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Issue #217 August 2008
A Reactive Necessity
by Steve Ciarcia
I don’t know how we got started on this conversation at the party last night, but we were all sitting around the fire pit and the subject of sports obsessions came up. Almost all of the wives started describing the various lifestyle accommodations they had to deal with during the baseball, basketball, and football seasons. Sympathy was offered to those dealing with multiple sports or seasons that never seemed to end. There was an almost unanimous opinion among the wives that their husbands were absent a good deal of the time fulfilling various obsessions.
While I do watch an occasional event, I felt confident that I was going to receive the “dream husband” award when I piped in with, “I don’t have any sports obsessions.”
But then, from the other side of the fire pit, my wife yelled, “You’d have to take time off from all your other compulsions to fit it.” Amid laughs from the crowd she continued, “Have you looked at all the black boxes, flashing lights, and wires around here lately?”
OK, Dear. You got me. ;-)
It’s laughable, but true. It seems like I’m always pulling wires to connect one thing or another. In the early days, it was RG-6 cable everywhere to distribute cable TV and C-band satellite signals. Then came speakers, security system sensors, video cameras, etc. The real volume wiring started with my forever-evolving and expanding home control system (HCS) iterations—and it has never stopped. Let’s just say that the latest wiring is mostly CAT-5, and I’m on my fifth 1,000' reel since I started expanding the HCS to include web-interactive monitoring about six years ago.
Call it an obsession if you want to, but I like the satisfaction of knowing what’s going on. There is nothing worse than coming home from a trip and finding a foot of water in the house or getting a call from one of those vacation home monitors screaming “low temperature alert”—then what? In fact, during the last two years, my oil heating system has hiccuped twice while I was 1,000 miles away.
Fortunately, I can diagnose a lot via the web through a combination of video cameras and temperature sensors. A web cam pointed at the boiler’s digital control panel shows the LEDs representing the current state of the thermostats, burner, and zone pumps. It’s a simplification, but let me just say that being told I have a “low temperature alert” is better understood when I can go see that the bedroom thermostat is indeed calling for heat, the zone pump for the bedroom is on, and the physical room temperature is 48°F—obviously, that particular zone pump is bad and I need to call the oil maintenance guys to replace it.
For the most part, the quantity and type of sensors I’ve connected to my HCS have been predicated on catastrophe avoidance rather than audio/video switching or lifestyle enhancement. I still feel that’s the way it should be; but today, there’s a new wrinkle. All of the energy monitoring electronics on my new PV system, along with the prospect of $5/gallon home heating oil, suggests that I should be doing more “oil monitoring” too. How long does each zone pump run (i.e., Which zone is sucking all of the heat?)? What is the total oil burner run time (i.e., How much total oil consumption?)? What are the temperature swings in each heating zone (i.e., Heat transfer rates, dirty filters?)?
The first rule in home-brew sensor connection: Do No Harm. Basically, try not to trash your heating system or blow up your house when you accidentally short the connecting wires or the HCS self-destructs. Recording zone run times requires HCS sensors on the pump control circuits or the physical zone pipes. My initial thought was just to bond a 140° bimetallic sensor on each zone pipe, but it was grossly inaccurate. The burner might run for 5 min. and the zone pump for 10, but the amount of time for the pipe to warm and cool enough to switch the sensor depends on the rate of heat transfer in each zone—too many variables. For later analysis, and specifically to see if there was any interesting correlation among those variables, I attached a Xytronics four-channel temperature monitor on the pipes. But as for connecting the HCS to the furnace, I was dealing with the 24-VAC actuator signals.
Converting a 24-VAC on/off actuator signal into isolated contact-closure output to the HCS is a relatively easy concept. Because there are six actuator signals, however, doing it with the least current requirement from the furnace power supply was the challenge. Ultimately, I chose to use capacitance reactance. Typically used in transformerless power supplies or trickle chargers, the technique utilizes capacitance reactance to limit current in a Zener-regulated AC-to-DC converter. The converter’s DC output powers a low-current reed relay whose isolated contacts go to an HCS input. Certainly, I could have dug through the junk box for some appropriate opto-22 commercial solution, but going back and experimenting with basic electronics was fun for an evening, just like reading Circuit Cellar. Besides, as a home-brew electronic answer, it truly does no harm.
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