Serviced Hayward H400FDN pool heater. Diagnosed faulty thermopile, replaced with OEM FDXLTER1930. Cleaned heat exchanger using muriatic acid solution. Adjusted gas pressure to 4" W.C. Replaced corroded igniter electrode. Verified proper voltage at control board: 24VAC. Tested safety switches: high limit, water pressure, and flow. Adjusted burner air intake for optimal combustion. Final test showed 140,000 BTU output and 84% efficiency.
