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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8430
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I am trying to install a in duct smoke detector a Air products

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I am trying to install a in duct smoke detector a Air products sl 2000, 24 v. system where on the furnace do I get the low 24 v. feed terminal and which terminals shuts down the unite or turns the fan off Thanks Ed.

Phil :

All work to be done by licensed professional. WE KEEP GOING UNTIL YOU ARE HAPPY . . . I come and go, be patient.

Phil :

you get the 24v from the control transformer, its the size of your fist and heavy. find it on the wiring diagram first.. then trace wires to find it

Phil :

the red thermostat wire goes directly to one side of the transformer... run that through the contacts on your smoke detector to kill the ac unit if there is smoke detected.

Phil :

read that over a few times, let me know if it makes sense on your end.

Phil :

we can go from there as necessary. phil

JACUSTOMER-q8iv5mip- :

I have no A.C. and no visable transformer, only a panel .but I will take anouther look. the three wires (thermostat ) White - com, red - - f an on manualy no heat -- black-- heat controled by thermostat. Ed.

Phil :

The furnace is fit with a 24v transformer.. and the controls are the same as with a unit with ac... the transformer is INSIDE the furnace, behind the cover that llifts up 2" then out, to expose the gas valve..

Phil :

the gas valve area can be located by noticing where the gas line enters the furnace.

Phil :

Look that over, tell me if it makes any sense on your end. Phil

JACUSTOMER-q8iv5mip- :

Sorry You Are Right, I found it on the wireing diag.

JACUSTOMER-q8iv5mip- :

Thanks Phil . So I can get my 24 v .to feed the smoke dect. from the trans , and switch on the red thermostat wire ? Ed.

Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8430
Experience: Retired HVAC/ Electrical & Boiler contractor. Industrial
Phil and other HVAC Specialists are ready to help you


Keep in mind if breaking 24 the fan will still run for 90 seconds. Most duct detectors are designed to stop the air flow asap as to not help spreads smoke.


If you want warning only then breaking 24 is sufficient...but for instant shut down as code requires breaking 24 does not cause instant shut down.


Just a bit of additional info on this unit.



Customer: replied 5 years ago.
The smoke dect. requires a pos. and neg. would the black therm.wire be the pos.and the white be neg. ? yes it requires a instant shut down,so there is no transfer of smoke.
Hello again, Billy is correct ! if the smoke detector goes off when the furnace is in the heating mode and you break the the red thermostat wire. the gas valve and the fan relay will go off,,, but the fan will keep running on its timer or furnace fan control intended to run the fan until the furnace cools down.

You will have to run line 1 from the 110v supply to the furnace through your smoke detector contacts in order to shut the fan off instantly'

Picking up the 24v to run the smoke detector from the furnace 24v transformer

regarding the positive and negative.... those are direct current voltage terms,

AC , alternating current is supplied by the utility the furnace transformer not DC power'

however with components coming from china lately many items are marked positive and negative when they are intended for AC use

look on the smoke detector wiring diagram for references to AC power

if ac is OK... then the red wire coming off the transformer to the thermostat marks
'positive'.. the other terminal will be called 'negative' even though its AC and there is no actual positive or negative

its a bit trickier than that but no need confuse the issue more than it is

verify the voltage spec on the detector is marked as AC

let me know

we can go from there

Customer: replied 5 years ago.
Yes it has a AC. contact Ed.

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