I couldn’t find installation manual for your Richmond water heater model but since it’s made by Rheem the information at this link - http://www.rheem.com/documents/power-vent-with-electronic-control-power-vent-use-and-care-manual will probably apply to it as well.
It appears that Rheem doesn’t put any length limitation on the exterior vertical section of the water heater’s PVC vent pipe
and your assembly should work fine.
But, there are a couple of things that might need attention and their importance would partially depend on your geographical location / climate zone.
Power vent equipped water heaters are not condensing type appliances / condensate formation does not occur during exhaust process. However, there are some conditions including ambient temperature, humidity, vent pipe length, and PVC vent pipe slope that might result in condensation formation inside the vent pipe.
To prevent any possible condensation from flowing back into the power vent’s compartment the horizontal vent pipe section should be sloped down towards the exterior 1/8” – ¼” per foot (so it drains outside) or up if the appliance has been equipped with optional condensate kit.
With that exterior vertical section installed (and especially in cold climates) the PVC vent pipe horizontal part must be sloped towards the power vent assembly and condensate drainage provided.
There are also items #7&8 on page 14 of the Rheem's manual that are important for cold climates and additional support requirement for vertical sections longer than 24” (applies to above the roof installations but vertical is vertical – page 15, item #3)
I would also verify vent pipe clearances recommended by the high efficiency furnace manufacturer.
Just click “comment” below if you have additional questions.