Winterization is required for areas with often drops in temperature and periodical cold seasons. It means manufacturers do not recommend running heaters if the air temperature stays below 50°F for a long time.
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Please mind that heaters must not freeze or work to maintain the water temperature just above freezing.
Winterization includes the following steps:
1) Drain water from the heater;
2) Turn off all valves and disconnect a gas tank.
3) Open drain plug located on the inlet/outlet header;
4) Remove the heat exchanger inspection panel on the side opposite water piping to gain access to the drain plug on the return header. Open drain plug on return header.
Please always read the manual beforehand to make sure your actions will not damage the heater or the pool.