Need to cut cabinet floor for access to kickspace heater

I have an under-cabinet hydronic kickspace heater which recently started leaking and will need repair/replacement. Unfortunately, the previous homeowner did not cut an access panel into the bottom floor of the cabinet when it was installed. It appears that the heater was plumbed in to the hot water baseboard heating loop before the cabinet was installed above it. To compound the problem, the tile floor was installed afterward so I'm not even sure that the unit can be slid out after the pipes are disconnected. I had to chip grout away in order to remove the grill from the front of the heater.

I can see underneath the cabinet where the heater is, and there appear to be 2 pieces of wood running front-to-back above the heater so that they support the cabinet floor. There is no evidence of nails or screws securing the cabinet floor to the frame or the the wood supports underneath, but it is attached somehow (perhaps glue?) and won't move.

Can anyone provide some insight as to how cabinets are usually constructed so that I can minimize the damage? I think the supports under the cabinet floor will have to come out if I want to replace the heating unit, but I need some mechanism to hold the floor up afterwards, while continuing to provide easy accessibility for future maintenance.

 

The traditional way cabinet floors were attached was in dados in the side panels and nailed/screwed through the sides into the edge of the bottom panel and glued. Yours could be assembled any number of ways with modern factory methods. How ever it is assembled, it is not made to be taken apart easily. You will probably have to cut the bottom panel out with a jigsaw leaving a few inches margin around all 4 sides

 

I would carefully measure from side to side halfway across the 2x lumber running front to back and strike a line on each side. Then determine the perpendicular lines where it will allow you to access the heater. It will take a multimaster tool to make an accurate cut on top of the two 2x pieces, but it will leave support in place for you to reinstall the piece that you cut out.

 

I would use a plunge router with a 1/4" bit as it will leave a much finer edge to the material in the base of the cabinet. Then make a cover panel probably the same dimensions as the base of the cabinet. It's a real pain, but it probably should have been done like that in the first place!