Interesting Temperature Response During Stagnation

I received a call the other day from a customer that was testing the stagnation response of a bank of solar collectors.  He filled the system completely with water and then just left the pump off while the sun baked the collectors.   He was doing a form of pressure testing of the system for leaks.  Since the system is ultimately going to be a closed loop glycol system he has a generously sized expansion tank connected to the system.  He is using an immersed thermal well with the temperature sensor for measuring the temperature of the solar collector array.  What he noticed was under stagnation conditions the temperature sensor went up to about 252 degrees Fahrenheit and then would drop down in temperature about 10 degrees for a period of time before it would build up to 252 degrees again.  The system was repeating this cycle over and over again and he wondered why.

What the installer was experiencing was the result of the fluid boiling, turning to steam, and then cooling as a result.  He confirmed that his solar system was operating around 30 psi when it was doing this.  You can see from the charts below that the boiling point of water at 30 psi is roughly 250 degrees.  This matters because any closed loop glycol system can exhibit the same behavior.

When the fluid in your system is undergoing this phase change it will experience significant amount of pulsing as the pressure spikes from the creation of steam.  During this pulsing you don’t want your pump running.  If the pump is running you end up rattling the pipes by continuously pumping cold fluid into the collector to experience boiling.  You also can damage the pump by having it operate when the shock waves of the fluid phase change are propagating through the system.  The issue comes in when you use the standard turn off point of a solar controller.  The different manufacturers of solar differential controls have different safety turn off points.  Steca uses 265 degrees as the point after which the pump will shut down.  Other manufacturers use 249 degrees.  All of these settings are adjustable.  To prevent customers complaining about the rattling of their pipes and to prevent damage to your installed equipment make sure that the turn off point for your differential control is set to keep the system off during these times.  I would recommend you set your turn off point for the system at 249 to be safe.

 

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.

Tank Rusts Out

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.

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.

How to Freeze a Brazed Plate Heat Exchanger

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.