Hot Water taps run dry

[Kraken]

Having a loft conversion done currently (by a professional company), and they've run into a problem today. As I don't want to pay £3.5k for a combi boiler they've moved the cold water storage tank to the eaves (lowering it about a metre in the process), and redone the pipework to the cylinder below. As a consequence the hot water taps don't work, as air is drawn into the system from the vent pipe instead of water flowing.

I didn't understand what the plumber said he was going to do, but he suggested something like a one-way valve on one of the pipes so the air couldn't drawn in. He didn't have one with him, so he's due back tomorrow to finish off (and I have no hot water for tonight).

Having read another thread on this subject I'm concerned he's suggesting doing exactly what was objected to in that thread, ie, adding an anti-gravity valve to the vent pipe.

Is this really a big no-no? I just need some evidence (plumbing regs?) to confront him with if that's the case. What other solutions are there? The tank can't be raised any more as there no more room in the eaves.

 

[waterworks]

No valve of any kind in open vent pipe.
If your cold water tank is higher than your cylinder it will work on gravity regardlest of it being lowered as you said you will just get less pressure,
if you have pumped supply then you can have problems.

 

[AshaDPlumber]

Big NO NO! It's very dangerous to obstruct a vent pipe with any form of valve, regardless of wether it should only work one way or not.

 

[Kraken]

I think the problem exists because the cold feed goes down from the storage tank to the foot of the cylinder, and the hot feed (and vent) both come back to the loft, the hot water being distributed from there.

The plumber mentioned something about the length of the pipework being more effort for the water to travel than just syphoning air into the vent pipe (or something....)

Is it really just a complicated airlock?