shut down relay for AHU

[Angel Electric]

I am installing a shut down relay which is addressed, (Mircom system ) the HAVC contractor left a single pair doorbell wire in the component area where i am supposed to connect to this bell wire... The wire is open ,with no voltage or continuity bwtn each of the two wires in the pair... I am wanting to understand how there bell wire shuts down the unit... My guess is somehow it shunts via the themostat? The bell wire runs to the thermostat. Any input is appriciated...

 

[kwired]

I am installing a shut down relay which is addressed, (Mircom system ) the HAVC contractor left a single pair doorbell wire in the component area where i am supposed to connect to this bell wire... The wire is open ,with no voltage or continuity bwtn each of the two wires in the pair... I am wanting to understand how there bell wire shuts down the unit... My guess is somehow it shunts via the themostat? The bell wire runs to the thermostat. Any input is appriciated...

What terminals is it connected to at the thermostat? All you can do is open the controls from a call for heat/cool or manual fan operation, any automatic delays will still happen and there will not be immediate shutdown which may not be acceptable. Isn't it more common to shut down the supply power to the HVAC equipment to ensure immediate shutdown? The issue is usually to shut down blowers quickly to prevent supplying more fresh air to a potential fire.

 

[nhfire77]

I am installing a shut down relay which is addressed, (Mircom system ) the HAVC contractor left a single pair doorbell wire in the component area where i am supposed to connect to this bell wire... The wire is open ,with no voltage or continuity bwtn each of the two wires in the pair... I am wanting to understand how there bell wire shuts down the unit... My guess is somehow it shunts via the themostat? The bell wire runs to the thermostat. Any input is appriciated...

Gonna take a guess as to what you have:

There may be some set up for an input at the t-stat, or the bell wire goes there then directly to the unit. There it would take an open or closing of your relay as an alarm input to shut down immediately.

Or

This arraignment may not work: If, the wire is to the t stat, they probably will be running the com leg through the pair. The idea is you hook up their t stat wire to the COM and NC terminals of the relay. On alarm it opens killing power to the thermostat. Which simuates to the AHU the t stat has stopped calling for heat or cooling. The issue is most hvac systems have a purge cycle of ~60 seconds. The unit needs to shut down immediately.

Check with the hvac guy. And test it after he's done and gone. some hvac guys have been known to remove wiring or change programming affecting the shut down.

 

[dhalleron]

Maybe the other end of that cable is not connected to anything yet. That?s why you don?t see continuity or voltage.

It?s been a few years since I connected a fan shutdown so maybe they do it differently now. I used to see a jumper on the fan controller and when it was removed the fan shutdown. We connected a normally closed contact in place of that jumper. There was always some kind of control voltage there. Opening our contact shut down the fan right away with no delay.

I get people all the time asking for a relay for fan shutdown. When I ask what voltage and current are they controlling they usually say it's just a dry contact. They must think the relay uses magic to shut down the unit.