Relay failure with inductive loads: arcing vs. surges
What is not good practice is to have a relay that must remain energized to shut off a system that must be shut down in case of fire. Cutting off power to the furnace may be ok.

The thermostat probably doesn't immediately shut off the fan. It tells the furnace to shut off the heat. That will first shut the fire off, then some time later, shut off the fan. That allows the remaining heat in the furnace to be circulated through the building.

You don't want to just shut off the fan without shutting off the fire in the furnace. The furnace must be designed to shut down safely in the event of power loss, so I would think that it would be ok to just cut off the power to the furnace for an emergency shut-down.

However, I would think that the furnace should have an emergency shut down provision that would allow connecting a low-current relay to a special place in the furnace control unit. You should probably investigate that. I would think that a heating contractor would know exactly what is the recommended way to accomplish what you want to do.

If you want to cut off the furnace power with a normally open relay contact, search for Hp (horsepower rated) relays. Note that a modern furnace will have both an air circulation fan motor and a forced draft fan motor. Since normally closed contacts are not usually used in motor power circuits, you may not find a Hp rated relay with NC contacts.


  The smoke detectors deliver 24V to the relay when they are triggered. Not sure how I could use them to keep the relay energized.


Use a latching circuit as shown below.