My system: Open loop,2 heat pumps, separate shallow well jet pumps(AY McDonald 8551)for each hp, each heat pump and jet pump has its own electrical circuit(4). Check valves in pump suction lines to prevent loss of prime. Not ideal system but it has been installed over 40 years and works..until now. I can run either system alone just fine. But, when I try to run them simultaneously pump 1 shuts down, tries to start, shuts down, tries to start and never runs. This happens whether pump 1 was already running or whether pump 1 is the second pump trying to start. Voltage is fine at 120v. A plumber tried to tell me perhaps there is not enough water to run both but low pressure should not shut down the pump and cycle it. One other tidbit. When we first moved in 20 years ago, these ran simultaneously. I replaced pump/motor 1 4 years ago and the units have never been run simultaneously since, until I tried today. Any thoughts? Larry

Your well may now have insufficient yield and the pump has a protection circuit that detects running dry (which is not the same as low pressure) or overheating. Does the problem occur if you partially throttle both pumps so that they aren't removing much water?

I do not have the ability to throttle, no valves in the system. I am not familiar with a pump protective device. What would that be and where would it be located. I have taken apart and rebuilt an identical pump. There is not much to it, only the mechanicals except the Square D box with the contactors and the pressure switch. But the pressure switch will not shutoff the pump on low pressure, only high pressure. Larry

You can check voltage right at the pump motor to verify if something external (pressure switch, protection device) is turning off power. Does the pump seem hot to the point that a thermal switch would turn it off?

Voltage is fine at both pumps, 120v. The pump is not hot, in fact it happened this morning when the pump had very little run time on it. Moreover the pump has been running just fine, as required by the heat pump, all day. It is hot here but the casing was about 95-100 degrees while running, way too low for a thermal cutoff switch. Besides, if it were the thermal it would have cut off with more run time today. No the cutoff only happens when the other pump is already running or when the other pump starts. But pump 1 tries to restart as soon as it cuts off. I am thinking of disconnecting the pressure switch entirely to see if that resolves the problem.
Larry

I'd seen a lack of a closely attached pressure tank cause a pressure switch to cycle rapidly. Pump turns on, output is slightly restricted, pressure immediately rises, pressure switch turns it off. Was easy to see with the pressure switch opened up. The solution was more hysteresis (by adjusting the screws). But if the output systems of the two pumps are completely isolated, then the link must be on the input side.

I just solved the problem by tightening down the cutoff pressure screw on the Square D. I suppose I could have just bypassed it. Something about the parallel suctions was causing a pressure spike on pump 1.
Larry