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ALeGatorAL · Feb 18, 2009, 10:14 AM

First off Thanks for reading. I was wondering if there is anyway you could run the AC unit normally with the Heater section of the furnace disabled? The Unit in question is a Bryant 394GAD024050. The Overall problem seems to be a hole in the heat exchanger that allows the flame from the burners to get back into the closet area. Since the cost of a new unit is pretty steep at the moment and summer is about here anyway I was wondering if I could just bypass the heater part and just run the AC. Any help in this matter would be appreciated.

Thank You in Advance
AL

KISS · Feb 18, 2009, 10:29 AM

No combustion. No danger of carbon monoxide.

Disconnect the W wire from the stat and turn off and tag out the fuel valve even though the unit is yours.

ALeGatorAL · Feb 18, 2009, 12:31 PM

Originally Posted by KeepItSimpleStupid

No combustion. No danger of carbon monoxide.

Disconnect the W wire from the stat and turn off and tag out the fuel valve even though the unit is yours.

Sounds like a simple solution but I'm afraid I will need a little more information as to what some of your terms are, e.g.. Stat ? I'm asuming by tag out you mean Cap off? Also there may be damage to the overheat switch could that have anything to do with the A/C?

Thank You Again for your time and patience in this matter!

AL

KISS · Feb 18, 2009, 01:27 PM

Stat - Thermostat. The W terminal or usually white wire controls heating. It can be disconnected at the thermostat or furnace. The furnace makes sense.

The overheat switch should not affect cooling.

There is an industry term used in industrial situations called "Lockout/tagout". When a machine goes down for maintenance a gismo that allows multiple locks to be placed on a device. So lets say that the plumbers, electricains and the plant manager is involved.

Each department puts his lock on say the power. As they are removed each group has finished working on their part.

That's lockout. There is not much you can do with that.

Tagout

Is a means where tags or paper luggage like tags are placed on the valve that might say "Do not operate. Cracked heat exchanger, (possible defective high limit switch).

Unhooking the furnace is probably not the best choice UNLESS you capped the line.

So, say you got hit by a car and if you have a wife, she doesn't know what went on. Just post a note in plain view.

Even if it's:

"Do not operate heater. Cracked heat exchanger, (possible defective high limit switch).

Heater has been disabled by turning off the gas valve and disconnecting and capping the white wire that went to terminal W. Gas has also been turned off.

It may seem stupid, but it makes sense to do it.

In a lab I worked at that used toxic (in parts per million concentratins) and pyrophoric gasses (catches fire when exposed to air), the keys stayed in the system at all times.
They were only removed about 6 times in 16 years for upgrades and 1 explosion (no burning). The safety device that was laying in a box hadn't been installed yet although it is unclear if it would have made a difference but it could have.

KC13 · Feb 18, 2009, 01:37 PM

Originally Posted by ALeGatorAL

Also there may be damage to the overheat switch could that have anything to do with the A/C ??

AL

If the high-limit switch is in series with the low-voltage control system, it may indeed prevent the a/c from operating.

cornerstonehvac · Feb 18, 2009, 06:02 PM

Originally Posted by KC13

If the high-limit switch is in series with the low-voltage control system, it may indeed prevent the a/c from operating.

No it won't. The high limit opens on high temp not high pressure. High limit shuts down the burner only it does not affect the fan or A/C operation.

KC13 · Feb 18, 2009, 06:23 PM

Originally Posted by cornerstonehvac

No it won't. The high limit opens on high temp not high pressure. High limit shuts down the burner only it does not affect the fan or A/C operation.

If there's one thing I know, it's Bryant. If the high-limit opens, it breaks the transformer circuit and the indoor blower runs continuously. Bank the farm on it.

mygirlsdad77 · Feb 18, 2009, 06:29 PM

Just to confirm. Shut off, or disconnect and cap gas to furnace. A/C should still work just fine. Also disconnect white wire(or wire at w terminal on stat or furnace) as Kiss suggested.

cornerstonehvac · Feb 18, 2009, 08:48 PM

Originally Posted by KC13

If there's one thing I know, it's Bryant. If the high-limit opens, it breaks the transformer circuit and the indoor blower runs continuously. Bank the farm on it.

You need to go back and read your manuals. The limit breaks the w circuit and closes the fan circuit, running the fan continuiously until the limit resets. It states that in the installation manual from bryant. If it broke the complete lowvoltage circuit , the thermostat display will go out (unless battery back up) and fan relay would never energize. If the fan circuit closes then there is 24V. Present

KC13 · Feb 18, 2009, 10:57 PM

Originally Posted by cornerstonehvac

You need to go back and read your manuals. The limit breaks the w circut and closes the fan circut, running the fan continuiously until the limit resets. It states that in the installation manual from bryant. If it broke the complete lowvoltage circuit , the thermostat display will go out (unless battery back up) and fan relay would never energize. If the fan circuit closes then there is 24V. present

Actually, the blower relay for heating is energized to break the NC circuit and turn the blower off. Loss of control voltage allows the relay NC contacts to close, powering the blower. Any digital/non battery-powered thermostat display will blank out until the limit circuit is restored. Maybe you aren't quite understanding the description of operation in the manual.

cornerstonehvac · Feb 19, 2009, 10:13 AM

Bryant manuals state:
Limit control:
If the furnace overheats for any reason, the limit control LS switches, breaking the circuit to the automatic gas valve.The gas valve closes immediately, stopping the gas flow to the burners and pilot. In addition, the blower motor continues to operate because heat relay HFR is deenergized to cool furnace down. It does break the low voltage circuit for heating but it has no effect on the blower in the cooling mode.
COOLING MODE:
When the t-stat calls for cooling the transformer energizes the CFR cooling relay by way of Gc, which energizes the Blower motor on high speed. The heating limit control is bypassed and has no effect on blower operation in the cooling mode. Totally seporate relay . Look at Bryants install manual for 394GAD page 6 Figure #6 it shows the complete diagram for this furnace and also explains the operation of both relays. I've been in business for 25 years and have worked on just about everything out there. I'm not saying I am the Master, I can screw things up just as well as anyone can, believe me I have messed up a few in my time.
Summary: the heat limit control has NO effect on the cooling operation of the system.

KC13 · Feb 19, 2009, 10:28 AM

Are you sure you are looking at the correct manual for a 394GAD? I have the ladder diagram in front of me right now. Following the low voltage circuit from the transformer, 24v is supplied to SEC-1, through board to LIM-1, through harness to 11C (fusible link), through harness to 7H (limit switch), through harness to LIM-2, through board to R. If the limit circuit opens, 24v is removed from R at the board, also de-energizing GH and allowing the heating blower relay NC contacts to close and power the blower. The W circuit never enters the limit circuit.

cornerstonehvac · Feb 19, 2009, 10:35 AM

KC13 My Bad. I bow down to you. I in fact read the diagram wrong. You are correct.

cornerstonehvac · Feb 19, 2009, 10:37 AM

For some reason I kept referring to the HFR relay as the limit switch. Some days I live in a Bubble.

ALeGatorAL · Feb 19, 2009, 12:03 PM

First Off, I would like to thank everybody for being very helpful and taking their time to help me out with this dilema. I will pass all info on to my maintaince guy and hope that helps him, If not I will probably have to take care of it myself! LOL

Thanks Again

AL

ALeGatorAL · Feb 19, 2009, 12:14 PM

First Off, I would like to thank everybody for being very helpful and taking their time to help me out with this dilema. I will pass all info on to my maintaince guy and hope that helps him.

Thanks Again,

AL

KC13 · Feb 19, 2009, 01:48 PM

Originally Posted by cornerstonehvac

KC13 My Bad. I bow down to you. I infact read the diagram wrong. You are correct.

No bowing necessary. There's nothing wrong with a good, spirited disagreement now and then, as long as the "participants" come away with their dignity and respect for each other intact. I don't claim to be a know-it-all (except for Bryant!:D), in fact I come here to learn just as much as to help. There's plenty of good-natured rib-poking to be had amongst the guys here - just wait until the next great variable-speed debate! (T.B.A.) Buy yourself a beer on me and don't be a stranger. Peace, out! Damn, that sounded ghetto...