I have a rheem powervent model# 21VP40-1P. My problem is this- no hot water. I turned off the unit and waited a few minutes and turned it back on. The vent fan comes on (and stays on non stop), and I hear clicking as though the igniter is going, but get nothing. No flame, no nothing.

Start with replacing Thermo-coupling first...

Milo

Do you know if the water heater has a glow plug (hot surface ignition) or spark ignition? If glow plug (most are) then there is no thermocouple, but there is a flame sensor. However, if the glow plug is burned out (never glows) then the flame can not ignite so the flame sensor has no flame to detect. If you are hearing a clicking noise, it may be your gas valve opening, but without a source of ignition, it will shut back off after a few seconds due to lack of flame sensed. If you have a multi meter and the basic knowledge of the sequence of operation and are mechanically inclined, you can test for power to the glow plug, to the gas valve, etc and locate the issue. If you don't feel comfy working on this yourself, you could call in a tech and they should be able to get you back in hot water in short order.

I have a possible answer and question myself. I've the identical water heater. All the cyles run through, vent, glow plug heat, but when the controller tries to turn on the gas valve, no 24V to it. Acting like the relay on the control module just can't switch. It' sounds like it's trying, and occasionally 'blips' up 1 - 5v but no more. Nothing better if disconnected from the valve and measured. Definite control module failure?

Sure sounds like a control module issue.