Some furnaces switch the fan internally when heat is being used, and require only R & W to be shorted.
I would assume Y would be connected to a relay coil for the AC compressor, and would be open not ground. It may be possible to use it for a ground, as long as the current drain in very low.
The first electronic thermostat I installed required an additional connection, which required running a 5th wire. The contact point wasn't hard to find, since I had a wiring diagram for that furnace. I think it was labeled "C" (24VAC common). The thermostat I have now (in a different house), uses a separate power supply (wall wart).
When a doorbell button is NOT being pushed, there will be 16VAC (or somewhat higher) across the contacts. A bulb could be connected across here. It would be in series with the bell, but this would still work if the bulb has a high enough resistance that the current would still be too little to ring the bell (and many doorbells are actually chimes, that make sound only when the current starts or stops, not when it's steady).
BTW, considering strange electrical stuff, what if you had an old (Edison base) fuse box that you could screw a light bulb into instead of a fuse?
On at least one furnace I've seen, the overheat switch would cut off the 24V. This disables the gas valve and fan relay. The fan relay (the one associated with heat, not "fan on" from thermostat) controls the fan through the NC (normally closed) contact, so the fan runs constantly in this case.
Well, the uncertainty about why this happened after two working heating seasons will be a great unanswered question in my life. (ok, well, not that great) BUT, the problem is solved.
So, as suspected, the digital therm was simply losing access to power when it routed R to W. The additional variable that I didn't realize is that there's a summer/winter hard switch on the furnace. I assumed that normally I had switched this to winter in the past. However, with meter in hand if the switch is on summer, I measure 24VAC across R and Y. If the switch is on summer, I measure 0VAC. I don't understand exactly what this switch does that other than I suppose forcibly disabling the compressor. But given this fact, there's no way for the therm to steal power when it requests heat, and it shuts off.
My only explanation for previous success is that perhaps I forgot to switch that to winter in the past. I suppose maybe that switch does something more complex, which USED to leave Y at ground but doesn't anymore. It's also a remote possibility that the power stealing can work by stealing from the fan input, and that in my reconfiguring the therms, I reset it to request fan and heat simultaneously thus killing 24VAC on G.
In any case, the solution was simple, run a dedicated C. That was sort of the obvious first fix, but it just concerned me why it was a problem now.
I'll use the money I saved by not calling a tech for this to call them for my other problem, which is a radiant baseboard unit that spews water way too often. :) Now *that* I have no chance of fixing myself.
You are on the right path. Just keep troubleshooting until you find the culprit. Look for a schematic on the furnace and you should find the controls in the circuit, one of which is malfunctioning. Wiring and/or connections could be the culprit too.
So which is it when it's on summer -- 24VAC or 0VAC? (I think you mis-typed in the above quote.) If this is an attic-mount furnace, then my guess would be zero, which keeps the heat from coming on when, e.g., an attic exhaust fan might be running (which could lead to dangerous levels of CO).
It sounds to me like whoever installed the digital thermostat didn't cover all the bases.
Well, I think now I'm the one who mis-spoke. :-)
Having 24 VAC across R and Y in the summer is what you want, because then you can effectively call for cooling. In winter, you'd want it across R and W instead, to allow calling for heat. This could still be part of an interlock with an attic fan.
When I was young (not too long ago) I read in some sort of "home emergencies" type of book something to the effect that you could screw in a light bulb in the fuse box to locate or diagnose a short. The bulb would glow brightly when there was too much current going through it.
I'm guessing that if you screwed in a light bulb in a fuse box, the brightness or dimness of the glow would depend on how much current was being drawn on that circuit.
That is true. While I don't recommend it to people that are not use to working with electricity, a trouble shooting practice is to subistute a lamp with for the fuse or breaker. If the circuit is suspose to be open the light will not burn. If you put in a 100 watt bulb in a 120 volt circuit and it lights up to full brightness then you have a short circuit or some device in the circuit that is drawing current. If you are sure there is nothing in the circuit you look around and disconnect things tuil the light goes out. If there could be a high current (more that an amp or two) device in the circuit, you will have to disconnect it. Anything in the circuit that uses less than a couple of amps will cause the light to burn dimmer.
Sounds like a good way to burn down a house or to smoke a few small appliances. For an encore you can slip a penny under the fuse. Dear readers, please don't try any of this at home!
Of course, the sensible thing to do would be to shut off the breaker, and test continuity from the nuetral to the hot, but that requires a hunk of equipment that costs several thousand pennies.
Are you somehow equating a penny and a light bulb? A light bulb has a much higher resistance, and (unless it got shorted somehow) could never (on 120V) pass a current higher than 833mA (for a 100W bulb). It wouldn't even conduct that much unless it was into a shorted circuit.
I used to work at Goodwill (for awhile), and they actually used light bulbs in series for testing appliances.
Two reasons its a stupid idea: Suppose the ohmic resistance of the light bulb is 240ohms (60watt incandescent). Now suppose that the resistance at the short is 240ohms when you have the bulb in place in the fuse box. Each load drops 60 volts for a total of 15watts dissipated at the short. Now suppose the short is where a wire nut has fallen off. The wire junction has gotten next to the fart fan housing and is arcing to the paper thin sheet metal. There is a paper label inside the housing of the fart, opposite the short, and the arcing sets this paper on fire. How long to you think it will be before 15 watts continuously dissipating will get a junction hot enough to ignite any combustible materials adjacent to it? Arcing shorts can cause fires, and are in fact responsible for a large number of electrical fires every year in the real world. The odds may be against a fire starting in any one particular instance with this method of short detection, but I'd hate like hell to be that one in ten-thousand statistic on anything that I was diagnosing.
Now let's suppose that there is a small appliance plugged into an outlet on that circuit. The voltage dropped across that appliance will vary from 0 to 120 depending upon its resistance and upon the nature of any upstream short which can vary from 0 to infinity ohms. Most appliance won't be harmed with continuous low voltage for 5 to 30 minutes, but some will. I also wouldn't want to be responsible for a smoked electronic or motorized appliance.
A volt/ohmmeter can be obtained from any Walmart store for $10, (or
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