When more is less: 4KW @ 240VAC hot tub vs. 1KW @ 120VAC

[soylentgreen]

I have one of those "plug N play" hot tubs that can run on 120VAC - it heats at about 1KW.

I realized after some energy auditing that this tub is using about 150 kWh/month (roughly 100 summer, 200 winter).

As a test, I tried putting it in Eco mode, which drops the temperature from 104F to 84F. I was expecting it to use less energy, but I was shocked at how much less:

It went from using about 3kWh/day to 300 Wh/day, a 90% decline, or a reduction to 1/10th of the energy use.

In hindsight, this makes sense: this time of year, ambient temperatures are in the 70s to 80s, and the tub is in direct sunlight for a few hours each day.

Naturally, I don't use the tub much in the summer, but I also don't use it that often in the winter: perhaps 2-5 times per week.

So I had an ephiphany: if I wired the tub for 240VAC, I could keep it at the low temperature (84F) year round, and only heat it when I planned to use it.

On 120VAC the tub takes about 16-20 hours to heat up, which makes the "on demand" plan silly. But on 240VAC the tub will heat up in about 4 hours, which is much more practical.

Now, I'm not expecting a 90% energy reduction in the winter, but I am curious how much I will save keeping the tub at 84F instead of 104F come wintertime.

 

[BentleyJ]

It really depends on how well insulated the tub is. Heat loss due to conduction is proportional to the temperature difference. For example if the ambient temp. averages 64 outside and the tub is at 84 that is a 20 deg differential. If the water in the tub is held at 104 deg that is a 40 deg differential and would take 2 times as much energy to maintain. This doesn't take into account evaporative energy loss which is more for higher temps and kind of depends on the cover being on or off.

 

[soylentgreen]

Interesting - I've also read that heat loss is proporational to the temperature difference to the 4th power for radiation, but I think that's for an ideal black-body in a vaccum.

Which dominates for a real-world hot tub?

 

[efficientPV]

I use my hot tub primarily in the winter Tues, Thur, Sat and Sun and only in the mornings. That allows it all night to heat up. It is more like a heated swimming pool since it gets set to 96. Minimum temp setting is 80F, but economy lets the temp drop another 20F. Temp raised about 5 degrees an hour. Turning it on before bed works. Many hot tub covers die an early death because the use a foam that absorbs water and the tape they use stops sticking not much after warrantee. I've delayed too long getting a new one and the electric bill shows it.

 

[soylentgreen]

For a birthday a few years ago, I got a FLIR camera, and it's really useful.

Here's a picture (during wintertime) of my tub. From the picture, it looks like my cover is still OK, but there's a lot of heat loss down low. In fact, when I look inside the tub, the insulation generally doesn't go to the bottom. This tub is sitting on a concrete slab. I wonder how much energy I'm losing out of the bottom? I've thought about raising the tub on 2" of styrofoam sheets.

 

[soylentgreen]

Interesting test result. After wiring it up, see https://diysolarforum.com/threads/critique-my-wiring-plan-hot-tub.66341/ I ran some tests.

in theory, I was expecting the heater to heat exactly 4x faster on 240VAC vs 120, since power P = V**2/R and the heater Resistance is the same, and 240 is exactly 2×120.

Actual data shows it heating 5x as fast.

Explanation? I’m pretty sure that while running on 120 V, I was actually getting a pretty substantial voltage sag, running on 12 gauge house wiring.

Since the voltage term is squared, when running on 240 V, it’s actually more like 2.2 to 2.3 times as high, meaning power is close to 5x!

Very pleased so far with how it’s working so far…

 

[efficientPV]

You are also losing heat all that time which may be the greater impact. "220 voltage" can widely vary depending on location. Some areas are 208, 220, 230, 240 as the standard and that can vary upon time of day an if the utility is browning the system.

 

[hwy17]

Yup, heat loss during the heating period is the third factor resulting in 5x results.

The most efficient heating scheme will always be zero heating on standby and heat on demand. Only heating on standby to protect from freezing, if necessary.

 

[soylentgreen]

Good point, if I assume that 100% of the heat usage is heat loss, and it was using a little over 100kWH/month in the summer, then that's a steady-state loss of roughly 150W, which applies to either voltage.

In theory:
• 1000W @ 120VAC vs.
• 4000W @ 240VAC
= a 4 to 1 ratio

In practice:
• 1000W*.85 (for voltage squared losses due to long wire run / too small gauge) - 150W (steady state heat loss) = about 600W 700W net on 120VAC
• 4000W *.98 (much smaller losses, shorter & larger wire) -150W = about 3770W net on 240VAC
= roughly 5.4 to 1 ratio.

(Edit: oops, bad math, corrected!)