ASK THE EXPERTS: Clogged Solar Water Heating Collector?


My technician inspected an antifreeze-based solar water heating system about nine months ago in which the heat-transfer fluid (HTF) wasn’t circulating. He measured the temperature at the collector, which was 180°F. The pump and control were located in a pump station and appeared to be OK. The pressure gauge indicated 10 psi, indicating a loss of some fluid. When he tried to pump HTF through the collector with his charge pump there was no circulation. This pump can develop 50 psi of pressure, which has always been sufficient to blow all the air out of the system, so I doubt that it is the problem. Can the antifreeze in a system get “cooked,” clogging the collectors’ piping?

Phil Parkinson • Austin, Texas

I have never seen a glycol solution (even with a decade or so of no flow through a collector) become so viscous that it stops flowing altogether. I have seen solutions that are dark brown, or black and gunky with a viscosity similar to old engine oil and with an accompanying burnt smell, but the solution still flowed enough to drain—though it’s a nasty job changing fluid like that!

I know you have confidence in your technician, but we can all make mistakes. I had a really great service guy (who had more than a dozen years of experience) once put in a pump backward on a large system. It took awhile to figure that one out since I thought he was too good a hand to do something like that.

My first suspicion would be that the charge and return hoses were connected to the wrong fittings in the pump station. I have done this myself a few times in my career. The result is no flow, since, no matter how large the pump, it is pumping against a check valve.

I would suggest checking the system again. First, ensure that the charge and return lines are correctly connected—only then should a technician try to flow the HTF solution through the collector. If there is no flow, they should check the valves—in particular gate or check valves—for being closed or not functioning. I’ve seen gate valves with the stem broken, and check valves stuck open or closed depending on whether it is a spring or swing type. Although it is a rarity, a check valve could be stuck closed by a really gunky antifreeze solution, but a large charge pump should open it.

The oddest clog I ever encountered was a silicone plug in a header pipe. It was left in at the factory, but not visible on installation. If you’ve exhausted all the other possibilities, look for this. Over many years, the plug had worked down the one-inch header tube on the collector to block the outlet or return connection.

It’s tough to troubleshoot a problem like this remotely, and all of these no-flow conditions can be tough to diagnose if you’ve eliminated the pump as the cause. Good luck!

Chuck Marken • Home Power solar thermal editor

Comments (2)

Mark Norman's picture

In the UK we have vacuum tubes a lot that stagnate at 200 C and above. The German antifreeze and all the rest can't take the heat. Turns to tar then rock slats. I have cut 22mm copper pipes that are full and it's like cutting solid copper. Can clog up the solar pump stations. There are solar cleaners, but they don't really work. You have to look for the rock in any brass valve and outlets to the solar panel. Big problem. But when you get it going again good feeling. Good luck.'s picture

If the system is old and has a cast iron air scoop, the problem could be in there.


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