Err!
The local utility company does not guarantee any pressure except a minimum of about seven metres static head in the communication pipe to the premises.
Companies try to keep an eye on the pressure at about ten metres head at the outside stop tap, with a flow rate of nine litres per minute about enough to fill a 4.5 lite can in 30 seconds.
So the guaranteed pressure ain't much and depending on the frictional head of the cold mains system pipework, by the time it gets upstairs it can be pretty weak.
But seven metres head is enough to get it upstairs in most properties, just!
2.5 metres per lift = 5 metres add floor void between floor's .2 metre = 5.2 metres about another metre for rising off the main to the Ground floor level = 6.2 metres.
So you ain't got much head to design a crm system on.
Incidentally 7 metres is just over .5 bar and combis usually have a minimum requirement of .5 bar. If I had a shilling for every combi installed on a low pressure main I've seen I'd be loaded.
Its partly because of low pressure in the mains, that some areas use storage cisterns.
Its also why you try to make sure your pipework have as low a frictional head loss as you can. Throw that bag of elbows away and use your bending machine if you can. Incidentally I don't know whether you can still get them or not, but the likes of Yorkshire once made and sold bends as well as elbows. They where much slower and obviously must have cut down the frictional head.
Fortunately most main water supplies are well above 7 metres static. If your at the bottom of a hill your pressure is probably higher as they have got to reach those at the top of the hill. It all depends of course on the mains hydraulic line.
Water companies don't like high pressures either, by keeping it as low as possible but still providing the minimum pressure, they reduce the water lost through leaks.
You can check all this out with OFWAT if you like.
Err!
I wonder if you could use a flow switch with the pump into the cylinder cold feed, this would turn it on only when water was being drawn and if set up right, may not be on long enough to pump out the vent? It depends of course how high the highest part of the vent is over the water level in the tanks. And if the resistance in the pipework to the tap is less than that to the vent. Mind you, the hot water in the cylinder usually expands a bit back up the cold feed into the storage tank and with a pump and flow switch in the system it may not be able too, so it would vent over instead.
You also don't mention how the water is heated, assuming say a back boiler and direct primaries. The boiler is fed directly from the hot water cylinder and the cylinder from the storage cistern and you would be blocking the boiler cold feed if you put a pump on it, a great big NO! NO!
I bet somebody comes on with an easy answer
