> > where do you want this to be triggered?
> I was thinking that, aside from inrush current, the pump will draw say 5
> amps when all is well. If there's a blockage somewhere, or the pump is
> starting to fail, then the draw might go up, say to 8 or 9 amps. This might
> be a good way to spot a problem early.

The other problem that comes up is if you use the pump controller. I still don't think I plan to install two of them, one for each pump - I still plan to consider the second pump a "boost / emergency secondary" device. But what if the pump controller slows the device down? What if the pump controller does this with pulse width modulation (PWM), "pulsing" the motor instead of reducing voltage to control its speed (this is quite common)? All of these things will cause problems with sensing flow via current.

I think the best answer is as others have responded, a real flow meter.

By the way, for whoever posted the original comment, there IS a good reason not to run two pumps at the same time all the time. The difference, statistically speaking, is that it is possible to have a simultaneous failure, although the chances for this are slim. But external events (surging alternator maybe?) could have some impact too, and having the second pump shut off could conceivably help isolate it from faults. Taking the surging alternator case, hypothetically, and assuming a crobwar OV module wasn't installed or is malfunctioning, one could still disable the alternator and then turn on the second pump even if the first was damaged. I prefer the isolation this provides, at the expense of additional pilot workload during an emergency.

Regards,
Chad