When the compressor ran from about 1:45-2:30 on Tuesday, the supply and return lines were at the same temperature. After we had coolant added in July, my old thermometer measured that drop at just about 10 degrees.
One typically expects a 10-15 degree temperature drop across an A/C unit’s cooling coil, though the actual drop from cold air return back to the room might be a little less. This is, in fact, the primary reason I wanted to start this project. But I ran it for three brief periods, about a half hour each, just to see what it looks like on the graph. Because it’s currently malfunctioning I don’t have the compressor running. One important measurement is the cooling drop produced by the A/C system. Blue is the air temperature at the cold air return directly above the desk (and thermostat), and red is the supply register (output vent) directly above a window, maybe 8 feet from the other three sensors.
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The green line is an outside temperature taken as an average of a few web-accessible weather stations in the area (a few in nearby neighborhoods, plus Dulles airport), so it’s a reasonable approximation of the temperature near my home. Here, the orange line is one of two sensors on a table (in the next graph they’re individually shown as red and blue). I’m still trying to figure out the best way to sense thermostat calls for compressor, heat, and fans – do I use clip-on current sensors, inline current sensors, voltage drop sensors, opto-isolaters – and how do I integrate those sensors into the 1-Wire bus… So for now, I only have a few temperature sensors. I just got it working this week, and already I’m finding some interesting results. After returning, it seemed…okay…but still not ideal, so I rushed a (greatly simplified) monitoring circuit into play. Then just before I went to DEF CON, I noticed (using my 20-year-old Radio Shack thermometer) that the AC unit didn’t seem to be cooling as much as before. Our HVAC company came out, pumped two pounds of freon into the system (I really gotta start doing that myself – far cheaper), and scheduled a comprehensive leak search for mid-September (just in case we have to disable the system for a long stretch, we wanted it to be in a season where we might not miss it).
Then…right on schedule, in the height of the summer heat, our upstairs system stopped cooling again. So I asked for some Arduino gear for Christmas, and earlier this summer, I finally started playing around with it. I’ve been thinking for a while that I should be able to build a temperature monitor to track how the system is running, to detect problems (loss of freon, etc.) early, and maybe even forestall costly repairs.
It seems that every year or two something else goes wrong, especially with the combined cooling / heat pump unit that handles the upstairs. It’s also got all the modern cut corners, including an air conditioning system (two, actually) that even 12 years later we’re still struggling with. We live in a house that was new-built, so it’s got all the modern trimmings.
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