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.

Flow Rate and Piping Size for a Solar Hot Water System

The flow rate and piping size are important considerations when designing and installing a solar hot water system.

The flow rate, measured in feet per second (fps), is generally recommended to be between 2 fps to 5 fps for a solar hot water system. If the flow rate is at the high end of this range, the heat exchanger will be more efficient and less scale will be created in the heat exchanger. A flow rate of greater than 2 fps is needed to entrain air through the piping. This is critical in a glycol system since a glycol solar water heating system will use some form of air elimination.  In order to make effective use of the air elimination feature in the system the air needs to be carried to the device that will capture and release the air.  If the flow rate is over 5 fps, excessive flow noise may be detected.  When you get beyond 8 fps erosion corrosion may be produced inside the piping as well as noise.  This internal corrosion of the pipes will ultimately lead to the system springing a leak.

Where solar collector manufacturers certify their product at a given flow rate solar collectors will operate well over a wide range of flows.  If you understand the trade-offs between; 1) entraining air, 2) noisy/corrosive flow, and 3) pump energy you will be able to select the appropriate line size.  The smaller the line the greater the pressure drop at a given flow rate.  The smaller the line the lower the cost for the line set as well as the insulation.  For most residential solar hot water systems, the inside diameter piping size should be between 1/2 inch to 1 inch. In addition to flow rate, piping size should also be determined by the length of piping needed, the type of pump used, the capacity of the collectors and whether the system is an open or closed loop.  As a general rule the following is the maximum flow rate you should plan on for different size copper pipe

Pipe Diameter

Max Flow rate (gpm)

3/8”

2

½”

3

¾”

6

1”

12

1 ¼”

18

Generally, in designing a solar hot water system, using a larger pipe size will give you lower pressure drop.  The lower pressure drop will result in less pump required to overcome the pipe resistance.  This may (or may not) result in lower energy consumption for the pump.   However, using the minimum pipe size will be the most cost effective.

Flow Rates of Solar Collectors

The efficiency of solar collectors is dependent on several features in a solar hot water system. In addition to the amount of solar radiation absorbed by the solar collector and the temperature of the ambient air, the efficiency is also determined by the flow rates of the heat transfer fluid in the solar hot water system.

At it’s most basic level any solar collector is nothing more than an air to liquid heat exchanger.  The sun provides the heating on the outside of the collector (air side) and the fluid flowing through the collector picks up the heat as it passes through.  In any air to water heat exchanger the amount of heat that is transferred over time increases as the flow rate increases. Along the same lines, any heat exchanger that the solar fluid passes through (either in a tank or external) increases it’s rate of heat transfer with higher flow as well.  So, higher flow rates increase both the amount of heat that is extracted from the collectors as well as the amount of heat that is passed into storage.  In general, the efficiency of solar water heating systems improve as flow rates increase. The reason all systems aren’t pumped at the maximum flow rate is because as the flow rate increases the pumping power required generally increases as well.  At a certain point the increased efficiency you achieve through higher flow rates is offset by the greater pumping power.

While we get the question all the time “what is the right flow rate for this collector?”  The real answer is hidden in the details.  We do not like to see systems that are pumped at a fluid velocity beyond what the piping can support (see previous blog).  That being said adding a flow meter to a system so you can make sure it matches exactly the “recommended” flow is counter productive.  Flow meters to confirm flow make sense.  Flow meters to control flow don’t.  Pumps come in a finite number of sizes and the best answer is to choose a pump that matches your system design.  When in doubt choose a larger pump (but not beyond the flow limits of the pipe) and the little you pay extra in pump energy will more than be made up in system output.