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.