When more pumping means less.

Problem:  A customer has a drainback system that turns on properly when the differential is achieved.  Once the system turns on the pump starts pumping and then shortly thereafter the flow can be heard dropping into the drainback tank.  Everything is working according to design.  A short while later (5 to 10 minutes) while the pump is still running the system ceases pumping over.  When the system originally started the site glass was at the top of the site glass.  After the water started falling back into the drainback tank the water level in the site glass was down about 6 inches from the full level.  After the system ceased pumping around the water level in the site glass was now 3 inches below the full level.  This situation repeated any time the system turned on.

The installer, thinking there was a problem with the pump, replaced the pump.  No change.  The installer then added another pump in series to address the problem.  The system operated identically except it “lost prime” faster than with a single pump.  What was the problem?

Answer:  The particular drainback tank that the installer was using had the return from the collectors coming straight into the top of the tank immediately above the line leaving the drainback tank going to the heat exchanger and then back to the collector.  After the pump started running the fluid coming from the collectors picked up air as it splashed in the drainback tank.  Enough of this air flowed out of the drainback tank and ultimately collected in the pump.  With a small amount of air in the pump body the pump was no longer able to generate enough lift to get the water past the highest point in the system and the prime was broken.  The solution to the problem was to reduce the flow out of the pump by partially closing the ball valve on the exit side of the pump.  By doing this the volume of flow going through the drainback tank was reduced.  This allowed enough of the air to come out of solution to prevent the pump from air locking.