Rust and Hole in Heat Exchanger Envelope

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RyanDonovan

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Dec 9, 2015
2
Houston, TX
I purchased a pre-owned home in early 2014 that came with a pool and spa, including a Mastertemp 400 heater. I recently opened up the heater for the first time and found considerable rusting on the "tank" that contains the heat exchanger near the exhaust pipe and on the exhaust pipe itself.

Picture:
<a href='http://postimg.org/image/rinx48juj/' target='_blank'><img src='http://s1.postimg.org/rinx48juj/IMG_20151125_101851_1_Copy.jpg' border='0' alt="IMG 20151125 101851 (1) Copy" /></a>
View image: IMG 20151125 101828

I took the following actions:



  • sand off as much rust as I could with steel wool
  • apply "Ospho" to the rusted areas
  • patch hole with JB Weld

I've run the heater a bit now and the patch seems to be holding.

I suspect that some of the rusting may have been caused or accelerated by the hole allowing exhaust to vent into the body of the heater, but that doesn't explain why there was a hole in the first place. Does anyone know what may have caused this hole and/or rust? Any recommendations for other immediate maintenance? Any recommendations for ongoing maintenance to extend the life of the heater?

I have an in-ground chlorine pool.

Thank you!
 
Welcome to TFP! :wave:

Since the rust is on the outside of the pipe (see if you can look inside the pipe to see if the rusting is less, the same, or more), I suspect it is caused by the external environment. One possible cause is chemicals are stored in the same room as the heater, such chemicals being either chlorine or acid, but particularly acid. Fumes from Muriatic Acid will rust metal nearby.

The other possible cause is corrosion from condensation, but that corrosion occurs from inside the pipe. The exhaust has water vapor in it and as long as the exhaust temperature is high enough it pushes this water out of the exhaust pipe as a gas (i.e. it heats the pipe enough so that water will not condense during its operation. However, if the heater is cycled too quickly or if the heater is "too" efficient, then water vapor can condense on the relatively cool pipe surface in the interior of the pipe and can accelerate rusting of the pipe. It's the ironic situation that you don't want the heater to be so efficient to not produce hot exhaust gas. Very high efficiency heaters (this one is not) need special exhaust materials or systems to prevent condensation from being an issue.
 
The biggest cause of the rust you see is trying to heat water that is too cold. What, you say!!! I thought that was the purpose of these things.

The problem is physics. Water that is too cold causes the flue gas to be too cold. This in turn causes all of the moisture (from the hydrogen that is released and combined with the oxygen present in the air during the combustion process) to condensate out inside the burner/heat exchanger area. Once the flue gas temp exceeds ~150 deg f the water stays in the form of vapor and is carried out the chimney. This is also the number one cause of sooted up heat exchangers. The incoming water temp needs to be at least 55 deg in most cases for this condensation to not occur. Ambient air temp and water flow can also contribute to this
 
danpik said:
The biggest cause of the rust you see is trying to heat water that is too cold. What, you say!!! I thought that was the purpose of these things.

The problem is physics. Water that is too cold causes the flue gas to be too cold. This in turn causes all of the moisture (from the hydrogen that is released and combined with the oxygen present in the air during the combustion process) to condensate out inside the burner/heat exchanger area. Once the flue gas temp exceeds ~150 deg f the water stays in the form of vapor and is carried out the chimney. This is also the number one cause of sooted up heat exchangers. The incoming water temp needs to be at least 55 deg in most cases for this condensation to not occur. Ambient air temp and water flow can also contribute to this
Click to expand...

This would cause corrosion from inside the flue pipe which is why I described that as a possibility but not likely given that it looks like the corrosion he is seeing seems to be outside all piping. Would the process you describe have corrosion on the outside or only on the inside? You are right that if the incoming water temp is too cold then the flue gasses will be too cool and make condensation more likely, but corrosion should be from the inside out in that case.
 
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