Pressure variation

Could anyone tell me why my GlowWorm combi boiler central heating system is now varying from 1.5bar to 3.0bar, now that I have bled the radiators? Previously it was varying from 1.5 to 2.5.
Which expands more with heat?: Water or air?
What are safe limits? What happens if you go above these limits?
Thanks for your help. AD
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