tanks thereby trapping air in the system as a consequence.

I tried to get this rectified by arranging another appointment, but instead the plumber that turned up didn't even attempt to clear the airlock, but instead rewired the timer/programmer, thermostat for c/h and replaced the junction box for some reason. As a consequence the boiler kept tripping all the electrics in the flat each time it was plugged in. He initially tried to blame the electrics in the flat, but I was able to demonstrate that it was down to wiring he had done, not to mention I had the electrics checked and consumer box changed a year ago. He eventually rewired the system again, but this time the electrics didn't trip. However, he didn't fix the airlock and to be honest I'm not sure I had faith that he would do a good job.

Currently when I now turn on the h/w the boiler fires up for about a minute or so and in that time the h/w feed pipe from the boiler to the cylinder gets extremely hot. The h/w zone valve opens and the rest of the remaining h/w feed pipe heats up all the way to the connection to the h/w cylinder. The cylinder and the return pipe gets slightly lukewarm, but no where near hot. When I switch on the c/h the boiler fires up and remains on once the zone valve opens and the radiators get hot.

There is water in both tanks when I checked so am I right in thinking it is likely an airlock in the h/w cylinder? My question is how do I resolve this myself. Can I just turn off the main supply, manually open the c/h zone valve and drain the water from the system using the drain cock located at the bottom inlet of the first radiator. Once it is completely drained, do I refill it from the drain cock by attaching a hose pipe to a mains fed cold water tap rather how the plumb filled the system before? Would there be an issues affecting the central heating if I'm draining/refilling the system effectively from the c/h side? For example do I need to open the first radiator bleed valve until the water reaches the top of the radiator? Any assistance would be great and appreciated!

I've attached a diagram of my h/w & c/h system. Thanks.
 

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Nice sketch. Can you carefully loosen/slacken the coil inlet (top) pipe connection to try and vent any trapped air, repeat with the bottom return coil connection, try it with boiler/circ pump off first and then with boiler on, with both CH & HW on, then with HW only on, manually latch open the zone valve if venting with boiler off.
 
Are there any air venting points on the pipework feeding the coil ?
Is the gate valve on the cylinder return open ? When turning it anticlockwise does it continually rotate ,or come to a stop ?
 
Unfortunately I did contemplate loosening the connections but they are too close to the side wall to get leverage and there is another pipe in front causing a slight obstruction. I’ve read about forcing mains water down the open vent, but I don’t think that will work based on the layout.
 
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Are there any air venting points on the pipework feeding the coil ?
Is the gate valve on the cylinder return open ? When turning it anticlockwise does it continually rotate ,or come to a stop ?
The only air vent is the overhanging vent above the f&e tank pointing downwards. Other than that there is no other air vents I can find. The gate valve is open as you turn it anti-clockwise. It also stops and doesn’t continue turning.
 
If you close the gate valve ( don't overtighten it) and loosen the nut on the gate valve ( cylinder side ) the coil should vent of air.
 
It should, especially if the HW zone valve is closed, it shouldn't be installed in the vent pipe, no vent when closed, but should be on the horizontal leg into the cylinder coil inlet.
 
If the gate valve is below the cylinder will it still vent as it seems the the air is higher up in the cylinder coil? I suppose the logic is closing the valve and loosening the compression nut on the cylinder side will force the air to travel down attracted by the opening?
 
Water pressure would push the air out if the joint is opened by releasing the compression nut....have towels to hand !!
Is the zone valve for cylinder actually on the vent pipe ,position as per your diagram ?
 
I’ve tried to layout the diagram exactly how the pipes and joins are connected and the position of the the system components like the pump and valves. So the vented pipe hanging over the f&e is represented correctly.
 
I should mention now that it has been suggested that I open the compression joint on the cylinder side that the plumber when draining the system opened the compression joint on the boiler side as I can see the marks left on the compression joint and it was leaking until it was tighten a few days afters. I'm assuming that he did this to control the flow of the water when draining the system? I'm not sure if that is significant or not.
 
if you can get a hose on the drain off at the rad and connect the other end onto a cold mains tap open the drain with the zone valves closed and the mains water will fill from the bottom via the return and will force any air out, only do it for a short burst
 
if you can get a hose on the drain off at the rad and connect the other end onto a cold mains tap open the drain with the zone valves closed and the mains water will fill from the bottom via the return and will force any air out, only do it for a short burst
If I read your suggestion correctly you are basically saying to not drain the system but rather skip that and fill from the drain cock. Instead of refilling from the feed side you are suggesting leaving the zone valves closed and filling the system from the return side via the c/h? This will force the airlock to shift out of the cylinder and assuming into the air vent?
 
Zone valves must be latched open whatever route you choose (that's why that facility is on them, for filling the system), that HW zone valve should be repositioned ASAP, presently, if CH only on then the only expansion/vent path is back through the return through the HW coil via its gate valve (tie wrap this fully open, as a temporary measure). My 50 year old oil fired system is similar to yours and on the very rare occasions I have fully drained it down, I latch the zone valves open and I just barely open a service valve that I installed on the F&E tank cold water supply. You can just drain the system down again to the gate valve and then with both M.valves latched open just allow a trickle of water into the F&E tank until "full", that will IMO do the trick since the CH system is OK.
You could syphon or bale out the 5/10 litres of water in the F&E tank, you only then have to drain off a few litres at the gate valve, less of a mess.

Post #13 suggestion would probably work OK but I wouldn't like taking the risk of pushing any sludge which may be in the bottom of the rad(s) elsewhere.
 
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