Question about Hayward H Series Pool & Spa Heater 400k Btu Propane Gas Milivolt Ignition H400p1

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Hayward spa heater about 5 years old, when we turned it on for the first time in probably a year as soon as the water heater came on it started making a loud clanging sound

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For starters you will need to proceed carefully...the noise you hear is water beginning to boil within the heat exchanger. The pressure that results can cause the unit to rupture or worse, blow. DO NOT ALLOW THE HEATER TO OPERATE IN THIS CONDITION FOR MORE THAN A MOMENT OR TWO as you attempt to repair it.

Water boils in a heater due to low flow, allowing the water to get too hot as it passes thru, or a faulty part or parts within the unit. We'll start with parts:

The bypass valve inside the header may be the issue if it is stuck closed. You will also want to inspect the thermostatic control valve in the header (if one is present) and inspect the overall condition of the heat exchanger. It is likely aggressive water has damaged some or all of these parts causing the banging noise you hear.

If the heater checks out then flow is the likely cause. It is possible there is an issue with the pump and/or filter not getting enough flow thru the heater. Check the overall system flow. If it is reduced too low the water inside the heater gets too hot and begins to boil causing the noise you here. Speeding the flow back up will eliminate the issue. Check all the trap baskets and the filter for debris and clean up what you find; backwash a sand filter or clean a cartridge or DE unit. If your pressure gauge is broke replace it and check system pressure. If it is low again that is a sign of a flow problem. If flow is low and all these items check out open up the pump and inspect the impeller for clogged vanes or a blocked inlet eye.

If this answer does not fix your problem, please comment with additional details prior to rating the answer. Positive feedback is appreciated once your problem is solved!

Posted on Mar 14, 2011

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