## Thursday, September 16, 2010

### Figuring Out Hot-Tub Costs

It’s fairly easy to figure out what a restaurant lunch and a fancy coffee a day are costing you, but other things are a little harder, such as the cost of bumping your house thermostat up a degree in winter. I have a hot tub attached to my pool that I use in late spring and early fall, but it isn’t obvious what it costs to use.

Most of the time, water circulates through the hot tub and pool, so that both are at the same temperature. To use the hot tub, I have to divert all water flow to the hot tub and heat the water. I’ve always had the vague feeling that this may be more costly than it is worth, but hadn’t really worked it out.

Since I use the hot tub during cooler weather when I’m no longer heating the rest of the pool, the amount of heating required varies with the pool’s temperature. The amount of heating required ranges from about 30 to 60 degrees Fahrenheit. So the question now is how much it costs to warm the hot tub by a degree Fahrenheit.

After some experimentation, I’ve found that the hot tub heats up 20 degrees in 13 minutes and the heater takes 37 seconds to consume one-tenth of a cubic meter of natural gas (based on timing the spinning pointers on the gas meter). The last part of the equation is that natural gas costs me about 37 cents per cubic meter right now. This includes all extra taxes and charges that are proportional to my gas usage.

So the marginal cost of heating up my hot tub is between \$1.17 and \$2.34. This is less than I feared, and I’ll probably worry less about using it. Of course, this is just the extra cost of actually using the hot tub. I pay other fixed costs that I haven’t figured out yet.