Long shower or warmer air temperature?

I try to keep my thermostat set low to save energy. Instead I take a long, hot shower. Would I be better off turning up the heat and taking a shorter shower? Which will save the most energy?

Answer from Green Energy Efficient Homes

Figuring out the answer to this question is complicated by many factors. It depends on:

How much water you use in your shower (how long)
How much heat is applied to it, ie. the difference between the temperature at which the water enters your dwelling, and the temperature at which it exits the water heater
How big the heating area is that you are keeping cooler than you otherwise would, and what temperature you keep it at, and what warmer temperature you would feel you had to keep it at if you didn't take your hot shower.
How well insulated and sealed your dwelling is - since if it's very well insulated and sealed you wouldn't be wasting much heat by heating the air space, but if it's very poorly insulated you'll be losing a lot of the heat through walls, windows, gaps etc.

Since I don't know the answers to these questions you raise, I'll make some assumptions that fit my own dwelling just to illustrate the complexities of the question:

You live in a 1300 square foot home with 8 foot ceilings, which translates into about 10,000 cubic feet of air space
You are keeping the heat at 62F instead of raising it to a more comfortable 70F (so you are avoiding raising the temperature of the air space by 8 degrees)
Your shower lasts 10 minutes and uses a low-flow shower head, at 2.5 gallons per minute for a total of 25 gallons. The water entering the house is 40F and the water coming out the tap is 110F, so that's an increase of 70F.

One easy way to look at this is to consider the mass of the objects being heated. When you turn up the thermostat for a short period (say the time it takes you to get out of bed, quickly shower, get dressed, eat breakfast, and leave) almost all the energy of increasing the indoor temperature just goes to warming the air (with a small amount going to warming up the objects that are warmed by the warming air). In the above example, you're warming 10,400 cubic feet of air by 8 degrees F. One cubic foot of air at standard pressure and temperature weighs about 0.0807 lbs, so your air space of 10,400 cubic feet weighs 839 lbs. One gallon of water weighs about 8.35 lbs, so your 25 gallon shower weighs about 209 lbs. You're heating the air 8 degrees and the water 70 degrees. If you measure in the arbitrary unit of degree-pounds (measures the energy required to raise one pound of matter by one degree) you'll see the following:

Air: 10400 cuft x .0807 = 839 lbs x 8 F = 6,714 degree-pounds
Water: 25 gal x 8.35 = 209 lbs x 70 F = 14,613 degree-pounds

In this scenario, keeping the thermostat low in a three-bedroom, 1,300 square foot house, and instead having a 15 minute shower, uses nearly three times as much energy, if we don't include factors such as the extra heat required to warm objects in the air, heat escaping out walls and through drafts around windows and doors, or, for the water, heat loss through the pipes. Of course, the relative efficiency of the air heater (furnace, electric heater, etc.) versus the water heater also comes into play.

If you used an ultra low flow shower head, at as little as 1.5 gallons per minute, and the water coming into the dwelling is warmer (e.g. from a ground source, which tends to be warmer in winter than a lake or river source) you'll shave some of the energy off the hot water side. But if you are concerned only about your own shower I'd guess that there aren't other people living with you, which means your dwelling might be smaller than 1,300 square feet, either that or several people in your dwelling will be showering longer for the same reason (to stay warm). That means it's fairly likely that the energy required to warm your home a few extra degrees is still substantially lower than what it takes for you to have a long shower, so you'll do better to turn the thermostat up (while you're around) and take shorter showers.

Now, if you were to shower when you get home, and in a bathtub, you could leave the plug in the bathtub while you shower, so that the heat in the tub eventually dissipates into the air within your home. That way you're getting both the comfort of a hot shower, and the residual heat from the hot water. What goes down the drain is room temperature - at least better than hot water from a shower going down the drain.

Make sure you use a setback thermostat or programmable thermostat - have it turn the heat on just before you get out of bed, and off about 15-30 minutes before you leave the dwelling for the day. There's no point paying good money to heat an empty home.

Green Energy Efficient Homes articles to view

Click here to post comments.

Join in and write your own page! It's easy to do. How?
Simply click here to return to Home Energy Efficiency Questions.

Feel free to link to this page to spread the word about energy efficiency