Elk924 and Nest - What am I doing wrong?

[Madcodger]

Hi all - It's been a while since I've been on the board, but hope all are well.
 
I'm trying to hack the ability to close an Elk Zone whenever my Nest thermostat turns on a particular stage, and I'm having a heck of a time with it.  Hoping someone can help, please.
 
I connected the Nest stage 1 output (W1) to the +_trigger of an Elk 924 relay, and connected the 924 to the Nest's 24volt power (RH and Common).  I then connected the NO and Com on one side of the 924 relay to an open zone on my Elk panel.  But when the Nest calls for heat and energizes that +trigger on the 924, it buzzes like a madman.  I have cut the resistor that allows for 24volt on the 924.  I have also tried a jumper between the positive power terminal and the +trigger, and between the negative power terminal and the -trigger.  No dice.  What on earth am I doing wrong?
 
Thanks in advance...
 

 

[RAL]

Most HVAC systems operate on 24VAC, not DC.  That's why the 924 is buzzing. 

 

[Madcodger]

Ah!  I have crossed into idiot land...  Must be the cold medicine.  Thank You. 
 
Could have sworn it took 24VAC.  Blast...

 

[RAL]

There are relays with 24VAC coils.  Most require a fair amount of current, though (100 mA is common).  You might be able to connect one of those to the W1 wire.  But without knowing more, I would be concerned about whether the HVAC system can provide enough current to drive both whatever W1 is controlling as well as the added relay.    My guess is that it probably can, but would want to dig further to be sure.  Also, I don't know what load the contacts in the NEST are rated for.
 
Another solution would be to put a current sense relay on whatever load W1 is controlling.

 

[Madcodger]

Thank you. I had a some RIBU1C's around, and they worked perfectly. I just wanted to use the 924s I had, as they fit better in the Elk can. I'm using the relays to monitor each of three heat stages called by the Nest to close a relay, which in turn closes dry contacts wired into the Elk as three separate zones.

Which brings up a question... I could not have just wired the 24VAC into the Elk zones, could I? I've been away from all this for way too long... Pretty sure that's a bad idea...