Help with cold HW cylinder/coil

[Vee239]

Good morning, a little guidance would be appreciated if anyone has the time.

System details ..
-Potterton Netaheat 10-16 boiler
-HW cylinder in the loft
-Pump located in the bathroom, directly above the boiler
-It looks like there is a 2 port valve fitted but its stuck open. If hot water is selected the rads heat up too. As a result my parents have had the rads turned off all summer.

X400 was put in a month ago and the system drained and refilled last weekend. I planned on doing this a couple of times and then putting in some inhibitor.
The rads heat up very well and have all been bled.
The cold feed from the f/e tank doesn't appear to be blocked - if I drain the c/h it fills up again as it should.

My problem is there is no hot water.
Both pipes going into the coil are cold.
Presuming an air locked coil I opened the bleed valve on the upper pipe - only a little air came out and then stopped. No water is coming out of this bleed valve at all.

The 1" pipes going up to the loft seem to be on a circuit including the pump and the 2 port valve branching off this circuit. These 1" pipes are hot about halfway up only.

Does this sound like an air lock in the coil ?
Should I just open one connection where it goes into the hw cylinder ? If so which one, upper or lower, and, should the hw be switched on ?

Alternatively, any other suggestions ?

 

[ChrisR]

Does this sound like an air lock in the coil ?

yes

Should I just open one connection where it goes into the hw cylinder ?

No - risk of damage to an old cyl at connector is high.

Try harder with the vent point. It may be blocked. Water should come out.

 

[Vee239]

Thanks Chris.

WRT the bleed point - it's a screw cap which I can undo by hand.
Once loose, very little air came out and then nothing at all.

If its blocked, what can I actually do to it ?
Stick an unwound paperclip down the hole ?
Replace it ? If so I assume the system needs to be allowed to go cold first and everything switched off ?

 

[D_Hailsham]

WRT the bleed point - it's a screw cap which I can undo by hand.

This sounds like an automatic air valve (AAV). Works a bit like the float in a carburettor. If there is air in the system the valve opens to let it out the water level rises and the valve closes. Any chance of a photo of the valve, if not, is there a maker's name on it?

 

[Vee239]

Not sure if its an AAV going by the diagram in the Wiki.
I'm in the office at the moment so no pic til tonight.

In the meantime I can try to describe it . .

The upper pipe into the coil is approx 1" copper.
At the point it turns 90 degrees to connect to the the hw cylinder there is a thinner copper pipe, approx 1/2" diameter coming off it, sticking up about 3".
At the end of this there is a screw cap which I can undo by hand.

This system was installed late 1970s/early 1980s.

EDITED TO ADD : IT LOOKS MORE A MANUAL AIR VENT (from google images)