Water circulator flow vs. Amps used?

Question

If I add more restriction (system head) to a water circulator pump, the flow goes down. Do the Amps drawn by the pump go up or down? I have seen conflicting charts on this. Does a deep well pump act differently than a circulator (Taco 007)? Why would it? Thanks BBP

Answer 1

Amps stay about the same (on alternating current) on circ pumps and fans as back pressure / delta P / output head increases. What does happen is the phase difference between volts and amps shifts such that more power is consumed. (In AC you have to multiple vectors to get power consumed). You have to size the wires for amps, but you pay for watts (power).

Deep well pumps (Grundfos, Flint &Walling, etc) have multiple stages - little, 1/2" thick impellors stacked up and rotated on a common shaft. There can be 6 to 20 stages, depending on the specs. Some produce 100 psi, some produce 300 psi or more.

Circ pumps (Grundfos, Taco, etc) usually have only one impellor - they are designed for lower delta P. They are also designed for some input pressure. Usually 15 psig system pressure is recommended (although you can get away with a little less). If you don't give it enough input head, the pump will take a really long time to prime (totally fill with fluid) and cavitation can erode the impellor over time. (Cavitation is when the impellor moving away from the liquid causes the liquid to vaporize/evaporate into a gas. A mixture of gas and liquid is much more wearing on the metal than all liquid. Circ pumps in heating systems, especially, are at risk because of the higher vapor pressure at elevated temperatures.

In contrast, a deep-well pump self-primes about instantly even in a few feet of water. It is not great for them to dewater often (mixture of air/water), and there are "pump protectors" to shut them down if they do (Coyote is one brand).

One analogy would to piston air compressors. A circ pump is like a low-pressure (low compression ratio) single-piston air pump. A deep-well pump is like a multiple-stage, multi-piston pump (like a SCUBA compressor that makes 3000 psi).

Answer 2

Fluid horsepower is calculated as Flow x pressure / 1714. If you boost pressure and drop flow, you can get constant, increased or decreased power consumption - it depends on the pump characteristics. Amps (power) follow the horsepower needed to move the water.

The pump curve is used to determine the exact response, and each pump is unique. This is what you need to check for this specific pump you are troubleshooting. It will have a horsepower curve on it along with an NPSH, Head, and Flow line.

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Answer 3

Taco 007 Pump Curve