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Treebeard's Stumper Answer
2 March 2001

Keeping Warm

With continuing cold weather and California power shortages, it's a good time to think about how we use energy at home and school. I only need hot water at home in the morning and evening. Should I leave my water heater on all the time, because it takes so much energy to heat up all that water after it cools down? Or should I just heat the water when I actually need it? Similarly, if no one will be in my classroom at school during lunch on a cold day, should I turn down the thermostat for the hour, or will that require even more energy to reheat the building when we return?

Ry and Mojo in the 105° hot tub during a recent snowfall at our home in the mountains behind Santa Barbara. This is an extreme heat loss situation! It felt great after clearing the damaged trees blocking our road. Maybe redwood hot tubs in the snow will become the new Santa Barbara tradition.

Weather Note: We literally had Seattle's weather in Santa Barbara this February, with rain or snow on 15 of the last 20 days of the month. (And Seattle got our earthquake! Jim B. pointed me to this interesting site on Earthquake As Artist.) The local Santa Barbara Newspress gave this account on March 2, 2001:

February was also colder and drearier than usual, said meteorologist Bill Hoffer at the National Weather Service office in Oxnard. High temperatures averaged 59.9 degrees, or 5 degrees below normal. "It was one wicked month," said Hoffer. "We had winter storm watches and advisories out much of the month. It was not the exception. It was the usual."

The reason: The semi-permanent high-pressure area that normally sits off our coast "almost complete dissipated," he said. That opened the door for a huge low-pressure area in the Gulf of Alaska to move in. "The jet stream aloft which moved south and came up over our area caused a tremendous amount of cold temperatures and moisture throughout the month," he said. "Normally this weather would go to Seattle and the Pacific Northwest. We received all of Seattle's weather."

This resulted in one of the oddest months in memory in terms of the weather. Not only did Santa Barbara Municipal Airport set a new high temperature record of 81 degrees on Feb. 4, but four days later, it tied a 43-year-old low temperature mark of 33 degrees. That ushered in a remarkable 20 days when daily temperatures topped 60 degrees only twice. On Feb. 13, snow fell down to the 900-foot level below San Marcos Pass and Highway 154 was closed for about 12 hours.

The preliminary NWS February 2001 Monthly Summary for Santa Barbara is available. What a change from our very dry autumn this year! (That was my Rain or Shine (5 Jan 2001) stumper, which has links and much info about our local weather.) We've now had 4 to 12 inches of snow on San Marcos Pass in each of the past three years: 15 March 1999, 6 March 2000, and now 13 February 2001. This time it snowed all day and stayed on the ground, so I had two snow days home from school! Before this, the last heavy snowfall was twenty years ago in 1981. And last Sunday we got 13+ inches of rain in one storm. This is global warming?!

To save energy, I should always turn the heat off when I don't need it. This follows from Newton's law of cooling, that heat loss is proportional to the temperature difference with the surroundings. Houses and water heaters hold heat like a leaky bucket -- not very well! And the more there is, the more they leak. Keeping water or a building warm takes even more energy than reheating it after it cools. Of course it's a convenience to come home to a warm house, but we pay for it. I'm surprised that heaters with built-in programmable timers aren't more common!


I think of Newton's law of cooling as the fourth "Law of Motion" that governs the motion of heat between more and less. The greater the difference, the greater the flow, at least as regards conduction and convection. (Heat radiation follows it's own rules.)

Think of a warm house or water heater as a full bucket of water, with the water level being like temperature. If insulation was perfect and the bucket didn't leak, the level would stay the same and it wouldn't matter what we did. But real houses and water heaters always lose some heat. That bucket is leaky and the water level always goes down. The more water in the bucket, the faster it leaks because of the higher pressure, so more water per minute is required to maintain a high level than a lower one. If you want a full bucket an hour from now, the best bet is to let it drain and refill it when you need it. As the level goes down, so does the leakage, so the total leakage-replacement over time is less.

Graybear sent this explanation which explains the physics and considers the reality:

Heat loss (actually heat transfer) occurs because of a temperature differential between two areas. The greater the temperature differential, the faster the rate of heat transfer. This is why energy guidelines say your hot water heater thermostat should be set at 120 degrees F instead of 140 degrees, after all, 120 is plenty hot. Depending on where your water heater is located, the ambient temperature is 55-70 degrees F, let's say it's 60 degrees for this example. You can see that there would be more heat loss in a (140-60=) 80 degree differential than a (120-60=) 60 degree differential. If you turn off the water heater when not in use, the temperature differential between the inside and the outside of the water heater decreases as the water cools, so less heat energy is lost. The same is true for the house thermostat - the smaller the differential between inside and outside, the slower the rate of heat transfer (true for summertime also, when the heat is trying to come in).

If you have an electric water heater, and if you are going to be gone for more than a couple of hours, it's probably worth turning it off, especially in the current crisis. Likewise, you could turn it off when you are sleeping. The main disadvantage is not the amount of energy it takes to heat up all that water, but the time involved. Typical electric water heaters require two hours to heat 55 degree water to 120 degrees. The secondary disadvantage is the method you use to turn it on and off. In order to turn it off at the thermostat, you must remove some of the thermal insulation. Turning it off at the circuit breaker decreases the life of the circuit breaker because of the spark that occurs when you turn it back on again. The ideal solution to both problems is to install a time clock with a power relay (also called a contactor) in the electric line, and then program the clock to turn off the water heater an hour before you leave for school in the morning, back on an hour before you get home (two hours if you take a shower as soon as you get home), off again before you go to bed, and on again an hour ot two before you wake up. Your electric water heater will then be on a programmable thermostat similar to your house's furnace/boiler/ heat pump. If you have a gas- or oil-fired water heater, it's probably not worth turning it off at this time. Other ways to save energy are to install plenty of thermal insulation around your water heater. Kits are available from DIY home centers and installation is simple.

I know your school building is an old house in Southern California, but I hope there's enough insulation in the walls to where this wouldn't save very much energy, but it still would save some.

Our only heat at home is a great old Vermont Castings wood-burning stove. By it's nature, this is a heat-on-demand device, though we can turn down the draft at night and bank the fire with ashes to keep some heat until morning. I've regreted our decision to stick with wood heat on a few cold mornings, but we always get by. As they say, splitting firewood warms you twice! We live in an oak forest, and wood smoke has hazards too, so it's not for everyone.

It's easy to turn down the thermostat in my classroom, but turning heaters off is difficult and not advised. In fact, I rarely touch the thermostat at school except over vacations, because it's so nice to come into a warm classroom in the morning, especially when someone else is paying for it. Power generation has environmental consequences, but it's easy not to think about them. One benefit of the California power crises is that conservation is at least being talked about again. Power generation and the environment will always be at odds, at least until fusion power plants are a reality. I think Graybear and I agree that programmable timers should be more common.

Saving energy is not the only concern. Turning down the thermostat might have other effects that are not acceptable, for example larger temperature swings are harder on house plants and aquariums. Light bulbs usually burn out when first turned on. I usually leave my computer on during vacations (and all summer) to minimize stress from temperature changes. Turning off the TV for half an hour will save energy, but is it harder on the picture tube? Could fiddling with thermostats manually or automatically stress other parts of our household and be even more wasteful? I don't know! Better insulation is the only practical answer I'm sure of.

A couple of other real stumpers are based on the same principle of heat cooling.

I think I should add the cream to the coffee before the phone call, and I should get up and pee now, even if it's freezing outside. In both cases, losing the extra heat immediately will result is less heat loss in the long run.

Here are some links for your own research:

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