Why Pool Heaters Fail

The question came up as to why pool heaters fail. This is not a simple question to answer as there are several causes for this and it can be just one or a combination of things which will damage a pool heater. Older pool heater were more forgiving but the new low NOX, high efficiency heaters are much more prone to failure under less than ideal conditions.

Improper water balance. If the water is maintained in the negative range  of the saturation index, this will cause the copper in the tube assembly to be dissolved. This can be identified by blue copper stains on the plaster.

Common causes are the use of chlorine or bromine tablets which lower Total Alkalinity and pH. This will quickly dissolve the copper especially if the Calcium Hardness is low. Spas can also lose alkalinity when the jets are running or just from fill water with low Total Alkalinity and Calcium Hardness. For spas with low alkalinity problems it is important to check alkalinity weekly and add sodium bicarbonate to keep it in the 80-120 range for liquid chlorine or calcium hypochlorite. If using tri-chlor or bromine tablets it would is recommended to keep it in the 140-160 range.

When using chemical automation, a trick to slowing down the degradation of alkalinity in pools and spas that have a tendency to have low alkalinity is to program the acid feed for small amounts of acid at a time. Usually around 8-10 seconds on with a 45 second to 1.5 minute off time as well as a set point of 7.6.

Salt water pools can also be hard on pool heaters if the hardness levels and alkalinity are low. I have personally found that when using salt it is advantageous to maintain a hardness of at least 300 ppm.

Sometimes the opposite can happen which will scale the inside of the tubes when in a hard water situation from fill water or using calcium hypo chlorite. The calcium buildup inside the exchanger will lower the heat transfer rate and potentially reduce water flow through the heater causing damage to the heater.

2. Improper water flow through the heater. This is caused by having too much or too little flow through the heater.

Heaters in the 125-405k BTU’s which are common i back yards, hotels and apartment buildings have a built in flow regulator at the inlet header which can get stuck in the full open or closed position or the disk and spring regulating water flow can be damaged.

Too much water flowing through the exchanger can erode the copper causing the tube assembly to leak. This condition will also drop efficiency. Too little flow can cause the copper tubes to become weak and brittle, eventually causing them to leak.

If you are using a high output pump it is important to have a manual bi pass to ensure proper flow through the heater. A heat rise test will ensure proper flow through the heater to achieve maximum efficiency and longevity.

Heaters 500k BTU’s and up are typically installed with a manual bi pass.   

3. Improper air flow through the heater. This is caused by improper venting and can apply indoors or outdoors.

Many heaters now are fan assisted. Some even have air filters which can become dirty and clogged which will prevent proper burning of fuel. It is not uncommon to see heaters installed indoors to not have enough air flow into the room ad/or improper exhaust venting to support proper combustion which causes the heat exchanger to soot, causing flame roll out which damages components, greatly reduces efficiency, weakens tubes and are a potential fire hazard.

I have seen many pump rooms built with insufficient air available to the room.

4. Improper gas pressure to heater. This can be caused by an improperly sized gas meter, gas line that is too small or both.

The most common problem is insufficient gas flow. This will cause the heater to condensate. The condensate is corrosive which will damage the heater.

Gas pressure that is too high can cause fires and explosions.

5. Bad heater design. There have been particular heaters that are prone to failure just like any other appliance. If none of the problems listed above apply to your situation, you may have a lemon. A properly installed and maintained modern pool heater should give you 7-10 years of service.

‘This is not meant to be an all inclusive cause and effect but does state the most common issues leading to pool heater failure.

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