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.