We had a new worcester bosch greenstar combi-boiler and new radiators with thermostatic valves installed one year ago. Now that we have started to switch the heating back on after the summer, I find that I'm having to bleed the radiators every day. The towel warmer in the bathroom only gets hot at the bottom, and you can unscrew the bleed screw on the top of it completely without water coming out. The temperature control on the boiler is set to maximum and the pressure dial is reading zero. Is it a lack of pressure/ not enough water in the system, or both ? I can see the adjustable valves at the bottom of the boiler, but don't want to fiddle with any of them without knowing what I'm doing, and the manual doesn't mention how to remedy lack of pressure or not enough water. I'd be grateful for any advice.