Well, the difference between the natural draft and power vent water heaters is in their design and of course venting. A natural draft water heater operation sequence has fewer components, when the thermostat senses temperature drop inside the tank, it opens the valve and ignites the burner. Hot exhaust gasses rise through the vent tube into the connector / chimney.
In a power vented water heater this process is slightly more complex. The thermostat sensing a need for heat activates the controller which checks the pressure switch state. If the pressure switch is open the controller will send a signal and activate the blower motor.
Once the blower motor is activated and starts circulating air through the combustion chamber (sometimes it takes a few minutes) the pressure switch closes sending a signal back to the controller which starts the ignition procedure and the burner is lit.
In order to install a power vent unit on top of a natural draft water heater you’d have to make sure that the natural draft WH has been approved for such installation which I doubt it is (you would have to call the manufacturer to verify).
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