Heater Condensation - Further Reading

Gas Heater Corrosive Condensation

Corrosive condensation can form within gas heaters when they are run with water below 70F degrees. Incomplete combustion of natural gas from cold air can lead to the formation of very corrosive condensates on the heat exchanger.

Some heaters use a unitherm governor. A unitherm governor blocks water from exiting the heat exchanger until the water warms up in the exchanger.

It's similar to the thermostat in a car that regulates the coolant circulating through the engine. This prevents cold water from circulating through the heat exchanger.

Cold water circulating through the heat exchanger creates condensation that can drip off of the exchanger and onto the burner tray. So, you can get condensation any time that the conditions are conducive to condensation.

If you have ever seen condensation on your plumbing and system equipment, then you probably have had condensation in the heater.

Usually you can hear a sizzle sound which is the condensation coming off of the heat exchanger and dripping onto the burners.[1] If you have the FD model of Hayward, it would be harder to hear this based upon the construction of the unit. The older ED2 style units, you can hear the sizzle sound when it happens.

How to Prevent Corrosive Condensation

Adding an external bypass can lesson condensation as you are bypassing some of the cold water from the heater so the water that does transfer thru the heat exchanger has a better chance of heating without condensation happening. However, it will take a bit longer to heat the water at times because you are sending unheated water back to the location.

Manufacturers say prolonged operation with water temperatures below 50°F is not recommended. When starting the heater with water temperatures below 50°F, operate the heater continuously until higher temperatures are reached. Operating the heater for prolonged periods with pool water below 50°F can seriously damage the heater, and is not covered by the warranty.

If this happens once, or once a year, it is no big deal and there shouldn't be any significant damage. But if you do this regularly, for example daily for weeks or months at a time, it will significantly shorten the life of the heater.[2]

Effectively you are cooling the exhaust gas below the condensation temp of 135-140 deg. When this happens the water vapor in the exhaust forms water droplets and runs down inside the unit. You do not want this to happen too often as the condensed water is slightly acidic and will start to corrode the sheet metal parts inside the unit. The water also contains carbon which will start to build up on the heat exchanger and plug it up. High efficiency house furnaces furnaces are designed with stainless components and drains to combat this issue. Pool heaters are not.[3]

A low volume water body like a spa will heat up quickly enough to not matter. A large volume pool will not and the heater flue gas will have a longer exposure to condensing temperatures. The water vapor in the gas will mix with NOx and sulfurous compounds forming a very acidic mixture.[4]

The Pentair MasterTemp Heaters say:[5]

  • Operating this heater continuously at water temperatures below 68° F. (20° C) will cause harmful condensation and will damage the heater and void the warranty.
  • When starting the heater for the swimming season with a water temperature below 50° F (10° C), the heater may be used to heat the water; however, make sure that the heater operates continuously until the water temperature reaches the heater’s minimum setting of 68° F (20° C).

The Hayward H-Series heaters[6] say "Do not use the heater to maintain the water temperature just above freezing or for freeze protection."


With the heater off, place a mirror into the area, above the burner if possible, or below up through burner rods. Likewise for an endoscope.[7]

What you're looking for in terms of identifying a combustion/condensate problem is: A) evidence of sooting (black) and, or, if viewable, b) condensate itself, which tends to be a darker green, sticky substance

If with your type of heater you are able to view the burner while its burning, also look for a good blue field of flame with only a bit of orange. It there's lots of orange and particularly yellow flame, plus any sizzling as others mentioned, that's a sign of improper combustion which causes the condensate -- or the condensate causing improper combustion.