Hot Tub vs. Bath Tub

Ok, I was pretty sure I knew the answer to this question before I started to do any research: which consumed more energy: running the hot tub full bore all winter or taking baths? But, I had to do the research anyway (see The MATH! below).

Ultimately, I’d just like to live life with a sense of which options are more environmentally friendly. Some things are obvious such as hanging laundry to dry versus throwing everything into a clothes drier. Along those lines, I found a cheat sheet on Sopris Foundation’s website, which gives some guidelines for home energy use, travel and food. Its handy, but I think if you have a private jet or heli-ski or a regular basis, which are listed as examples of large carbon footprint activities, you’ve either snubbed your nose at the environment or don’t keep up with current events.

Yesterday, at a Sustainable Bainbridge board retreat, Dana mentioned that she measures/monitors her CO2 footprint by keeping track of three main contributors: kwh of electricity used in her home, miles driven, and air travel.  Brilliant.

In the meantime, I still wanted to get into the weeds and see if I could figure out the environmental impact my hot tub habit was having. We use our hot tub a lot during our chilly NW winters so maybe maintaining a body of hot water at a constant temperature takes less energy than drawing a bath and heating water from 50 to 104 degrees each time we need to warm up. If we run the same number of baths as we use the hot tub, is it a wash?

The MATH!
If you’re not a math person, you may want to skip ahead to the alternative hot tub video below. First some basic info:

• One gallon of water weighs 8.34lb
• One BTU (British Thermal Unit) is required to raise the temperature one degree Fahrenheit
• One KWH = 3413 BTU

Bath tub:
Increase temperature from 50 degrees to 104 (54 degree increase).

54 degree increase x 8.34  (gallon of water in pounds) takes 450.36 BTU
One kwh = 3413 British Thermal Unit
450.36 BTU required / 3413 BTU per kwh = .313 kwh per gallon from 50 deg to 104 deg

.313 x 25gallons = 3.29 kwh to heat 25 gallons to 104 degrees per use.

So, the bath uses 3.29 kwh per use for half-bath of 25 gallons (yes, I have a big bath tub) heated to 104 degrees at $.1035 per kwh = $.34 cents per use. If I took as many baths as I do dunks in the hot tub (average 2 day) = $.68 and 6.58 kwh

Hot tub:
Let’s assume the heat element can recover 4 degrees per hour. Let’s also assume the tub loses 2 degrees per hour in 30 degree weather the heater will have to run 30 minutes for every hour of heat loss.

So, that’s 48 degrees that the hot tub needs to make up to keep at a constant 104 degrees 24/7 – 48 x 8.34 = 400.32 btu required x 3413 BTU per kwh = .117 kwh per gallon or 52.78 kwh/24hr at $.1035 per kwh = $5.46/day in the coldest winter days to maybe half of that on warmer days $2.73. If we figure two tubs a day that’s 26.39-13.2kwh per use.

I thought it would be closer. I suppose if I averaged five hot tubs a day, kept the hot tub at a lower temperature or if I filled the bath tub up to my chin then the two might be more of a wash. Hmmm! Other than putting on another sweater, I suppose the most carbon-neutral thing to do would be some burpees to warm up.

Compost Hot Tub

Alternative Hot Tub
Compost-heated bath thanks to Little Eco Footprints blog. What a stitch. Love those Australians – so creative: http://vimeo.com/36538338

We need to keep an eye on those Australians anyway because they HAVE to solve water and energy problems right NOW. They are foreshadowing what the rest of the world will need to do in the future, mainly, collect rainwater and use it for flushing toilets, cleaning cars, watering gardens – it’s crazy that we use potable water to flush toilets. If you’re not aware of the scorching heat and raging fires (adding harmful greenhouse gasses into our atmosphere) in Australia, Bill McKibben of 350.org has been following them pretty closely on Twitter.