Duct detectors + imediate shutdown.

[budman21901]

Lennox RTU's, and duct (smoke) detectors. I broke red, but the fan had a delay of 90 seconds. They want immediate shutdown. There is no emergency stop on these lennox rooftops. I wired into a molex connector (fan override). This still caused a 5 second delay. Can someone tell me the correct way to totally kill a Lennox RTU with a duct detector?

[bascontrol]

Don't know if this is the "factory approved" method, but it will work.
In a nutshell, break power to the factory circuit boards. There are typically 2 transformers in the unit and one will feed the boards. We install a relay that is held in from the smoke detector contact using power from the other transformer (or other power source) and break the power to the boards.
Hope this helps

[just_opinion]

Lennox RTU's, and duct (smoke) detectors. I broke red, but the fan had a delay of 90 seconds. They want immediate shutdown. There is no emergency stop on these lennox rooftops. I wired into a molex connector (fan override). This still caused a 5 second delay. Can someone tell me the correct way to totally kill a Lennox RTU with a duct detector?

If any unit has heat sequencer, then nothing you can do except killing the complete line voltage.

Else, kill the xformer secondary is fine. With one problem on some unit that you may clear the fault code. But when it comes to fire, fault code is your least concern.

[snowboarder1]

i think you can change that in the programing call lennox tech support they are really good

[DeltaT]

Had this same situation with fire inspectors years ago and the only way I found to work is to kill the common to the fan. Shut off the power to the fan and it will stop on the spot.

[lwarren]

Had this same situation with fire inspectors years ago and the only way I found to work is to kill the common to the fan. Shut off the power to the fan and it will stop on the spot.


What if the compressors are running when you kill the fan? Do the units have proof of air flow to stop mechanical cooling?

[DeltaT]

What if the compressors are running when you kill the fan? Do the units have proof of air flow to stop mechanical cooling?

On a typical wiring set up to a smoke detector the control voltage to the unit is broken thereby stopping everything including the fan. Then manufacturers, desiring to have a higher energy rating by allowing the supply fan to "wipe" the excess energy off of the heating/cooling coil, added a delay within the units circuit control board allowing the supply fan to continue to run for 30/60/90 seconds typically after the control board has lost its power or lost a heating/cooling signal.

This goes directly against fire codes in that the fire people want the supply fan shut down immeidately to keep from feeding/circulating the fire and smoke.

They do not care or are concerned with the status of the compressors/heaters. They just want the fan to stop immediately when a supply air or return air and both smoke detectors go off.

The only way to bypass, without killing the main power to the unit itself, is to open the common to the supply air blower motor in a single phase motor. Doing that bypasses the units delay of the control circuit board.

Remember, this only happens when a smoke detector goes off. It's not in the normal control circuit application.

Oh, and the normal wiring of the smoke detector is also included in killing the control voltage to the circuit board so the compressors/heaters also cycle off.

So it's wiring the smoke detector normally to kill the power to the thermostat, typically the easiest way, AND an additional relay to open the common to the supply fan for immediate stoppage.

[ticotech cayman]

with a axu 2 pole relay,kill the common of the fan and in the other contactor the yellow wire for cooling

[DeltaT]

with a axu 2 pole relay,kill the common of the fan and in the other contactor the yellow wire for cooling

Once the power is stopped to the thermostat by the triggered smoke detector there will be no heating or cooling or fan signal to the unit so everything will shut down normally.

Only an additional relay is needed to open the common to the fan on a single phase motor.

[ticotech cayman]

yeah thats true.Good point

[lwarren]

On a typical wiring set up to a smoke detector the control voltage to the unit is broken thereby stopping everything including the fan. Then manufacturers, desiring to have a higher energy rating by allowing the supply fan to "wipe" the excess energy off of the heating/cooling coil, added a delay within the units circuit control board allowing the supply fan to continue to run for 30/60/90 seconds typically after the control board has lost its power or lost a heating/cooling signal.

This goes directly against fire codes in that the fire people want the supply fan shut down immeidately to keep from feeding/circulating the fire and smoke.

They do not care or are concerned with the status of the compressors/heaters. They just want the fan to stop immediately when a supply air or return air and both smoke detectors go off.

The only way to bypass, without killing the main power to the unit itself, is to open the common to the supply air blower motor in a single phase motor. Doing that bypasses the units delay of the control circuit board.

Remember, this only happens when a smoke detector goes off. It's not in the normal control circuit application.

Oh, and the normal wiring of the smoke detector is also included in killing the control voltage to the circuit board so the compressors/heaters also cycle off.

So it's wiring the smoke detector normally to kill the power to the thermostat, typically the easiest way, AND an additional relay to open the common to the supply fan for immediate stoppage.

I understand the proper way to shut a unit down on a smoke detector, I just wanted you to qualify your statement so others who read the post will not just assume that you only need to kill the fan and not make provisions to stop the entire unit.

[budman21901]

Thanks for all the reply's. I will let you know how it goes.

[budman21901]

Thought i would update on what i found just in case anyone runs into this.


I ran into the same thing last week and finally called Lennox to learn how to do it properly. Inside the Lennox RTU there is a 3 wire connector. R,C,M1. R and C send power to the detectors. You wire M1 to a normally open set of contacts in the detector. When the detector trips it sends 24v to M1 and shuts the unit down completely. Of course its no were in the manual to be found, and i would have never figured it out because i am not sending any power to a board unless a expert tells me too. LOL

New Carrier units have a Jumper that you cut on the board and then wire the smokes to a normally closed set of contacts and then to those pins.

Trane has a emergency stop terminals.