Jack N. writes in: We rent a home and are responsible for all utilities. It came with a rather nice 8 person hot tub, currently, we will turn it on (warm it up) only when we plan on using it that evening. In the winter it takes about 8-10 hours to warm up. My question is this, should we leave it warm year-round, or only heat it up when we plan on using it. I think currently we use it 2-3 times a month, if we had it warm all the time, we would probably use it 1-2 times a week. Maybe more during the winter. During the summer it retains heat thanks to the cover and the temperature outside, and during the winter, it will retain the heat for maybe a day at most.
Without running the numbers at all, I can tell you that it’s cheaper to heat the water on occasion rather than keep it constantly hot. Hot water will constantly lose heat to the environment, and the greater the difference between the environment and your hot tub, the greater the heat loss at any given time.
Think about a hot cup of coffee. It’s initially very hot, but it doesn’t take too long for it to cool down to the point where it’s drinkable. However, after that, it doesn’t cool down at a very fast rate at all. It does eventually reach room temperature, but it stays at an acceptable heat for quite a while as you’re drinking it. That’s because the closer the liquid gets to room temperature, the slower the heat loss is.
Since the exact heating and cooling of water in a hot tub varies a great deal depending on the model, the insulation, and other factors, the best I can do is look strictly at the heating of the water.
A hot tub of the size you describe holds about 500 gallons of water. It takes about 8.34 BTUs to raise one gallon of water one degree Fahrenheit. So, if you’re raising that water from 40 F to 100 F (in the winter), you’re using about 250,000 BTUs, which is roughly 75 kWh of energy. A kWh costs about $0.12 from your electric company, so your energy cost for heating that much water in the winter is about $6.25 just to heat it for one use.
Now, your major issue with the hot water is your insulation. How much of it are you losing to the environment? For most hot tubs, the hot tub cover has a much lower R-value than the hot tub itself, so that’s how most of your heat is lost.
In your example, you mention that the heat in the tub is completely lost after a day or so if you’re not running the heating. Just using extreme back-of-the-envelope math, you’d essentially be heating the tub’s water each day during the winter, costing you $6.25 per day to keep it hot all the time. That’s going to add up quite fast.
My suspicion is that either it’s retaining at least some heat for longer than that day or your hot tub cover is a pretty thin one with a low R-value – or, most likely, both. I sent an email out to a friend who owns a hot tub in the southern part of the United States and who keeps a pretty close tab on his energy use and he estimates it costs him about $30 per month to keep his tub hot around the clock, but it would cost about $50 a month with a “typical” cover.
Compare that to $18 a month heating the tub for the three times you’d use it during that month.
You might want to stop by a hot tub dealership just to get an idea as to the R-value of the insulating cover on your tub. You can get a good ballpark estimate just by the feel of the covers they sell.
While the math isn’t exact (again, there are so many variables here and the math gets quite complicated quickly), you can compare the cover you do have with some of the others that they sell. If you see one with an R-value twice as much as the cover you do have, it’ll hold in the heat for roughly twice as long as your current cover.
Is a new cover an investment you want to make? If you really desire to leave your hot tub on all the time in the winter, it probably is at least worth looking into, as you’ll repay that cover’s cost eventually in the money you save.
However, the best approach is to simply turn on the tub in the morning if you plan to use it that evening. If you’re using the tub more than six or seven times a month, you may want to consider running it all the time and investing in a very high R-value cover for the hot tub, particularly if you’re going to live there for a long time or if your landlord will help with the cost of the cover (provided it stays with the house, I’m sure).
The savings for making the right choice here is going to be on the order of $20 a month, so it pays to think this one out.
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