Daily Archives: March 12, 2010

We can heat our house with a hair dryer. Is that inexpensive?

The house we are building here in lovely Massachusetts, USA… almost a Passive House, but not quite certifiable… well, we can heat it with a hair dryer (1500 W of heat). And two on a very cold day (6F design temperature). Does this mean our heating bills will be low? Pretty low, I imagine, yes. But not zero.

Let’s do some math… And first let’s simplify things… the HDD (heating degree days) for Stow, MA is somewhere around 7200 (base 68F) meaning that if you multiply the days of the year we need heat times the temperature differential between the inside and outside temperature on these days, you’ll get 7200. It’s a little more exact than this (think “area under the curve” from calculus — using hour by hour measurements), but that’s the basic idea.

So let’s simplify it even more. Let’s call every winter day exactly 32F outside vs 68F inside and assume our 1500 W can keep up with that (which from my back-of-the-envelope calculations seems about right…) So that’s a delta of 36F. Now how many days is that? 7200 F*days / 36 F = 200 days = 6.66 months. Let’s go crazy and call it 7 months.

OK, so let’s say one really did run a hair dryer for 7 months, 24 hours a day. What would that cost to run 1500 W (1.5kW) that whole time? Well, we pay $0.20/KWh. So using the factor-label method to keep track of our units… making sure numerators and denominators cancel out, that’s:

7 months * 30 days/month * 24 h / day * 1.5 KW * $0.20 / KWh = $1,512.00 per year

In other words, kinda a lot!!!!

But that’s why people don’t typically use electric heat, it’s expensive. (It’s also not a great idea because fossil-fuel power plants are roughly 33% efficient in converting the fuel to electricity… I imagine that’s exactly why it’s expensive! So… it’s better to use it in direct form at your house…) That’s where the heat pump comes in. If we factor in the 2.7 COP (coefficient of performance) of our Mitsubishi “Mr Slim” air-source heat pumps that’s:

$ 1512 / 2.7 = $560

OK, now we’re talking! This also happens to be almost exactly the number you would get if you calculated the cost of delivering 1500W of heat via propane or natural gas. That would be fine and dandy too. So the 2.7 COP mainly serves to green up the electricity use, getting back to parity with using propane directly. 2.7 * 0.33 = 0.8991 (probably about the efficiency of a Rinnai propane direct-vent heater)

Now, the reality is that I hope most of this heat comes from active solar heating. But more on that later!

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Filed under building science, erik-green, house, money, passive house, solar, superinsulation

Don’t forget your jacket! Why Sudbury Valley School makes sense to me.

When I arrive with A at Sudbury Valley School, I often have try to convince him to put on a jacket before we walk to the school from the parking lot. Depending on the day, it’s a tough sell. But see, the thing is… I can’t really blame him. Logically this happens primarily on those border-line days where I myself wouldn’t necessarily see a need to wear a coat, so why should he? But still… usually my thinking is… yes, but later maybe he’ll wish he had one! But 1) if that turns out to be the case, and he cares, then surely he won’t be so casual about passing on the jacket offer in the future! And 2) as we walk to the school building on days like this (usually in the late fall or late winter) one often sees about 1.2 million other kids who are wearing even LESS clothing than A, or, if it’s a rainy day… out in the rain completely soaked, but loving it. Obviously the kids aren’t exactly suffering from their (chosen) lack out outerwear.

In other words, Time to get over it… the jacket is MY issue, not his. Kids People learn fast, so if he needs a jacket, he certainly knows how this works.

And it gets better… on days when it is REALLY REALLY cold, the school has a strict policy of the under-8 kids needing to check in with a staff member before going out and the kids know it because there is a huge sign on the front door… and anytime there is a sign on the front door A of course asks “what does that sign say?” These are the days when basically no one in their right mind goes outside anyway, but it’s a good policy, and of course, like all other school rules/laws/policies, it’s one the School Meeting (think… New England-style Town Meeting) came up with. (Consent of the governed!)

I mean, how awesome is THAT?! This place just makes sense.

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Filed under contrarian, school, Sudbury Schools and Sudbury Valley School