Putting a Smart Switch on my Hot Tub heater

[wayner]

Why do I want to do this? I live in Toronto and I have a Jacuzzi J345 tub that is from about 2006 and it does not have any sort of timer or scheduler functionality. My electricity costs vary a lot by time of day, so I want to control when my tub’s heater is running. Overnight my costs are 2.4c/kWh. From 4-9pm the cost is 24c/kWh – 10X higher. I chose this rate plan because I have an EV and it makes it cheaper for me to charge my car, but it also lets me think about time-shifting other electrical power consumption. So, I want to heat my tub when the electricity is cheap. (Note that the costs quoted exclude indirect electricity costs like distribution and transmission fees but I am more considered about the differential between the different times and those fees are constant throughout the day).

What did I do? I put a Sonoff POWR3 smart switch between the power leads and the heater power input. Smart switch means that the tub connects via wifi to my LAN so it can be controlled by apps, an API, a home automation system, etc. So, when the switch is off the heater does not get any power. But the rest of the tub is getting power so the pumps will still run, and the control unit works. If the water temperature calls for the heater to be on the tub’s control unit thinks that it is heating the tub, but it really isn’t.

This smart switch has a power rating of 5500W which is the same as the power output of my heater – so that is cutting it close. This switch can be controlled by the eWeLink app on my phone and it can handle 240V which is what the heater uses. Having a smart switch allows you to set up sophisticated scheduling, where you can turn it on or off multiple times per day and have different schedules for different days of the week. The smart switch also monitors power consumption so you can see how much energy the heater is using. I am planning to flash the firmware of this switch to Tasmota which is an open-source firmware which will allow me to have more direct local control of the switch, and to monitor the power consumption by storing it in a database of text file. I will also then control the timer from my Control4 home automation system rather than the eWeLink app.

What exactly did I do? I disconnected the power leads that went from the power block (orange circle) to the heater power inputs on the circuit board (purple circle). I ran new 10 AWG wire from the power block up to the input of the smart switch. Then I ran two other 10 AWG wires from the output of the smart switch to the heater power input on the circuit board. I put the wires in liquidtight and used a pre-existing hole from the wiring box with a liquidtight connector to provide a waterproof seal for the wiring box.

What is left to do? I should put the switch in some sort of project box to protect it from potential water exposure. But I am worried that it might get too hot if it is in a watertight project box. I also want to put a temperature probe into the tub so that I can monitor the water temperature. Then I can keep a log of water temperature, power usage, and ambient temperature to also see how the heater’s power consumption varies with the outdoor temperature.

Aren’t I worried that the water will freeze? The main freezing worry is with the water in the pipes that go from the pumps to the heater. The tub runs the main pumps every hour or so to recirculate the water. And there is a smaller pump that continually recirculates the water. Having the main tub not be heated for 12 hours, or more, is not going to drop the water more than a few degrees and won’t significantly affect the temperature of the water in these pipes. By putting the power switch only on the heater and not on the rest of the tub system the pumps will still continue to run.

 

[kimkats]

This will be interesting to see happen and for a very good reason!

 

[Forest_Spa]

Let us know how it goes!
This is very nice in that it allows remote control, correcting the time at your phone, and usage data collection.

 

[Mdragger88]

I just wonder is it that much of a savings to require such efforts?
If the tub is at temperature & closed, maintaining that temp is not that hard & requires little energy. You can further assist this (prevent heat loss) by adding a solar cover under the regular cover.
Its been hashed out here time and again that it’s much cheaper to maintain the temp of your tub than to turn it down then back up again which is what this is essentially doing.
The only time it is more cost effective is if you will be not using the tub for an extended period. So we suggest turning it down while on vacation.
If energy consumption is an issue reducing the circulation pump frequency will help. Most people find the factory default schedule is a bit overkill.

 

[Mdragger88]

@wayner not trying to be a Debbie Downer with my previous post, just realistic. I think you’re idea is inventive.
I am quite curious to see the energy results as compared to previous years in the same months without this modification.

 

[Newdude]

+1. We are 'enthusiasts' here and do many things because we can, without necessarily having a need.

 

[Forest_Spa]

I just wonder is it that much of a savings to require such efforts?
If the tub is at temperature & closed, maintaining that temp is not that hard & requires little energy. You can further assist this (prevent heat loss) by adding a solar cover under the regular cover.
Its been hashed out here time and again that it’s much cheaper to maintain the temp of your tub than to turn it down then back up again which is what this is essentially doing.
The only time it is more cost effective is if you will be not using the tub for an extended period. So we suggest turning it down while on vacation.
If energy consumption is an issue reducing the circulation pump frequency will help. Most people find the factory default schedule is a bit overkill.

I believe the poster's intent was to shift power usage to a (much cheaper) time of day, not to save energy overall.
"Overnight my costs are 2.4c/kWh. From 4-9pm the cost is 24c/kWh – 10X higher."

 

[JoyfulNoise]

Have you checked the in-rush current of the heater when it turns on at different temperatures?

Resistive loads don’t have as much of a problem with in-rush current as reactive loads do but it’s not zero. You’re already using that switch at nearly its entire load capacity which is not good electrical practice (most would say you should never exceed 80% of the design capacity). With in-rush current, you could be hammering that switch every time the heater kicks on. Cumulatively that could lead to premature failure. A clamp on ammeter should give you a good idea of the max in-rush current (I wouldn’t trust the reading from the switch as you are using it at max load).

 

[Dirk]

Nice project. Results will be interesting, so I'm mostly just following along.

This smart switch has a power rating of 5500W which is the same as the power output of my heater – so that is cutting it close.

I would say that is too close (agreeing with Matt). That 80% rule is what I would have also quoted. A bigger margin would have been even better, to cover the hammering he's describing. I would have chosen a lesser device (cost and ratings) and then had it operate a high-wattage relay. That would have taken all the heat stress off the device, and placed it on a dumb relay, properly rated to take it. You'd lose the monitoring, but I suspect you're going to eventually lose that device anyway. Mounting it in a water tight (air tight) box will hasten its demise.

With the relay, the "smart unit" could have been indoors, safe from all the elements. It would probably be a simple matter to find a waterproof-ish relay, which wouldn't need an additional enclosure, and then it could shed its heat as designed.

2¢

 

[Dirk]

5500 Watts ÷ 240 Volts = 23 Amps. A 40 amp rating would be a good buffer.

I can't say this is the exact right part, but you can see that you can get into the ballpark for only about 16 bucks. There are 120V and 240V versions:

Amazon.com

Something like that might even fit in the steel box with all the other electronics (as long as it doesn't get too hot, which it shouldn't). A relay is only generating the heat put out by its coil, it doesn't matter how much watts is running through the contacts. I'm not sure the same is true for your Sonoff, which has all sorts of electronics in it.

 

[ajw22]

A relay is only generating the heat put out by its coil, it doesn't matter how much watts is running through the contacts.

It does matter how much amps are running through relay contacts. The relay contacts need to be rated for the amps otherwise too high amps will burn up or weld closed relay contacts.

It is the same as running amps through #10 wire versus #24 wire.

 

[Mdragger88]

I believe the poster's intent was to shift power usage to a (much cheaper) time of day, not to save energy overall.
"Overnight my costs are 2.4c/kWh. From 4-9pm the cost is 24c/kWh – 10X higher."

I realize this - when I said energy savings I meant $$ on energy which is the goal in general.
Many do this with timers/automation for their pool pumps/heaters for this purpose but not sure how much savings will be yielded with a spa heater alone.

 

[Newdude]

but not sure how much savings will be yielded with a spa heater alone

Win, lose, or. Um. Draw. (Hehe), one data point is better than no date points.

 

[JoyfulNoise]

.... no date points.

Is that like when you take your first date to The Cheesecake Factory and you're wearing sweat pants?

 

[Newdude]

Is that like when you take your first date to The Cheesecake Factory and you're wearing sweat pants?

Now that autocorrect gifted us with that chuckle, it's did point out a critical flaw in the plan. When she wants to use the hot tub and it's not at temp due to peak electricity time, there will be no date points awarded.

 

[JamesW]

Is that like when you take your first date to The Cheesecake Factory and you're wearing sweat pants?

That’s better than going to the McDonald’s Drive-Through and trying to use an expired 2 for 1 coupon for a Happy Meal.

 

[JoyfulNoise]

That’s better than going to the McDonald’s Drive-Through and trying to use an expired 2 for 1 coupon for a Happy Meal.

It just shows how frugal you are and that you appreciate a good deal … all fine characteristics in a prospective suitor … hopefully she understands that splitting the kids-sized drink with two straws is not only a romantic gesture but very cost effective as well!!

 

[Dirk]

It does matter how much amps are running through relay contacts. The relay contacts need to be rated for the amps otherwise too high amps will burn up or weld closed relay contacts.

It is the same as running amps through #10 wire versus #24 wire.

Yes, or course, that's what I meant by "properly rated to take it," and the math I showed with a recommendation of a 40 amp rating. By "it doesn't matter how much watts is running through the contacts," I was referring to the heat generated by the relay. The heat generated by a relay is determined by the watts being used by the relay's coil, not the amps running through its contacts. The OP voiced a specific concern about where to mount his Sonoff, and the heat it might generate. A properly rated relay would solve for that, because it would generate less heat than the Sonoff.

 

[ajw22]

The heat generated by a relay is determined by the watts being used by the relay's coil, not the amps running through its contacts.

Power (watts) running through switch contacts does generate heat.

That is the way circuit breakers work. Circuit breakers are temperature sensitive switches.

 

[JamesW]

The heat generated by a relay is determined by the watts being used by the relay's coil

The coil uses very little power and generates very little heat.

The coil uses less than 0.079 amps at 24 volts DC.

79 ma x 24 v = 1.896 W (watts).

The majority of the heat is generated by the current going through the contacts and wires and it depends on the resistance.

Larger wires and contacts have less resistance and generate less heat.

If the contacts get corroded or burned, the resistance goes up and the amount of heat generated also goes up.

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