Tuesday, December 18, 2007

One Size Fits All, or Reflections on EPA guidelines

So here cometh a fresh pair of thermostats (Honeywell RTH7500 and RiteTemp GPMG8085C), both equipped with default schedules taken from EnergyStar ® Program Requirements for Programmable Thermostats: Partner Commitments (look for Table 2: Acceptable Setpoint Times and Temperature Settings). It was summer, and the default settings were too cold for us, so we changed them.

But the defaults for the heating season were left in place.

Now that the heating season is here, and it is eventually getting quite cold outside, several interesting things are popping up.

First of all, let's take a look at default EPA compliant settings.

Wake: 6AM, 70°F.
Day: 8AM, 62°F.
Evening: 6PM, 70°F.
Sleep: 10PM, 62°F.

Then, let's go back and read the long rant about whether you should shut off your A/C or leave it running.

Then, let's take a look at the temperature spread for the schedule above. 8°F.

That's quite a lot.

The very reason I've started thinking about writing this article is that one of my units (Lennox split), being perfectly capable in cooling mode, seems to either hit the balance point, or otherwise severely degrade its performance, when the ambient temperature drops lower than about 45°F - and, as a result, it is unable to bring the zone it serves from 62°F to 70°F in two hours.

Even worse the temperature actually drops to about 65°F by 6AM, and it barely makes it to 68°F by 8AM - forget 62 to 70.

While this is definitely quite uncool, it also points out another fact that is not on the surface: the unit works at the top of its efficiency curve. It doesn't cycle, it spends initial 10-20 minutes approaching the design efficiency and stays there.

That was the positive, now, another negative - since it serves two rooms, one of which is about five times size the other - guess what, by the end of the two hour run the smaller room is HOT. Balancing the dampers manually will not help since it'll shoot the balance for other conditions - like, the evening, when the ambient temperature is significantly higher and the runtime of the unit is very short in comparison.

So, what's the point?

  • One should carefully examine defaults;
  • You can't get away without actually zoning the house - unless you want to shuffle everything all the day or suffer;
  • It would *really* pay back to come up with an idea to figure out to anticipate the *actual* performance of the unit for varying circumstances and make the system issue recommendations to you about what you should do in any particular case;
  • Which would imply the knowledge base and rule engines to analyze input and figure out dependencies (homework: see Google's statements how having access to massive amounts of data helps to figure out the trends and make correct decisions);
  • It would help to share data between installations (see how Valve managed to make Half Life 2 the most playable game for the same hardware utilizing statistics) and applying some brains;
Damn! DZ is going enterprise.

Hmm... Turned out not quite the way I started it - but oh well, I'll just leave it at that.

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