Yeah, there is ample water in the loft cold tank, no where near empty. There is actually 2 x 50 gal tanks and the shower starts drawing air from the HW cylinder after about 7 mins of operation.
I think the problem is that the pump is taking the water out of the hot cylinder faster than gravity can get the cold back into it from the cold tank. When the hot tank level drops, you can hear the pump running dry and only pumping from the cold supply.
Pretty sure the float is flowing as fast as it can.... had to drop the tank by about 500mm so there was a very noticeable pressure drop when using a hot tap in the bathroom where the cylinder is. Will post some pics shortly.
I have recently had to lower the height of my cold water tank (in the loft), which has resulted in a lower pressure feeding my hot water cylinder, which is on the 1st floor. I have a pumped shower at roughly the same height as the top of the cylinder. This now seems to be pumping water out of...