Disclaimer: I can tell right now that this is one of those late-night posts where I should be sleeping, not posting about a technical topic. But these not-entirely-lucid ones are sometimes the most fun to read.

I consider myself extremely tech-savvy. I can build a computer from parts, make my own Ethernet cables, run some performance tuning on interactive websites, write applications in numerous programming languages (as well as SQL and HTML), and much more.

But I still don’t get our digital thermostat. They’re programmed to go down to 58 at night, come up to 67 on weekends and from something like 6 to 9 a.m., and 3 to 9 p.m. on weekdays. In other words, when people are home.

Of course, me being home on vacation isn’t quite compatible with this. There’s a simple override, where you can hit the up or down arrows to set it to a temperature. While I use (and appreciate!) this, it’s also a pain. It’s really no fun waking up and having it be 58. I’d really like to reprogram it to automatically come up to 63 or so around 10:30.

I still don’t get why the whole thing isn’t on the LAN. This would have two obvious benefits right out of the gate–it’d be much easier to configure (even if you let someone with no clue about usability design the GUI, it’ll be better than the myriad knobs, switches, and buttons on our thermostat!), and it’d be more convenient in many cases to pull up a new tab in your web browser than to walk down the hall to the thermostat. (Plus, the thermostat is in my parents’ bedroom. I’d have loved to have turned the heat up a few degrees around 11 tonight, since it’s 9 outside and almost as cold inside. But something tells me they really wouldn’t have appreciated it.)

I’m also not sure that the ‘simple’ thermostat algorithm is that efficient. You figure it works something like:

while(1) {
$temp = getTemperature();
$desired = readDial();
if($temp<$desired) furnace.enable;
if($temp>$desired) furnace.disable;
}

When we view it at ‘computer speed,’ I think we can see one of the basic problems: in theory, the furnace could start flapping, where on one loop iteration it turns the furnace on, and just a fraction of a second later, it turns it off. I don’t profess to know a lot about the overhead in starting a furnace, but I’d imagine that it’s most efficient to let it run for a few minutes.

I think a much better system would be to have a programmed minimum run time: if the furnace is turned on, we should run it for at least 5 minutes. After 5 minutes, we again evaluate the temperature: if it’s at the target, we turn it off. If not, we drop into a quicker polling, maybe once every minute. Incidentally, this is much better for the thermostat’s processor, but if its sole purpose is determining whether to turn something on or off, no one really cares about minimizing overhead.

So you give it a secondary purpose: handling a TCP/IP stack and a basic webserver! All of a sudden, instead of an infinite loop, you run a tiny bit of code every 30 seconds.

You can also generate some interesting statistics. For example, how long does the furnace need to run to raise the temperature one degree? How does this scale–if you want to raise it three degrees, does it take three times as long? How does the temperature of my house look when graphed across a day? How about telling me how long the furnace ran yesterday? And, given information about my furnace’s oil consumption and our fuel costs, it’d be cool to see how much it’s costing. And it could give us suggestions: “If you drop the temperature from 68 to 67, you’ll save $13.50 a month,” or such. This would require some storage, but a gig of solid-state media (e.g., a camera’s SD or CF card) is around $10-20 now. Plus, with the advent of AJAX, you can push some of the processing off to the client–let the client use a Flash applet or some good Javascript to draw the graphs if the thermostat is underpowered!

In conclusion, I’m freezing.