Is the flow rate over 0.75 gpm? See if the unit initiates when you increase demand of water being drawn through the unit (e.g. opening up more hot fixtures).
Check that the gas line is open and is purged of air (p. 22).
Check that the filter on the cold water inlet is clean (p. 21).
Check for reverse connection and cross connection (p. 21).
Problems with recirculation system
o See that a check valve is installed between the pump discharge and the connection to the cold water supply. A malfunctioning or missing check valve will cause cold water to back?feed into the system.
If you use the remote controller, is the remote power button turned on?
Check to see if the Computer board is receiving power. This can be verified by observing the computer board red LED.
o If it is not lit, the computer board is not being powered. Check to see if the ON/OFF switch below the fuse box is set to the on position, or if the fuse in the fuse box is blown.
If the fan does not initiate at all and it is verified that the unit is powered(see above), the flow sensor may not be working properly. Check the flow sensor for proper impeller functionality.