

Saving energy with a netduino - gspyrou
http://fabienroyer.wordpress.com/2011/02/22/saving-energy-with-a-netduino/

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crux_
So ... any actual measurements of how much energy was actually saved? Neat
hacking for sure, but it seems somewhere along the line thinking clearly about
the goal of "saving energy" got lost in the details.

Although I'm not exactly experienced in the field, I'm going to guess it's
quite unlikely the water heater's energy consumption was reduced by anything
close to 2/3, or even by a significant amount.

Most obviously, the author completely overlooked that there are two types of
energy use by a tank-style water heater:

1) Heating incoming water to the desired temperature.

2) Holding the tank at the desired temperature.

This setup does nothing to address the first energy need, which is the larger
of the two.

And, unless the timing is spot-on, it will heat a tank which is not
immediately used -- which then cools off, and finally needs reheating later.

If you make the (incorrect) simplifying assumptions that (a) a tank loses
energy at the same rate regardless of its current temperature, and (b) the
burner is instant-on/off to full efficiency, then my pre-coffee mind arrives
at the conclusion that reheating a stale tank to a target temperature would
use exactly the same energy input as holding it at the target temperature the
whole time anyway.

Without those simplifications, it still seems likely that the 'stale tank re-
heat' will use a decently large fraction of (2)'s energy.

tldr; To take large chunks out of your consumption of heating energy, insulate
your home better instead. Bonus: It saves you cooling energy during the
summer, too!

~~~
dmlorenzetti
_reheating a stale tank to a target temperature would use exactly the same
energy input as holding it at the target temperature the whole time anyway_

Essentially this same line of reasoning was used during the 1970's energy
crisis to argue that night setback of a thermostat wouldn't save energy. "You
spend as much energy to reheat the house as you save by letting it cool down."

Unfortunately, that argument is incorrect.

Think about the overnight cycle, during which no water flows in or out of the
tank, and the water ends up at the same temperature as it started. The total
energy you have to put into the tank equals the heat lost through its
insulation. That heat loss is driven by the temperature difference between the
hot water and the surrounding air. Lowering the water temperature means the
tank loses less energy over that time. Therefore you have to put less energy
in.

In other words, you were right to identify your assumption (a) as incorrect.
The tank's heat loss does, in fact, depend in a very direct way on the water
temperature.

~~~
adrianN
There certainly are real energy savings. But without any numbers we can only
speculate whether this whole setup is worth the effort.

A simpler way to save energy would have been to wrap a thick blanket around
the tank to improve the insulation.

A better way would have been to replace the tank with a tankless heater.

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jsulak
I've read that turning down the temperature in the tank can increase the risk
of Legionnaires disease. I don't know how much of a risk that actually is,
though: [http://www.treehugger.com/files/2009/03/turning-down-
water-h...](http://www.treehugger.com/files/2009/03/turning-down-water-heater-
safe.php)

~~~
kordless
It's a big risk because most people that get Legionnaires get Pneumonia. A few
years ago my neighbor got Pnemonia and almost died. He was 41, fit, and
otherwise healthy. It's nasty business, and you do not want to risk it just to
save a few bucks a year.

If you want to do your part, go out and buy a new efficient tank, or even a
tankless heater. Both of them will save you a bundle over that older model.

~~~
Swannie
+1 for the tankless heater +1 for the efficient tank

Great hack, but get a modern heater and a shiny micro controller for it, that
may even speak ZigBee or some other home automation protocol.

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patrickgzill
I seem to see an uninsulated water heater tank - if so, the first thing is to
add more insulation around the tank first.

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tocomment
This is really interesting. I'm assuming this fine gentleman (or lady folk)
reads this thread. Dude, you should totally post back in a month or two and
report your actual energy savings.

I'm wondering if it's actually inefficient to turn the setting lower and
actually heat the water back up later. Does anyone know?

~~~
crux_
Disclaimer: This comment, and my other one, are the first time I've actually
thought about this.

The amount of tank heat loss would depend on the temperature difference
between the water & the environment; a tank that's kept hot will lose more
heat than one that's warm.

So I think it's a marginal gain to let it cool, but only marginal. And: the
better-insulated your tank & plumbing are, the less you gain.

~~~
tocomment
That sounds correct. I wonder if you're missing anything?

Hopefully he'll post his savings in a few months.

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bingaman
Neat hack, but here is a timer for hot water heaters on Amazon:
[http://www.amazon.com/Intermatic-WH21-Electric-Water-
Heater/...](http://www.amazon.com/Intermatic-WH21-Electric-Water-
Heater/dp/B00002N5FP/)

Anyone considering this should also invest in some insulation as others have
said.

