I am installing a floating bamboo floor on concrete,with appropriate underlayment.The house has hot water baseboard heat.There is a small baseboard molding under the heater now,should that be removed and do I need to put the molding back cut smaller to take into consideration the thickness of the new floor or can it be left without a molding under the baseboard heat.
Molding is mostly for aesthetic purposes, so you can do whatever you think looks acceptable. With a floating floor, the baseboard hides the edge since generally there's a gap. Presumably you'll have to reattach all your baseboard molding after putting the new floor down.
I'd say if you can see underneath the radiator when you are standing in the room, you should probably install baseboard.
If the issue is the molding no longer fits, then trim the top edge off (assuming you can't see that unless you put your head down level with the radiator), or install a different type of molding there (such as just quarter-round).
If you can't really see underneath the radiator from any standing/sitting positions in the room, then you can get away without installing it, but I'd at least put some quarter-round up to keep dirt and dust from collecting in the expansion gap.