Brigadoon - An Occasional Corner on the Internet
Houseboat Hot Water System
Designed by Mark Little
Hot Water Pump Current Monitor
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To detect when the pump is working, a current sensor will be used to monitor the current flowing through the pump, using a Hall Effect monitor. More information about this sort of monitor can be found here.

To determine what model of current monitor is required, it is necessary to know how much current the pump motor will draw. The pumps used on the houseboat are ShurFlo 2088 model pumps and the data sheet indicates that they can draw a maximum of 7.5A.
Assuming the lowest running current is about 5A (to be confirmed), the voltage difference is only about 140 millivolts. This can partially over come by using the 20A model, but the voltage change between the pump off and on will still only be a couple of hundred millivolts, and offset by a couple of volts.

The readily (and cheaply) available current sensors come in three models: 5A, 20A, 30A. The 5A model does not have enough current capacity, and, as indicated from the data provided in the link above, the voltage difference between the 0A and 7.5A for the 30A model is less than a couple of hundred millivolts.