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. The Raypak Avia Installation Manual says:

The patented Unitherm Governor is a thermostatic mixing valve specifically designed to maintain constant heater internal temperature between 120°F (49°C) and 135°F (57°C) despite continually changing flow rates from the filter and changing pool temperatures. This range is needed to reduce the amount of condensation from the fin tubes which will occur if the heater runs for any length of time below 100°F (38°C) water from the pool. It is also needed to inhibit scale formation in the tubes by maintaining temperatures well below accelerated scaling temperatures.

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. 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.

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.

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.

What Manufacturers Say About Corrosive Condensation

Pentair and Hayward have included Condensation Management in their latest model heaters. Raypak has their patented Unitherm Governor to reduce the amount of condensation.

Pentair

The Pentair MasterTemp Heaters says:

• 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 Pentair ETI Heaters and UltraTemp ETI Hybrid Heater do Condensation Management. Their manuals say:

The ETi® 400 Heater is a condensing appliance. The flue gases will produce condensate while in operation and must be drained correctly. Note: The condensate pH level is between 3.1 and 4.2, Pentair recommends to neutralize the condensate to avoid potential damage over time to the drainage system, and to comply with local water authorities where applicable. To neutralize the condensate, use an optional Condensate Neutralizer Kit P/N 475612 or similar. The condensate drain must be installed so as to prevent accumulation of condensate.

Jandy

Jandy gives the following guidance in the JXI heater manual:

• Do not operate the heater if the water temperature is below 40 °F (4°C)
• Short term operation of heater when temps are below 40F can continue but continuous flow must be maintained.
• Prolonged operation with water below 50F can seriously damage the heater and is not covered by warranty.
• It is recommended that vent runs over 18 feet be insulated to reduce condensation and use a condensate trap in the vent run close to the heater, especially in cold climate installations.
• If the heater is to be operated in below freezing conditions it should be installed in a protected outdoor shelter.

Hayward

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

The Hayward Universal HC Series Heater includes Condensate Management. The UHC Installation and Operation Manual says:

In certain running conditions, any pool heater can produce condensate while it is running or sweat during an off-cycle. The formation of moisture on the heat exchanger depends on many factors including water temperature, humidity, flow rate, and ambient temperature which cannot be controlled. Because of this, observations of how much condensation is created by the unit will vary greatly. Consideration has gone into the design of Hayward’s HDF series pool heater to allow any moisture in the combustion chamber to drain out of the unit. Not allowing moisture to dwell in the unit will increase the life and reliability of your new pool heater. The condensate formed during combustion will be slightly acidic, about the same pH as a cup of black coffee. If condensate from the unit needs to be routed off the equipment pad or into a drain, the unit has been supplied with a 6 foot length of rubber hosing and a connection on the bottom of the exhaust side of the unit.

Note that the pH of black coffee is around 5 and Hayward is being disingenuous by not simply stating the low pH which is clearly acidic.

Raypak

Raypak says:

Important Freeze Information

MODERATE CLIMATE: Heater operation can continue during short-term cold spells. When temperatures are between 0°F (-18°C) and 32°F (0°C), flow (continuous pump operation) must be maintained.

A CAUTION: Do not use the heater to maintain water temperatures just above freezing or for freeze protection. When heater is used during freezing weather, care must be taken to avoid freeze-ups. Continuous pump operation is a must. Additional protection may be required. The heater is not warranted against freeze-ups.

COLD CLIMATE: Prolonged operation with water temperatures below 50°F (10°C) is not recommended. When starting the heater with water temperatures below 50°F (10°C), operate the heater continuously until higher temperatures are reached. Operating the heater for prolonged periods with pool water below 50°F (10°C) can seriously damage the heater, and is not covered by the warranty. For cold climate areas, please follow the winterizing procedures.

Raypak claims their patented Unitherm Governor will reduce the amount of condensation:

CAUTION: The patented Unitherm Governor is a thermostatic mixing valve specifically designed to maintain constant heater internal temperature between 120°F (49°C) and 135°F (57°C) despite continually changing flow rates from the filter and changing pool temperatures. This range is needed to reduce the amount of condensation from the fin tubes which will occur if the heater runs for any length of time below 100°F (38°C) water from the pool. It is also needed to inhibit scale formation in the tubes by maintaining temperatures well below accelerated scaling temperatures.

Diagnostics

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.

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.

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