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This blog post is about the chemistry of combustion, not the chemistry of the pool and spa water that is being heated.

Combustion:

Pool and spa heaters burn either natural gas—typically methane (CH₄) or propane (C₃H₈). Since both of these fuels are made up of the same basic stuff—carbon and hydrogen, just in different proportions, the combustion reactions are going to be similar. For the sake of simplicity, I will just focus on the combustion of natural gas.

Ideally, the combustion reaction is as follows:

CH₄ + 2 O₂ → CO₂ + 2 H₂O (ΔH = −891 kJ/mol (at standard conditions))

In other words, the combustion (i.e. oxidation) of natural gas yields carbon dioxide, water vapor, and a lot of heat.

Water vapor—a common byproduct and its disposal problem:

As noted above, one very common byproducts of combustion is water vapor: Put your hand a foot our two above a heater that is running and it will feel wet from the hot exhaust. If the heater’s combustion chamber and exhaust vent are hot enough, this water will remain vaporized long enough to exit the heater.

Interestingly, one of the key reasons that double wall vent pipe is recommended is not that it keeps the outside of the pipe cooler and thus safer, but that it keeps the inside hotter. A hot inner pipe improves its natural drafting ability due to the fact that hot air rises. The double wall pipe acts as insulation and the exhaust hot. The hot exhaust keeps the water in vapor form as it travels through the vent pipe. If you choose to use single wall vent pipe instead, the exhaust will lose its heat through the single wall pipe. As a result the exhaust will cool and the water vapor inside will condense while still in the pipe. This condensation explains why single wall vent pipe rusts more than double wall pipe.  

Low gas volume—a first obstacle to water vapor disposal:

If the volume of gas available to the heater is insufficient to provide for the minimum requirements, the heater may still light, but if it does, it will not burn as hot as it should. As a result, some of the water vapor in the exhaust will cool enough to condense before it rises completely out of the heater. This results in excessive rust-causing moisture remaining in the heater. Low gas pressure also produces an excessively yellow flame that emits a black soot that will stick to and eventually build up on the heat exchanger. Over several months or years, this soot will eventually block the exit of the exhaust. It will also trap excess heat in the heater causing it to become more efficient than it was designed to be. This efficiency is not as good as it sounds as the heater is destroying itself in the process (more on this later).   

Excessive water flow—a second obstacle to water vapor disposal:

Heaters are designed with a specific water flow in mind and have automatic valves that control the flow of water through the heater. Some of the better heaters will have two types of these valves. One type of valve is spring operated and is designed to produce a consistent flow of pool water through the heater despite the heater being used with a variety of pump sizes and with different amounts of restrictions due to the filter.

In addition to the pressure operated valve, there is often a thermally controlled valve that is designed to provide a constant heat exchanger temperature. If this thermally controlled valve malfunctions (or is not part of a particular heater’s design—it is absent from the Series One Laars, the Laars Lite, and the new Universal Forced Draft Hayward (UFD)), then the temperature of the heat exchanger will vary as the water heats up. If the initial water temperature is low, so too will be the temperature of the heat exchanger. And, if the heat exchanger is too cool, the water vapor from the combustion will condense on it and “rain” back into the heater. If you see a rusted burner tray on a Laars, this is probably the cause. Hayward claims that their new UFD heater is built so well, that this condensate “rain” will occur, but will not damage anything.

More than 84% efficient—a third obstacle to water vapor disposal:

Have you ever wondered why even some of the best heaters are “only” 84% efficient? Have you wondered why there is a major price jump upwards between these commonly used heaters and the rarely used “high efficiency” heaters? There is a good reason for this and it is all about the disposal of combustion water vapor.

As I said above, a heater’s exhaust needs to remain sufficiently hot for it to keep the combustion water vapor from condensing inside the heater. If the heater transfers more than 84% of its heat to the pool water, the exhaust becomes too cool to carry away this water as vapor. As a result, the water vapor from the combustion cools, condenses, and “rains” back into the heater. The condensing water (H₂O) combines with carbon monoxide (CO) to form carbonic acid (H₂CO₃). As a result, the condensate from a high efficiency heater has a pH that is typically in the 3.5 to 6.0 range.  

Efficiency in excess of 84% can come about by design—as in the Laars Hi-E2—or due to an unintended blockage that occurs within a standard heater. Heaters like the Hi-E2 that are designed to be highly efficient are manufactured with stainless steel burners and combustion chamber components to withstand the acidic condensate. Additionally, they have condensate traps and neutralization basins. These basins utilize limestone to neutralize the acidity of the condensate so that it can safely be disposed.

Standard heaters can inadvertently become highly efficient as the result of low gas volume—measured as low pressure with a manometer. Insufficient gas volume produces a flame that emits soot. This soot sticks to the fins of the heat exchanger thereby blocking the passage of exhaust gases. In blocking the exhaust, the soot slows the flow of the exhaust allowing it extra time to transfer heat to the water—thereby resulting in a highly efficient heater. Unfortunately, heaters that become highly efficient in this manner are not designed to handle the acidic condensate. As a result, the acidic condensate attacks the metal components and results in excessive rust and corrosion. These unintended high efficient heaters have only a short life left before they destroy themselves.

Formaldehyde—an occasional nasty byproduct of combustion:

There is one particularly nasty byproduct of combustion that sometimes occurs: Formaldehyde (HCHO, also written H₂CO) is one of these. Typically, formaldehyde is a short lived byproduct that quickly oxidizes in the extreme heat of the combustion chamber to form carbon monoxide (CO) and water vapor (H₂0). The carbon monoxide (CO) then oxidizes into carbon dioxide (CO₂). Sometimes, however, if conditions are wrong—if the combustion chamber is not hot enough—the formaldehyde doesn’t get oxidized and exits the heater as a smelly, eye-irritating gas. This mostly happens on low NO_x heaters.

The reason for this is that low NO_x heaters have blowers that are designed to lower the temperature of combustion—thereby reducing the formation of oxides of nitrogen (NO_x) that are known to form in small quantities at certain temperatures. The fans lower the combustion temperature and thus lower the quantity of oxides of nitrogen (NO_x) that are produced. Occasionally, some of the gas orifices on these heaters become clogged with spider webs. When this happens, only a fraction of the intended gas actually gets to the burners. As a result, the temperature within the combustion chamber is reduced even further. At this lower temperature, the formaldehyde doesn’t oxidize as it normally does in the combustion chamber and exits the heater. This results in a heater that smells particularly foul and will cause eye irritation. This problem is particularly prone to occur on low NO_x style RayPak heaters. See my YouTube video of how to correct this problem.  

Conclusion:

Understanding the chemistry of combustion can help when trouble-shooting a heater. The presence of rust—and its location—is a major clue never to be overlooked, so too are noxious formaldehyde odors and sizzling sounds caused by condensate “raining” down on the burners.