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.

Purging your Collector Loop

We have come across a couple of cases lately where we have had the chance to dissect a system that has been installed for a number of years.  Both systems were drainback solar water heating systems.  One was installed with a solar pumping station and the other was a system that was erected on site by the solar installer.  When both systems were inspected, we discovered copious amounts of residue in the system.  In the drainback solar system that used a pump station we found the residue below the clean out screen on the pump station (where it should be located).  On the system that was erected on site we found the residue in the collector.  In the latter case, we found about ½ cup of silt in the collector.  See the attached pictures to get a sense of the volume of material that was caught in the screen of the pump station.

The question that follows is; 1) how did it get in there, 2) does it matter, and 3) how to get rid of it if you don’t want it.  I don’t know that ultimately the first question really matters.  It is good practice to cap any pipes you might have to keep dirt and bugs from getting in while you are placing your pipes.  The residue could also come from solder bubbles, flux residue, copper oxides from soldering, Teflon tape or whatever.  In this particular case, we suspect a portion of the residue came from the installer using a standard store bought tank for the drainback reservoir.  This means that the anode rod in the store bought tank plus any corrosion of the steel fittings could have ended up in the screen.  Getting residue in the system matters because ultimately the residue wears down the components in the system including pumps, seals, pipes, etc..  The residue serves as an abrasive in the solar fluid eroding anything that it comes in contact with.  The residue in the system will cause premature failure of pumps, heat exchangers, and valves.

How to eliminate the residue is the ultimate question.  Any solar pumping station you use, or any system you piece together on the job, should have the ports necessary to run a cleaning solution through the collector loop.  If you are using a pump station insist that the pump station have screens and clean out ports with hose bibb connections.  Whether you run water, TSP, or water/glycol mixture through the system before commissioning make sure you connect your lines to the system in such a way to flush all of the residue from the system before attempting to charge the system.

Venting Pumps

The most common pump in the hydronic market today is the wet rotor circulating pump.  The pump also goes by the name circ pump.  They are made by various manufacturers including Armstrong, Grundfos, Wilo, Taco, and Bell and Gossett.  The pumps are common and fairly inexpensive.  Even though they are relatively inexpensive having to return to the installation to replace one within a few years can drive up the cost of the pump far beyond the initial purchase price.

The basic design principle behind a wet rotor pump is that the rotor and shaft are enclosed in a stainless steel can.  The fluid that is pumped has access to the inside of the can including the rotor, shaft, front and rear bearings.  Different manufacturers accomplish this different ways but primarily through holes in the face of the can and by the clearance around the shaft.  By allowing the pumped fluid access to this area the fluid acts as both a lubricant as well as a coolant for the heat generated while the pump spins  (see Figure 1).  The problem arises when fluid doesn’t displace all of the air that is initially occupying the empty space in the can.  If air is left in the can then you end up running the pump with insufficient lubricant and insufficient coolant.  The double whammy leads to premature wear on the bearings as well as superheated fluid/steam in the can.  If the fluid that you are pumping has any minerals in it they will come out of solution under the extreme heat and end up precipitating on the bearing which in turn leads the bearing and shaft to lock up.  This problem is exacerbated in solar heating applications because of the naturally high temperatures that are present.  A drainback solar heating system would be the worst case scenario for this type of premature pump failure since you are frequently dealing with unpressurized loops.  When a pump is installed and the fluid is under pressure the higher pressure of the fluid compresses the air in the can significantly during start up.  This fluid being forced into the can helps to flush out the residual air in the can when the impeller starts to spin.  If you don’t have any pressure (or very little pressure in a drainback situation) you don’t enjoy the benefit of forcing water into the pump.

Figure 1:

There are several steps that can be taken during the original installation of the system to minimize or eliminate this problem.  They are:

  1. Pressurize the pumps during the charging/priming of the system.  This includes “non-pressurized” drainback systems.  As the installer you should still prime the system to 15 psi of air pressure minimum  to insure that you get as much water into the can as possible initially.
  2. Using demineralized water when you charge a drainback solar  system.  While you can use tap water, you want to make sure that you don’t have high mineral content in your water that can cause you problems.
  3. Once you have pressurized the system be sure to vent your pumps to flush out any residual air.  This serves to insure that the pump can is fully primed.
  4. Keep the high limit temperature on your solar system lower than 150 degrees.  Historically solar heating people have tried to get the tank as hot as possible.  While this is good for putting BTUs into the tank, this approach puts the system at greater risk of failure from hard water in the pumps.  The European solar differential controls come with a pre-set high limit of either 140 or 149.  Except under extreme circumstances I don’t see a reason to make it any higher.

Next time you install a solar water heating system be sure to take these simple steps to give yourself and your customer the most robust system possible.

Tilting of a Collector

One of the issues that we see come up from time to time is when people need or want to install a flat plate collector flat on a flat or very low pitch roof. While this is easier to install it brings with it it’s own set of problems.

Most flat plate collectors on the market today have some gasket or seal around the glass. This gasket or seal has on top of it a cap rail or a portion of the extrusion. This build up of material on top of the collector (gasket and top rail) creates a lip that will hold a puddle of water. Pretty much every collector on the market has a sealing system that isn’t designed to keep water out when it is fully submerged in water for a period of time. What this translates into is more than a normal amount of fluid entering the collector box.

Flat plate collectors should have ventilation holes that will enable the collector to breath and exhale any excess moisture that gets into the box as a result of humid conditions. The ventilation holes rarely can accommodate the extra moisture that would end up in the collector as a result of the collector sitting flat on its back in a rainy environment. If too much moisture gets into the collector box then the collector will have a tendency to sweat on the inside as the moisture evaporates and then condenses on the glass. Over time this will leave dirt and contaminants on the glass making the glass dingy and difficult to see.

For those of you that think this spells an advantage for evacuated tubes you would be mistaken. Evacuated tubes that use heat pipes require the collectors to be tilted up at 35 degrees in order to allow the heat to move successfully from the bottom of the tube to the heat exchanger at the top.

The long and the short of it is. Tilt your collectors if you want them to last and not have crud build up on the inside that could hamper future performance. We recommend a minimum of 8 degree tilt.