The Baseboard.php is the form for the radiator output calculator.   It name RadiatorCalc as the action to take when submit is pushed.

RadiatorCalc.php pulls the values from the form using $_GET, does the calculatoions, and will send the result back to the form when done.

Got the form working, and the read from form in RadiatorCalc.  Got the calculation of the HeatOut working correctly.

Just need to get the HeatOut value back to the form.

Also, need to do validation, and prevent bad inputs from getting in.