Over the years we have received many customer service calls about this or that solar heating system not working. As we have continued to work with the installers we have developed patterns of problems that follow share a high correlation to the season. The problems we have faced recently (no surprise) are what I will call the summer problem. You might be thinking that the summer problem is systems blowing off because they have too much heat but you would be wrong. Several years ago we figured out how to mitigate that problem and have seen very few cases of it since. No, the summer problem I am talking about today has to do with differential controllers.
A high percentage of our summer problem calls are related to differential controllers. The problem solar installers report is that their sensors or controller have stopped working. Believing the product to be faulty (this is always the first assumption) they call seeking to get a replacement for whatever they believe is defective. In pretty much every case the solar contractor is providing a temporary patch to the problem rather than actually fixing it. The actual problem is that the solar loop piping isn’t grounded. Since the solar loop piping isn’t electrically grounded whenever there is a weather event the collectors on the roof and associated piping acts as a grounding rod for all of the electrostatic energy in the air. If the piping isn’t grounded enough electrical potential will develop until the static electricity shorts across the sensor or the control to reach electrical ground. This static discharge is enough to ruin the sensor or the control. The reason I call this a summer problem is because we get this call in the summer time particularly during periods when there has been a lot of electrical storms.
It doesn’t take a lightning strike to cause your system to fail so be sure to use a ground strap and grounding rod to electrically ground your solar piping to eliminate this problem By the way, this problem can also kill solar monitoring systems as well.
We got a call the other day from a homeowner that had a contractor install a solar water heating system. The system had been installed for just over a year and had been installed with a new water heater. The water heater had rusted out and failed in that period of time. The homeowner was concerned that somehow the solar system caused the problem. After some discussion back and forth we discovered that the installing contractor had removed the anode rod from the tank in order to provide an extra port to the tank. That was a major mistake. You should never remove the anode rod from a water heater. The anode rod is made from a metal that is more reactive than the steel of the tank so it serves to react (or sacrifice) with the ions in the water to prevent the ions from attacking the steel. Without the anode rod, the ions in the water will react with the steel of the tank and prematurely rust out the tank.
If you need to add an additional port to a two port tank you have several options for doing it. First, purchase a tank with 1 or 2 extra ports. The American water heater tanks (sold as Whirlpool at Lowes) generally come with a third port in the top of the tank. Rheem, Bradford White, and AO Smith all offer special solar tanks with extra fittings in the top. Generally the “solar” labeled tanks are sold at a premium so I avoid them if possible. Finally, even if a tank has only two fittings (one for hot and one for cold) you can create an extra fitting in the tank by using the P&T port. Remove the P&T, install a nipple and tee, install an extended probe P&T (Watts 100XL-8) in the end of the tee and then the side port to the tee becomes an extra hot out from the tank. A final option that is available is to use an outlet anode rod that suspends the anode rod below the anode connection while providing you a port to remove water from the tank.
You can now have the ports that you need in a standard tank without sacrificing the longevity of the tank or your solar heating system.
In our experience brazed plate heat exchangers offer the best heat exchanger value on the market. With solar heating systems we are trying to deliver renewable energy to the customer at the best value proposition. This frequently leads to using brazed plate heat exchangers which we carry and integrate into many of our packaged solar solutions.
One of the problems possible with a brazed plate heat exchanger is it is possible to freeze damage the heat exchanger. While this is a rare occurence, it can be done. In the thousands of heat exchangers we have used and sold we have only seen it one time. See the attached picture. This failure occurred on a glycol system where the heat exchanger was in a heated mechanical room. If the heat exchanger had glycol on one side of the system and was in a heated room how could we have caused the heat exchanger to freeze?
The problem was caused when the system circulated cold glycol fluid on the solar side (at night) while the water side didn’t circulate at all. The system had a faulty check valve on the solar loop. Without the check valve working, the solar side thermo-siphoned at night when the outside temperatures were extremely cold. This super cool fluid moved through the heat exchanger causing the stagnant water on the other side of the heat exchanger to cool and freeze. When the water froze the frozen water expanded and burst the heat exchanger between the plates. The fact that it was freezing water causing the heat exchanger to burst was proven by the distortion in the stainless plates as well as the failure of the pressure relieve valve to release.
So, the moral of the story is…… Be sure that you have a well installed check valve on the solar loop to prevent thermosiphoning and the consequent failure of an external heat exchanger.