Single wall –vs- double wall heat exchangers

As a regular part of our business we interact with contractors, engineers, architects and solar distributors from all around the United States.  It is not uncommon for us to run into one of these customers that indicates that they are having a problem with a code official in their jurisdiction.  When we hear about this it is generally the result of the contractor and the inspector not having a good rapport.  Frequently these issues don’t rise to the level of being of concern.  Occasionally though, we get reports back where either the inspector or the customer are insistent that a certain standard be maintained that from our perspective unnecessarily adds to the cost or complexity of the job.  One of these issues that pop up from time to time is the requirement (or perceived requirement) for a double wall heat exchanger.

The difference between a single wall heat exchanger and a double wall heat exchanger is the double wall heat exchanger would require the failure of multiple barriers in order for the solar fluid to contaminate the potable fluid.  A single wall heat exchanger can allow contamination with the failure of a single barrier.  These barriers are generally metal (can be copper, steel or stainless steel) and are what keep the solar (or hydronic) fluid separate from the potable fluid.

Our problem with the customer trying to force a double wall heat exchanger into a solar application is multi-fold:

  1. We are trying to achieve cost effective deployment of clean, renewable energy.  Double wall heat exchangers significantly add to the cost of a system.
  2. We are trying to maximize the amount of free energy the customer ultimately receives from their solar system.  Double wall heat exchangers significantly degrade the performance of the system because the gap between the two walls acts as an insulator between the hot solar fluid and the cold potable fluid we are trying to heat thereby reducing our ability to move heat from the solar loop to the potable storage.
  3. Neither of the two most universally accepted plumbing/solar codes in the U.S. (IAPMO and ICC) require double wall heat exchangers.

If there isn’t any requirement for it, if it costs more, and if it performs worse why do we keep seeing this issue crop up over and over again.  I suspect that the reason is because the people that say it is required aren’t familiar with the IAPMO or ICC code.  If the codes were copyrighted I would post it on the blog and be done with it.  Suffice it to say that they both mirror each other.  If in a solar system you are using propylene glycol (the only fluid you should ever use in a pressurized system) then the requirements boil down to this:

  • Label your system so nobody goes and puts a toxic fluid in the system 10 years from now.
  • Use an appropriately rated pressure relief valve on your system.  Some manufacturers like to use the highest rated pressure relief valve possible so they can claim some level of moral superiority over their competition.  You should limit the pressure in your solar heating system to a normal pressure that is below the standard operating pressure of your water heater.  That means using a pressure relief valve that matches your water pressure in your area.  Typically, we see PRVs being used that are either 75 psi or as low as 30 psi for low water pressure applications.

As you can see the requirements for using a single wall heat exchanger are pretty common sense and not too onerous.  So, the next time you hear someone say that a double wall heat exchanger is required challenge them on it.

We want to see efficient, cost effective solar water heating systems installed and adding useless requirements isn’t the way to achieve that.

The Seasons of Solar Thermal Problems – Summer

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.