Why does the power vent keep running?

Do the easy stuff first! Is this a new install or has it been working well for a time and now it is not? Is the entire unit continuing to run or just the fan? Thermostats generally turn the unit on and off when the desired temp is reached but the fan itself (not the compressor or heat strips) typically operates in 3 modes, off, on or auto. check the fan setting.

It has to cool down for safety reason unit still to hot

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

Rheem does not post service manuals
Gene

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Check the shut off valve to the tank, older models with washers cause problems when the washer deteriorates, splits, or the washer retaining screw breaks off. It will give you intermittent water supply or no supply at all.

Water heater and brand are not identified:
Question posted under electric water heater, but electric water heaters are not power vent.

First suspect is gas control valve thermostat.
Or burned points on contactor that turns on the blower.
Check if the burner is still going so you are not getting runaway heating that will trip the ECO and require possible gas valve replacement.

Open following link for older Reliance troubleshoot manual, plus some troubleshooting resources and illustrations, plus link to find age of water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

Add a comment and say what you did to restore functionality.

manufacturing oils burning off of the heat exchanger. it will continue for about an hour of run time. if it continues longer than that, contact your installer.

flue is overheating make sure outlet is not obstructed outside and if not that you feel a good air flow coming from it

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I have a problem with what you are asking,, the numbers you have are for a fan convection ROOM heater,, does the fan come on within this 3 minutes of running, if not it could be over heating and shutting down on a safety, if it does the thermostat could be bad and it is reaching the desired temp and shutting off like it shoud,, check the thermostat to make sure it is working properly,, set it at it's lowest temp turn on the unit have a themometer infront of the air flow see if the temp is the same as the setting,, how old is this unit, it may just be time to change with new,, if the controls are bad this unit sells for about 600 and to replace these parts is about 400, go to there web site,, write back and let me know how it goes,, good luck

If the blower continues after the end of the burn cycle it is the cool the heater down. Over running would indicate a defective end cycle temp relay.

Good Luck, Phil