The F&E tank should be sized and set up to accommodate (without overflowing) at least 5% expansion of the total system
contents, ie boiler(s), pipework, rads and cylinder coil or thermal store etc. 

When the system is filled and cold the water
level should be set (by bending the ball valve arm) to give around 100mm of water in bottom of the F&E tank, (enough to 
cover the F&E pipe connection and allow for a possible small drop in water level when the pump is started), so that when 
the system is heated to it's max temperature (boiling) and the water expands, the resulting water level should be at least 
a inch or two below the overflow outlet. Also the ball valve outlet should be at least a inch or two above the top of the 
overflow pipe/max water level to ensure there is no cross contamination between the system water and the incoming cold mains
 supply. If the F&E tank is too small and constantly overflows and refills with fresh oxygenated water, then this will lead
 to or increase system corrosion.

A 22mm open vent, 15mm feed & expansion and 22mm overflow pipe is OK for boilers up to 
25 kW, above that will require a 28mm open vent, 22mm feed & expansion and 28mm overflow pipe.

The F&E tank should be well 
supported and capable of withstanding boiling water, as should all components, ie the ball valve float and overflow pipe 
must be copper etc.


Pros and COns of an F&E Tank, versus using a closed loop expansion vessel:

Advantages of an F&E tank for solar:
	-simple
	-low tech
	-no equipment needed to get your antifreeze in (you need at least a modified garden sprayer to pressurise an EV)
	-automatic air bubble removal, hence fewer air vents needed  

Disadvantages 
	-more oxygenated air in solar loop, possibly eventually more air getting trapped at high points like pipes on roof
	-some heat loss via the vent pipe
	-no pressure guage and consequently harder to see if you've got a leak in your solar circuit. EV uses a pressure guage.
	(small leaks will get topped up automatically by your ballcock, diluting the antifreeze, and exposing you to risk of freezing)
	-need to run more pipework into loft.
	-you must run the solar loop at <100 deg c. (EV running at >1 bar raises boiling point of solar fluid)