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Jon Titus

Hot-Tub Pump Teardown & Lessons Learned

October 27, 2009

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

Posted by Jon Titus on October 27, 2009 | Comments (16)

November 11, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
G711 commented:

John,
You may not want to use that motor in "the shop". It's possible that it was wound particularly inefficient on purpose so that it would also function as a heater.
But how does the temperature regulation work? Cycling the pump doesn’t make sense – don’t people in the tub want the water to circulate all the time? Or is that the tradeoff you make with a cheaper tub? Could it be done by varying the air injection?
Or, here’s a question for motor designers: is it possible to switch something in and out of the motor circuit, maybe a phasing capacitor, such that it that would vary the efficiency, and therefore the heat generated, of the motor?
Perhaps a search of patents assigned to the tub manufacturer would yield an answer.

November 11, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
Barryfish commented:

It's also possible that the silicone on the capacitors is for mechanical reasons. If you don't support tall capacitors in a vibration prone environment the solder at the PCB will eventually crack.

November 11, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
David Monnier commented:

The motor is so inefficient that it produces enough heat to get the tub hot and keep it there. The company that makes the tub sells a replacement motor for big bucks. Put it on E-bay and you could get upwards of $500 for it.

November 11, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
iescontrols commented:

Two 1.5HP induction motors on a 1,200 gallon tank will raise the temperature 20°C in a day. During testing we come in on weekends to replace the evaporated water. This is impeller friction alone. The pumps are in free-space

November 11, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
William Ketel commented:

This is interesting, of course. The most likely source of the heavy silicone sealer is a dealer repair person. A suitable NEMA rated enclosure would make the price non-competitive, which is why you will never see it in consumer-targeted products. And as for pumping fluid to heat it, I did that on a very successful test stand years ago. It heated oil to 350 degrees F and then I was able to hold the temperature within about 1 degree by either relieving the pressure or running it over a relief valve. I think that the heating was about 1 degree per minute per gallon per minute per 1000PSI drop. That part was cesigned for me by a hydraulics wizard named UMA. With a 7HP motor we would get about 15 gallons of oil up to 350 in less than an hour. And we never had any carbon buildup on the system, because it distributed the heat so well.
BUT in the hot tub the coil is just there to cool the motor.

November 10, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
waternwaves commented:

all of the work done by the system will eventually translate to heat, add to that motor losses, and a 2 hp motor can keep a well insulated tub fairly warm.
It is not the most efficient way to do it....however it is possible.
most 110 V spas only have a 1200-1500 watt element and this is sufficient to heat a spa.
1000 watts of system heat in a well insulated tub would suffice. Tho I would guess there was a contactor available for an auxiliary element somewhere in there. Was there contacts for a small aux circulation pump? most of the times the small trace heater is paralleled with the circ pump.

November 10, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
pipedown commented:

If nothing else, the recycling value of the motor (specifically the Cu windings) is worth more than most junk we engineers salvage.

November 10, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
AV8R commented:

Some inexperienced service person did the repairs last time, the use of silicone tells you. In an enclosed environment the acetic acid in the silicone will cause corrosion to copper (wires, circuit boards) making more problems in the future.

November 10, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
Tim L commented:

How many amps is the motor rated for? I don't believe a typical hot tub pump motor can generate anywhere near enough heat to replace the heat that's lost by the water in the hot tub in most parts of the country.

November 9, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
DO commented:

I saw a hot tub at a show one time that was heavily insulated (3-6 in foam). They claimed the motor produced enough heat to keep the tub heated. I believe the cover was thick insulation too.

November 9, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
Fixedafew commented:

The fact that the manufacture states that the motor heats the water doesn’t necessarily mean that this is the only means of heating the hot tub. I suspect that the this system also incorporates a standard hot tub heater and that harvesting of heat from the motor may have a lot more to do with marketing than anything else.

November 9, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
MikeW commented:

Jon,
How big was that hot tub? The thermodynamics of a hot tub will require a separate heater to actually heat the water, typically electric resistance or gas fired. I suspect one of the relay outputs drives the main heater supply.
Running part of the water flow through the tube would alleviate the need for a separate mounted temperature sensor so this is probably a cost savings feature for the manufacturer. My tub has a separate RTD in a 1/4" stainless steel sheath that is submerged into the side of the tub. This design definitely had to cost more that an integrated sensor you have.
I agree water and chemicals are rough on the equipment - I usually loose a main pump shaft seal about every 3 -4 year. A $7 seal fails and it then shoots water into the pump motor which typically ruins the 2 HP motor.

November 9, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
Tsunami Spas commented:

I'm betting this is a VITA spa? Maybe a Duet model? All the white silicone on the control box is someones attempt to keep water out of the controller. That water is coming from a poorly sealed topside control mounted on the outside surface of the spa directly over the controller.
The stainless tubing captures waste heat from the motor and heats the water with it. The sensors you refer to are a thermostat sensor and a high-limit or overtemp sensor.
I'd like that motor if you aren't going to use it :) Contact me at 360-896-0070.
Thanks in advance.
John
Tsunami Spas
Vancouver, WA

November 5, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
Jon Titus commented:

Looks like two temp sensors connect to ADC inputs on the Microchip MCU. The control panel let people set the temperature and the MCU controls it by cycling the pump. I didn't have a 55-gallon barrel to test the pump with water and I didn't see it in use in the neighbors hot tub--they never installed after they moved to our street.

October 30, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
Stretchy commented:

Good info passed along!
Did you ever figure out how the hot tub temperature is regulated?

October 29, 2009

In response to: Hot-Tub Pump Teardown & Lessons Learned
WireMan commented:

Mr. Titus, you do cool stuff. This was a neat project because instead of just writing about taking something apart, you gave some important ideas for product improvements. Good job.

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