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Hot-Tub Pump Teardown & Lessons Learned

My neighbors decided to get rid of a small hot tub, so I put in my request for the pump and motor. I don’t like to waste a decent AC motor that might find use in my workshop. The pump arrived in a plastic-foam enclosure that included a control panel and an AC power cord. I pulled apart the enclosure and found a set of pipes and electronics. An aluminum box that contains the electronics came mounted on the motor with a very long worm-gear-type pipe clamp.

I was surprised to find water pipe wrapped around the motor, but according to the tub company’s Web site, the motor heats the tub’s water. That’s a neat idea. I figured the tubes might help cool the motor, but never thought the motor would create enough energy to heat the water to 104 degrees F. (Tub regulations set that as the upper temperature limit.) The pump outlet has two sensors that probably measure temperature and provide redundancy in case one fails.

It looks like the panel controls the water jets, temperature, and a light, and it has some sort of filter-monitor, too. A pointer on the control panel adjusts an inlet that injects varying amounts of air into the water stream. I didn’t get the air-injection part of the pump and didn’t get (or want) the filter assembly.

But how does the controller adjust the temperature? (The manufacturer doesn’t provide a schematic diagram or service information on its Web site.) Does the controller simply vary the speed or on-off cycle time of the motor and thus the amount of heat the motor produces, or does it somehow adjust the power to the pump motor to make the motor “overheat” and temporarily act more like a heater than a motor? I plugged in the electronics and the motor ran well at a constant speed. The motor’s innards look like those in a standard AC motor, so there’s nothing unusual about the motor itself.


Here’s the electronics “package.” (The backside looks similar, but without any pass-through wires.)  I’m surprised it conforms with the electrical code. It has silicone sealant slopped around the edges and commercial-style press-in strain reliefs for wires. One wire has electrical tape wound around it, probably to make a “seal” with an over-size grommet or too-small wire. I doubt the electronics box came from the factory in this “siliconed” condition.

Here’s a photo of the too-small wire fed through a strain relief:


And a photo of the opened strain relief and the wire wrapped with electrical tape:


Because someone used so much silicone material to partially seal the electronics package it took two X-Acto-knife blades to cut through to the package seams and a liberal use of my universal removal tool (hammer) to free the slide-out section so I could see the PCB. The circuit uses a two relays, one for motor control and the other for an accessory(?). So, nothing fancy as far as motor control goes–just on or off. A small-signal transistor, driven by an optical coupler, switches 12V AC from an off-board transformer to lights that illuminate the tub. (This signal might control a relay that turns the lights on and off rather than supply power to the lights itself.)


A second transformer attached to the PCB provided power for the on-board components that include a PIC16C73B MCU.

The back of the PCB shows signs of water depositing minerals and the burned areas indicate someone replaced three electrolytic capacitors in the power-supply section. They also used silicone on the caps. Several SMT transistors appear hand soldered, too.


Lessons learned:

1. In a water-pump environment, use an electrical box with the proper NEMA rating and properly-rated fittings. The pump-control box used press-in sealed fittings for mounting screws on the bottom (good), but the plastic PCB standoff holes could still admit moisture (bad).

2. Don’t apply a sealant that makes servicing difficult. Use an enclosure with an integral seal.

3. Match wire size to fitting or grommet characteristics.

4. Think about a conformal coating for circuit boards in a high-humidity environment.

I doubt I’ll use this motor after all. Some type of adhesive holds the stainless-steel tubing to the motor. I don’t want to make a mess unrolling the tubing and leaving it on could present a mounting-attachment challenge. On second thought… But for now, I’ll remove the pump and electronics and stick the motor in a corner. –Jon Titus

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