Furnace losing 24v when heat requested

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Hi. I have a Lenox furnace, about 15 years old I guess. It was working fine, but then all of the sudden the thermostat seemed to just die. So I swapped thermostats. It worked again for a couple of days, and then failed again. So I debugged it a bit more carefully, and it turns out the thermostat "resets" whenever it tries to call for heat. The fan will come on for about a tenth of a second, and then the display acts like it does when you first plug it in. I'm not sure exactly what's happening, but it SEEMS that the furnace is shutting off 24v when heat is called. It's not the blower though, because if I just turn the fan on (from the thermostat) it works fine. It's also possible that the therm is somehow resetting itself, the only real test I did here was connecting 24v to the heat line directly, and the heat didn't come on, but I'm not sure if that's the proper "protocol" for requesting heat.
Any thoughts before I call the techs?
Thanks, --Max
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Max Metral wrote:

Let me make a guess, You wired one of W, W2 to C?
:)
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No, not this time. :)

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I guess you gave up on those pricks in alt.hvac, eh?
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They were indeed remarkably rude. At least most of them, a couple were helpful.
Anyhow, the main question has now become, if my digital (i.e. needs power) thermostat says it only requires the following leads connected:
R - 24vac G - fan W - heat Y - cool
Where does it get power when it demands heat, thus shorting (?) G and W to R? I would assume (standard decomposition of assume applies) that it gets it from Y staying at ground, and perhaps that's where the fault has occurred. If not, where else can it pull the power?

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Where does a lighted doorbell button get its power from? It only has two wires and it lights without ringing the doorbell.
Mike
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Most digital thermostats require a ground (24V common) If you don't have one the batteries will power it for a few days, then it will die. Get a mercury non programmable thermostat or run at least a 5-wire cable and hook up 24V common.
You may be able to get your system to work with a power robbing thermostat. Those will work without a common. They use the Y & W as a ground (common) when heat or cool is off, using just a trickle of current to power the thermostat. Power robbing thermostats usually don't have back lit displays, as that requires more power than they can steal.
Stretch
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The light bulb is in series with the transformer and the door bell coil. The bulb does not draw enough current that would pull in the door bell coil. When you press the button, the bulb is shorted out and the only thing in the circuit with the transformer is the coil.
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The transformer that supplies the doorbell supplies the light. The current drain for the light also flows thru the bell, but is much lower than the current needed to ring the bell. When you press the button, the light is shorted and the current rises to enough to ring the bell.
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On 18 Dec 2005 17:07:46 -0800, upand_at snipped-for-privacy@yahoo.com wrote:

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?
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On Monday 19 December 2005 12:43, Mark Lloyd wrote:

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.
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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.
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Ralph Mowery wrote:

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!
hvacrmedic
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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.
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Goedjn wrote:

Before you do that, check the voltage between the two.
You're presumably working on a troubled circuit. Don't assume shutting off the breaker eliminates all sources of electricity.
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wrote:

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.

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Mark Lloyd wrote:

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 1,000 pennies even).
hvacrmedic
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If the thermostat connects R to W when it calls for heat that's not the same as shorting it to ground! A digital stat will still get power. If a call for heat is grounding out the 24V then presumably something is wired wrong, the stat is defective or something is wrong at the furnace end.
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If the thermostat does not have batteries, you may need to connect the other side of the transformer to C (common). Some thermostats I got for our warehouse need that if you want to use them without the batteries (although I think that is just mentioned, not shown in wiring diagram).
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On Sun, 18 Dec 2005 19:14:23 -0500, "Max Metral"

Is this a question with a built -in answer!
The way you have your question, after the chart, makes me think you don't underwstand.
First, I'm pretty sure the thermostat gets its power from the same source when it is "calling for heat" as any other time.**
Assuming your colors, when you turn the fan to On, it's going to connect the R to the G, so the fan terminal back at the furnace gets 24 volts.
But when you turn the thermostat switch to Heat, and it is cold enough to call for heat, it's going to connect the R to the W, so that the Heat terminal back at the furnace gets 24 volts. Unless the fan switch is on fan (and if that is the case, the fan is already running) it's not going to supply 24 volts to the G, because that would keep the fan running regardless of what the heating circuit wants, and the heating circuits in the last 30 years or more have wants. They want the fan OFF until the air in the furnace is hot, and they want the fan ON until the air in the furnace has cooled off a lot. The heat circuit is in charge of the entire heat functioning, including the fan. The Fan is directly controlled by the thermostat, only when the Fan/Auto switch is set on Fan.
** In my case, that is the furnace with battery backup. I know that because I haven't replaced the battery in more than a decade, and indeed when I turn off the power to the furnace, the thermostat forgets what time it is. So I know the furnace powers it, and I'm sure the battery would also if it weren't dead. But I get the impression that not all thermostats work the same way. (I only depend on the battery to keep the day and time. The temperature and setback times are set by mechanical multi-segment slide switches.)

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