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Hello, here to assist you,
unplug you hot surface Check for 120v after fan goes on,
If you do, replace hot surface, if you dont, fan pressure switch is not closing
Hope this was helpful
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If you need to replace any parts, I suggest that it is time to replace the whole tank as it is seriously outdated and may start leaking and do serious damage to you building.
When the pressure switch closes it sends 24v to gas valve ignition control, to start hot surface ignitor, if it does not, internal relay or electronic board in gas valve is defective and will need replacement.