Furnace losing 24v when heat requested

[Max Metral]

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

[Brian]

Let me make a guess, You wired one of W, W2 to C?

:)

[Max Metral]

No, not this time. :)

[vmravinec]

I guess you gave up on those pricks in alt.hvac, eh?

[Max Metral]

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?

[upand_at_them]

Where does a lighted doorbell button get its power from? It only has two wires and it lights without ringing the doorbell.

Mike

[Stretch]

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

[Ralph Mowery]

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.

[Steve Kraus]

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.

[David Efflandt]

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).

[mm]

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.)

Remove NOPSAM to email me. Please let me know if you have posted also.

[Tony Hwang]

Hi,

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

[RP]

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

[Max Metral]

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.

[upand_at_them]

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

[upand_at_them]

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

[RP]

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

[RP]

Call a tech. :)

hvacrmedic

[Rich Greenberg]

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.

[upand_at_them]

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