Here I don't know if there are any differences between the B and other Bonanza heater set-ups but one thing I found in mine was a cable for emergency heat cutoff in the circuit breaker panel on the right hand side. The original setup on this plane had the battery box on the back side of the panel with an access to the engine side.  Mine had let battery acid leak through the box and ate through the cable windings for the emergency shut off used to reduce smoke in the cabin in the event of a fire.  You could pull that knob all the way out and it had absolutely no effect on shutting off the heat access.

When I replaced my panel, the heater controls were moved to the opposite side of the panel and the cables replaced.  On mine there are actually two mechanisms that can affect heat entry. The first being the heater lever that controls flow of air past the exhaust and into the initial heater baffle and the second being the emergency heater shut-off  to the upper chamber entering into the cabin that was damaged.  Even with the the heat in the off position.... I guess due to age just don't seal off the air as well as they should, some heat was still entering the cabin. If you close the emergency heat shut-off, it does a much better job of stopping heat flow into the cabin. Hard to do if the cable is defective.

On the firewall on the left hand side you will see a small inspection port on the firewall to inspect the flap for closure fitness for the emergency shut-off. Making sure this device works properly can make a dramatic difference in unwanted heating of the cabin.