Hot water drawing air

Hi,
< apologies for the long post but I've tried to include as much detail as possible >
I've been working my way through a never ending list of jobs with my '30s bungalow. However, I'm a wee bit stumped with a couple of plumbing issues.
First my setup. I've a fully pumped open vented system (S-PLan Plus I think if that's what S-Plan with two heating zones is?). The water is heated via an indirect coil in an old style copper cylinder. By old I mean its got a separate hot water jacket - not one of the modern ones that come ready insulated, and I think it is formed from a number of bits rivetted together rather than the modern pressed ones. It is fed by a 22mm pipe from a header tank that sits just above it - maybe a foot from top of h/w cylinder to bottom of storage tank or 4 feet to top of water level. There is a 22mm pipe which carries the hot water which presumably tees down to 15mm for the taps at some point. There is also a 15mm expansion pipe right on the top of the cylinder.
I think this is all pretty standard stuff but I thought the detail might help the diagnosis.
When I run my hot water for a bath, it runs ok (not great, just ok) for the first 30 seconds or so. Then I get all sorts of gurling, spluttering, coughing. Consequently, the bath takes an age to fill.
Aha, I thought. Just do what they say on uk_diy i.e. get a washing machine hose and connect mains cold to hot under the kitchen sink. After much running up and down stairs (while SWMBO lies watching some garbage with Yvette Fielding and waits on me bringing her a cup of tea!) I'm pretty sure i've flushed any air out of the leg up to the loft and the leg off to the tap.
Result: continued spluttering etc.
It appears that air is being drawn in from the vent on the cylinder because when I temporarily bung it up and run the tap, the flow drops right down. So this is where the air is coming from I guess. There is a corresponding gurgling from this pipe as the water is drawn. I take it that's not right? I understand the principle of the vent for expansion but would have thought it would normally fill up with water up to the level of the top of the storage tank and therefore not allow air into the cylinder unless there is some sort of negative pressure drawing it down along with the air.
The only thing I could think of was that hot water was trying to get out quicker than cold is getting in (the tank is not running dry) but the inlet is 22m and the eventual draw off is 15mm so I would have thought this would cope. Oh yeah, the height from top of storage tank to bath tap is at most 20 feet.
Only thing I have to suspect is gate valve on the pipe going from tank to cylinder. It does close down ok but sticks at about 1 turn open and then with an extra bit of pressure opens quite easily up to fully open. Could it be that it is actually sticking internally at this 1 turn open position thus reducing flow and the handle is just spinning? why would it then stop spinning?.
My plan was to replace it with a full bore ball valve and see it that makes any difference.
However, I'd welcome any other suggestions/ideas.
Oh, and I'll save my other questions for later ;)
Mark.
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Full bore lever valves will spend their twilight years strumming harps on clouds.
Christian.
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They're not cheap. Almost entirely the same as an equivalent sized Megaflo.
See:
http://www.discountedheating.co.uk/shop/acatalog/Online_Catalogue_Heatrae_Megaflo_HE__Indirect__112.html
for hints.

That depends on the boiler. If you have a modern condensing modulating boiler, you are likely to have around 24kW available, which could power a standard shower indefinitely at almost 3 times the flow rate of your old electric. However, an old boiler is likely to be sized according to the central heating requirement and may be much lower, like 10kW or 12kW, so will need time to recover after a shower.
I went for a 180L. I've never run out since we actually plumbed in the boiler (before it was running on 3kW leccy), but don't have a shower. We can have baths back to back, though. They do a 250L version, if you're worried! Their full blown commercial models are 300L and 450L.
Christian.
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