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.

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