I have a rheem w/h Md 42vp50pf. It heats my shop floor through the radiant heat tubes. I noticed the shop was getting cold so I checked on it. The pump was on circulating water but the pipes are cold. The w/h would fire if I turned it up all the way , but go off after about 10 min. Now it wont fire at all. reset the power several times and the power vent purges then shuts down with the flashing code of #23 which is " lock out condition. FVD interface failure?miswiring (flammable vapor sensor failure)". I unpluged the wires and looked at the board for signs of being burnt, but no signs there. What am I missing??

Similar 42vp50 direct vent product appears on following link:
http://www.rheem.com/product.aspx?id=C24E3F52-A6A4-4BC7-AF5B-0DF23A7A0263
sample manual for power vent series:
http://globalimageserver.com/fetchDocument.aspx?id=c0471d58-5e59-4be6-96f7-68361b8fb515

Manual has troubleshoot section:
If this is combustion-shut-off or FVIR or flammable vapor event, then manual says to call qualified technician.
Service call is certainly less expensive than any fire or injury caused by fire.
Other brands of water heater, for example Bradford White, have customer-resettable FVIR on some models.

Another thing that can be causing problems is air intake, especially in dirty environment like a shop, or if shop chemicals are being inhaled by burner. Burner parts are susceptible to chemicals in trace amounts below odor threshold. You might need fresh air intake for water heater, and air intake needs to be cleaned.

'System lockout' shows on troubleshoot page 32:
1. Gas supply is off or too low to operate.
2. Hot surface ignitor not positioned correctly.
3. Low voltage to the water heater.
4. Electric polarity to unit is incorrect - test and correct.
5. Combustion shut-off device tripped, contact service
agency.
Rheem phone number: 1-800-432-8373

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