Saturday, November 26, 2011

Relay Computing FTW?

So we finally got around to getting a new humidifier to replace one that had died. Three times. (Hint: If you get the chance to put in a Honeywell humidifier? Don't. They suck.) The new one is an Aprilaire 700. Very nice, digital controls, outside temperature sensor, and the works.

We put it in ourselves.

Now, this doesn't seem like a big deal on the face of it. We had to cut the duct work a bit, and putting up the outdoor temp sensor was a joy, but by and large these are straightforward processes.

Wiring the thing into the furnace? That's where it got interesting. Our furnace is probably a decade and a half old. Unlike modern furnaces, where you open them and find yourself face to face with a printed circuit board (which, in fairness, I'd have been a lot more leery of working with), our contains relays and interrupt switches, and that's pretty much it. Working through the schematic has been interesting. Heaven forbid the HVAC industry should use electronics industry schematic symbols or anything. Once I figured out that those two vertical lines are contactor (relay) contacts and not a capacitor, the thing made more sense.

Once I sorted out which circuit did what, it dawned on me that I was looking on a string of and gates, effectively. If the white wire is at 24v, turn on the ventilator motor. If the flue pressure is correct (ventilator motor is running, flue is not blocked), turn on the ignitor and the gas. If the flame sensor indicates there's flame, turn the ignitor off. If the rollout sensor is off (that is, flame and/or gas are NOT coming out the back of the burners into the body of the furnace - yikes) keep the gas on. If the manifold temperature is above temperature x and the manifold temperature is below temperature y, turn on the blower. OR if the green wire is on, turn on the blower by itself, and bypass all the furnace start sequence logic. OR if the yellow wire is on, throw the relay to turn on the AC compressor. (note that these two ORs are not exclusive, and in fact the thermostat turns on both lines for AC.) It's all pretty straightforward. Theoretically.

Practically, I got the motor relay line plugged in the wrong place (on the yellow wire's terminal instead of the green) and it has been giving me bizarre results since yesterday for anything except heat. Now that that's straightened out, I just have to find out from Aprilaire how exactly I keep the humidifier from coming on when the call for heat comes down, humidifying the return plenum, switching itself off, then switching itself back on when the humidity in the plenum drops down, then switching off three or four times while the furnace goes through its startup sequence for heating. I don't even know if it's bad that it does this.

But it did amuse me once I realized that the furnace /does/ have a computer of sorts in it, and one of the more ancient types imaginable, at that. Flagging this post steampunk, since telegraphic relays most certainly did exist in the era, and you could do some interesting computing with them if you were so inclined.



Iceberg Greenberg said...

Best type of furnace. Mine's from 1985 and has the exact same goodies inside.

Like you, I had to figure out how it all worked when the flue pressure switch gave out. It's got a rubber diaphragm inside, and they sometimes get old and stop diaphragming. (spell-check just had a baby with that one...)

A new switch cost me precisely $0, and was provided courtesy of a guy who knows a guy who fixes all kinds of old furnaces. Cheapest furnace repair ever, and diagnosing it was kinda fun.

Good work on the DIY! I hate to see those furnace guys getting the easy money, especially when furnaces aren't that damn complicated in reality.

JRS said...

Oh, I tend to think their expertise is worth it. I wouldn't touch the flue pressure switch, or the flame sensor or that stuff, for fear that if I don't get them right, the furnace can blow up, burn the house down, etc etc.

The thermostat controls, worst case is the furnace just won't start, or oscillates between blower on/blower off (don't ask me how I know this) and stuff like that.


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