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I have a standard Johnson Controls Unitary Products TM9V*MP board in my furnace wired to a nest thermostat. Can I wire my duct smoke detector so that it splices the R wire going from my furnace to my nest. When the detector trips and breaks the R circuit, will that adequately shut off the furnace?

furnace board

furnace wiring diagram

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    do you also have AC?
    – Jasen
    May 1, 2021 at 0:53
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    Can you post photos of the thermostat wiring at both the air-handler and the thermostat? Also, I take it you are using a duct detector with a set of auxiliary (releasing) contacts, no? May 1, 2021 at 1:27
  • Sorry yes, its AC and furnace, Im going to throw up a picture in the main section May 3, 2021 at 22:32
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    What make/model is your duct detector, even? May 3, 2021 at 23:05
  • model number is TSD-CJ edwards-signals.com/files/… May 4, 2021 at 17:42

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Unfortunately, with a gas furnace, that's not quite enough

The bad news for your situation is that opening the 24V R wire to the thermostat may not do what you need it to do. Your furnace, according to pages 36 and 37 of its installation manual, and like most residential gas furnaces on the market, can and will run its blower autonomously if the plenum (fan) limit switch opens up without the thermostat calling for anything. This is a self-protection behavior designed to limit overheating damage to the furnace's heat exchanger should it fire uncommandedly; however, it would defeat the intended operation of your duct detector were you to wire it in the way you propose. You'd need to cut either 120V or 24V power to the furnace control in order to shut down blower operation altogether.

Nests don't like getting their plugs pulled on them, anyway

Furthermore, like most smart thermostats, the Nest assumes that its R/Rc line provides a steady supply of 24V power, especially when a C wire is provided to the thermostat. As a result, it's liable to throw error codes, lose track of time, and otherwise behave in awkward and annoying ways if you wire the duct detector to shut off the furnace control (or even just the thermostat, as you propose).

There's just not much good a duct detector can do in most places you'd want to use a residential furnace

The final rub is that a duct smoke detector is not a substitute for other forms of smoke detection, due to inherent detection delays compared to a smoke alarm or spot/area smoke detector in the room in question. This is stated outright in NFPA 72 17.7.5.2.1:

17.7.5.2.1 Detectors that are installed in the air duct system in accordance with 17.7.5.1(2) shall not be used as a substitute for open area protection.

Instead, they're intended to be used to shut down air-handling equipment in order to keep it from actively spreading smoke. However, this is only generally a concern for equipment that can move more than 2000cfm of air (the figure is from IMC 606.2.1), something that even the largest model in your furnace's product line has to strain mightily to achieve.

Nonetheless, if for some reason, you have a requirement to fit duct detection to this furnace, you can do it by cutting the 24VAC "hot" wire from the control transformer to the connector on the furnace control board and re-routing it to a NC auxiliary contact on your duct detector. This keeps the current draw within limits of the duct detector contacts and prevents the control board from overriding the detector's decision to stop furnace operation, but does have the drawback of confusing the thermostat if the duct detector ever has to operate in anger.

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  • Thanks for all the additional information. Unfortunately this is for code compliance, maybe related to multi-family dwelling code where I am locally. I was hesitant about wiring the 120V to NC-aux on the detector because the relay specs on that board only go up to 10A. I don't see a 24V transformer in my furnace, all I see is the one labelled 40VAC in the corner of my picture. Can you identify which connector the furnace board I can splice into? The only 24VAC line I see is going to the motor from the board, but I dont know where the board is getting its power from. May 7, 2021 at 16:00
  • The upper wiring diagram shows the XFMR as having the 115AC on White and Black wires and the 24VAC on Blue and Brown. The lower ladder diagram shows similar setup without the wire colors. You'll have to find the physical components.
    – DaveM
    May 7, 2021 at 19:03
  • @user2886057 -- the "40" is "40VA" not "40VAC" -- it's the power rating of the transformer. You'll have to figure out which wire is on the R ("hot") end and which wire is on the C (grounded or "cold") end of the secondary though May 8, 2021 at 0:19
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You're throwing around 3 different voltages here.

120V is the base power to the furnace and all associated items (except typically 240V to an air conditioning compressor).

40V is some internal voltage - not relevant here.

The 24V is the key - that is typical thermostat voltage, normally provided by a transformer "somewhere". The max. of 10Amps (current) on the relay is relevant but is a totally separate question from Volts.

From a practical standpoint you need to cut power to "everything" (which means usually 120V) or figure out how to both turn off the furnace burner - because if you turn off the fan but not the furnace burner then you will get overheating and a potentially serious fire, and if you turn off the furnace but not the fan then if the fire is from elsewhere the smoke will continue to be spread by the fan.

Using the thermostat to cut power to the furnace is not going to do it reliably. There are all kinds of delays for on and off built in to the controls of a modern HVAC system. For example, a furnace may start the burner and wait to turn on the fan until ignition is confirmed and things have started to heat up. This annoys people who can't understand why the AC goes on right away but not the heat. This is much like preheating an oven before you put in a tray of raw cookies. But all automatic. A furnace may keep the fan running after the thermostat has told it to turn off in order to make sure the burner has cooled down to a safe temperature - some ovens do that too. There are also emergency shutdowns of both the burner and fan if the control board detects problems (especially overheating).

A similar situation is the power switch on a typical modern computer - pressing the button doesn't actually turn the electricity on or off, it tells an always-on part of the computer that it should turn the rest of the computer on or off.

To have effective and fast shutdown, you will need to either install something that hooks in to the control board in a direct and factory-designed manner (something I'd expect more in a commercial or industrial unit) or just cut the 120V (which may need a different type of relay or switch) and hope that it doesn't mess up the thermostat or control board. My gut feeling is that if the automatic shutoffs are not frequent that they really won't cause any damage. If you have a real fire causing the smoke, damage from the fire is the big concern, not a small possibility of needing to reprogram the thermostat.

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this function of turning furnace off by sensing smoke in return air plenum is required in Ontario for any home that has an apartment within the dwelling. the purpose is to stop transfer of smoke from one unit to another.

Perhaps one could interrupt the furnace door safety switch, this would shut down the furnace completely

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    That's far better handled simply by giving the ADU/suite it's own HVAC system -- trying to send a duct across a fire partition between units requires faffing with fire dampers at a minimum anyway to keep a fire from using the duct as a "shortcut" around the fire partition, and may not be permitted at all depending on the nature of the fire separation and the codes you're operating under Jul 23, 2022 at 22:28

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