I have a Richmond power vent with vapor sensor just installed six weeks ago led light is flashing 3X burner wont light.

www.richmondwaterheaters.com/Contact-Richmond-Water-Heaters

The pilot lights then heats/sends a signal to open gas for burner then(burner)should stay on but if valve shuts the gas off,then have your unit checked. What ever you do,DON'T MESS WITH IT IF GAS/FLAME ON!!!!!. These systems come with a safety feature that allows gas to go on only when safe!!!!

Air intake is probably clogged, so combustion air flow slowed and caused overheating inside combustion chamber.
This tripped FV system and broke TRD, which is a small glass vial located under burner. The spring-activated air baffle snapped closed and shut off air supply, and also tripped FV lockout of gas control.
Remove burner, replace glass vial ordered from Richmond, and clean air intake.
http://waterheatertimer.org/How-to-repair-Rheem-TRD.html
If you have Intellivent gas control, then it can be reset.
Copy following link to identify gas control valve, and download possible service manual:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

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

Open following link for troubleshoot error codes showing 7 flashes on Honeywell gas control valve is system lockout due to flammable vapor:
http://waterheatertimer.org/pdf/Honeywell-gas-control_error-Damper-series.pdf
The FV system in Rheem can be tripped by presence of flammable vapors such as gasoline or paint fumes etc, or it can be tripped if water heater was dropped, or mis-manufactured, or if gas control valve is defective, or if water heater is not getting adequate fresh air, or there is inadequate venting of combustion by-product.
When combustion chamber overheats, then glass vial breaks and damper shuts off air supply to water heater and it will not light.
Open following link for possible help fixing the problem:
http://waterheatertimer.org/How-to-repair-Rheem-FVIR.html

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

Well I you have the problem resolved by now, but I will answer anyhow. This just happened to me yesterday with the same water heater that you have. My problem was that the flame sensor was dirty and not picking up that the burner was lit, so it shuts everything down and goes into lockout mode. I had a similar problem with my furnace lighting system. It is called flame rectification. What actually happens is the flame itself allows a small current to flow through it, so the flame sensor actuall makes electrical contact with the burner. The control senses this connection and knows then, that the flame is on. If it doesn't sense this connection, it thinks the flame is out, so it must shut off the gas valve to prevent a possible explosion. Take your burner out and clean up everything in there, being careful not to break the small glass bulb inside, which is another safety device. Fine emery cloth of a wire brush will clean up the flame sensor good. Also wire brush the burner itself, since this is the other end of the rectification circuit.

1) Could be the vapor sensor. That's a suspect.

2) General troubleshoot: clean burner and combustion parts, clean air intake, bleed air from gas line, check gas pressure. Color of gas flame should be clear blue, and flame should be strong, and no combustion by-product dripping back into combustion chamber. No chemicals of any kind stored in air supply. Plenty of fresh air. Water heater in hot attic needs more air. If water heater has igniter, then igniter has to be checked. Flame sensor must be positioned correctly and operational at time of combustion, or sensor will signal gas control to turn off gas. Gas control valve can be bad.
Open following link for possible illustrations and reading.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

3) Specific troubleshoot begins with identifying gas control valve and reading the service manual.
I think the Rheem/Richmond power vent has intellivent gas control
Open following link to identify gas valve and download possible service manual:
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html#intelli

4) Take a moment and rate answer:
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Fastest way to solve water heater problem is call plumber or service technician.

Open following link for typical error codes for intellivent gas valve found on newer Richmond/Rhemm power-vent gas water heaters:
http://waterheatertimer.org/pdf/Intelli-vent-gas-valve2.pdf
http://waterheatertimer.org/pdf/Intellivent-service-manual.pdf

There is no 100% sure way for novice to repair gas water heater.
This is because electrical parts have to ohm-tested using information from service manual that is typically not sent with new water heater.
Also parts have to be cleaned. The gas pressure checked, air in gas line removed, burner tubes and pilot tube and combustion chamber cleaned, burner parts inspected for problems, air intake cleaned, exhaust vent tested.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

Best way for do-it-yourselfer to solve water heater problems is to learn as much as possible about specific model by reading product manual that comes with new heater.
The manual will show error codes.
Then keep up with yearly service recommendations shown in manual.

The accumulation of condensation in the vent piping is more than likly the problem. I am guessing the vent piping is either 3-inch or 2-inch. You need to splice in a "T" fitting reduced down to 3/4-inch with a 3/4-inch tubing attached and run this to a sutible drain. This will drain the condensation elimiating the accumulation in the vent assembly. Install this "T" fitting about 3-feet down stream from the vent motor.These supples and fittings can be found at a Home Depot or Lowes store.

Certain water gas water heater models feature electronic controls with a flammable vapor sensor installed on the bottom portion of the water heater unit. This sensor is designed to shut off your water heater system to prevent fires in case flammable vapors are present within the area. If your flammable vapor sensor caused your water heater to shut down, check the area immediately for leaking gas or other flammable products. Common household chemicals such as bleach can also trip the safety system as well as unsuccessful burner ignition attempts. I'm not sure what control your unit has, but on a White Rodgers here's how it goes.


White Rodgers Intelli-Vent PowerVent Control Equipped Models

1. Step 1 Turn the water heater's blower switch off and disconnect the power cord for the water heater (refer to your particular model's operation guide for the exact location of the blower switch). Disconnect the Molex connector attached to the bottom of the control pad (the left most connector with five pins).
   
2. Step 2 Turn on your multi-meter and set it to read resistance or "ohms". Connect one of the multi-meter's probes to the number two pin and the other probe to the number three pin of the Molex connector (middle black wires). If the reading on your multi-meter reads below 9k or above 45k ohms, the flammable vapor sensor is faulty and in need of replacement; you cannot reset the system until this issue is resolved.
   
3. Step 3 Reconnect the Molex connector to the bottom of the control pad, reconnect the water heater's power if and turn the blower switch to the "ON" position. Press both the left and right buttons on the control pad at the same time. Wait for the green light on the control pad to blink and press the following buttons in sequence: Left, Right, Left, Left, Right, Left, Right, Right (you will need to perform these actions within 30 seconds of reconnecting the power cord to the water heater in order to be successful

I have a 40 gal richmond water heater with power vent and had a similar roblem. I call to talk to a tech and he suggested the vacuum switch. I ordered on line. It was $43 plus shipping and I installed it and it did the job. I happy.