If you keep dumping warm fuel into the bucket and re-circulating it, the temp. of the fuel should continue to rise... but it doesn't. The flapper valve at the bottom of the bucket serves two purposes. If you put an empty bucket into the fuel tank, as in a new installation, the fuel from the tank bleeds into the bucket, through the valve.
When you turn the pump on, at the junction of the fuel return tube and the flapper valve is an integral venturi. As the return fuel runs through the return pipe by the flapper valve, it creates a low pressure at the flapper valve and sucks in some cool fuel from the tank. Cool fuel from the tank is constantly mixing with the warm returned fuel in the bucket.
The fuel in the tank will never get hot. It's in contact with the large aluminum surface of the fuel tanks.
