Ben, Clay, a SolVelox on a tank and our customer Chris Allen made the front page of the Durham Herald last week. The article wasn’t really about solar but it Chris Allen of RTP Solar is interviewed about starting his solar installation business.

When trying to diagnose a system it is frequently important to determine whether the pumps are working in your solar heating system. The first (and least accurate) method most contractors go with is feeling the pump to see whether they are vibrating. This can be misleading because vibration from nearby mechanical equipment can cause you to assume the pump is spinning when it might not be. A very easy and accurate way to determine whether the pump is spinning is to remove the bleed screw from the top of the pump (see video). When you remove the bleed screw (not available on Taco pumps) you will see an inner slot on the end of the shaft. You can only see this when the pump is not spinning. Since the slot is directly connected to the shaft and impeller, if the slot is rotating you know the pump motor is spinning. Turn the power off and you should see the slot.

By removing the bleed screw and accessing the head of the pump you can also spin the shaft freely with a slotted screwdriver. You should be able to feel if the shaft is binding in any way.

A question we will get from time to time is “how do I know that my solar pump is running?”  There are several answers to this question so we will cover one way to know if the pump is operating properly.  When you look at it all a pump does is create a pressure differential across itself.  The pump (if working properly) will have lower pressure before the pump and higher pressure after the pump.  This pressure differential causes fluid to flow away from the higher pressure region towards the lower pressure region.  On a closed loop glycol system (or other hydronic systems) you should have the following components in addition to the pump: an expansion tank and a pressure gauge.  A properly designed system will place the expansion tank immediately prior to the inlet to the pump.  The expansion tank serves as the zero pressure change point of the hydraulic loop.  Since the expansion tank doesn’t see pressure change as flow is generated the only way for the pump to do its job (creating a pressure differential) is for the pump to create an increase in pressure on the outlet side of the pump.

This pressure increase on the outlet side of the pump can be easily observed by watching the pressure gauge when the pump is turned on and off.  Watch this video to get a better sense of what you are looking for.  The higher the head of the pump the larger the pressure spike you will see when the pump is turned on.

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.