Furnace losing 24v when heat requested

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mm wrote:

24V is coming from a step down transformer. Battery back up for 'stat is to keep the program settings. As long as main power breaker to furnace is on, 24V should be present. It's called logic sequence control power. If it goes missing, something is shorting it out or the transformer is bad. Or sosmething is loose. Tony
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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.
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On Sun, 18 Dec 2005 19:14:23 -0500, "Max Metral"

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).

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Max Metral wrote:

If your thermostat takes batteries then replace them. A battery powered t-stat with weak batteries will drop out when it attempts to bring in heating or cooling, having enough power for the display, but not enough for the internal relay circuits.
If your thermostat doesn't take batteries then it is a power stealing stat, using Y to tap into common back through the compressor contactor. Depending upon model Y may or may not go directly to the compressor contactor--it may go instead through an electronic circuit on a main control board. This type of system may require that a resistor be wired across Y and C in the unit in order to provide a low resistance path from Y to C. This type of stat usually comes with a large ceramic resistor packaged with it.
There are also power stealing stats that also use batteries for backup. The batteries in these are only provided to hold program and temp settings in the event of a power failure.
Barring all of this it may be that your unit is shorted somewhere internally which could cause power to drop at the transformer, and thus from the t-stat when it calls for heat.
hvacrmedic
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So, it turns out the therm IS a power stealing therm. What's strange is that when EVERYTHING is off, I still have 0V across Y and R**, so of course the therm has nowhere to steal power from when R and W are connected. The obvious fix is to connect C (I don't have a labeled C on the furnace board, will a screw terminal on the chassis do?) But the most confusing part is that this has worked for years, and now suddenly stopped. So I'm not sure if that means something has gone wrong with the unit and I should tread carefully. I'm also not sure whether this is just a little reminder that a 15 year old furnace should be scrapped.
** In truth, it's not 0, it's some barely readable amount. The meter moves, but hardly.

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It's my understanding that there is no C on most wiring, because there isn't a need for one. (Ungrounded light switches don't have a common wire; they just connect the hot wire to the light.) All of the appliances are grounded so you only need the power wire and a wire for each device. Connecting R (24VAC) to a appliance's wire completes the circuit and allows the appliance to run.
If it has worked for years then, yes, something has gone wrong. I would verify the wiring and then call the furnace guy to fix the problem.
Mike
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upand_at snipped-for-privacy@yahoo.com wrote:

Your understanding needs updating. In a typical resi HVAC system the 24v secondary from the transformer has a hot and a common. The common isn't necessarily grounded, but it is called common because it is fed directly to one side of every load. In some systems the common is also switched, but it is understood by any tech that common (C) is the transformer leg that is not fed to R on the t-stat. HTH. :)
hvacrmedic
(Ungrounded light switches don't have a common

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Yes, it was a dangerous mistake. I should have said common instead of ground. My point was that you don't need both sides of a power supply, because you only switch one. Thanks.
Mike
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Oh, and on a side note...
My mother's furnace behaved similarly years ago. The completely incompetent LICENSED HVAC guys that installed her furnace didn't ground it and it kept frying control boards. Fortunately, after their many stupid attempts to fix it and subsequent complaints to the manufacturer, Bryant sent their own techs out to fix it. And then Bryant stopped those lamebrains from using their products again.
Mike
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Max Metral wrote:

Call a tech. :)
hvacrmedic

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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.

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Max Metral wrote:

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.
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Yes, I mistyped, it should read:
...with meter in hand if the switch is on summer, I measure 24VAC across R and Y. If the switch is on winter, I measure 0VAC across R and Y.

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Max Metral wrote:

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.

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On Mon, 19 Dec 2005 09:37:45 -0500, "Max Metral"

Do you have a wiring diagram? It would show what the other side of the 24V transformer secondary is connected to.
If this furnace is connected to (or can be connected to) air conditioning, there should be a C connection to go to the compressor.

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On Sun, 18 Dec 2005 07:46:52 -0500, "Max Metral"

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.
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