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I recently installed a new Aprilaire 700 humidifier attached to a Carrier Infinity 96 furnace. It has been working perfectly for the past 3 days.

Considering the fact that there are 2 distinct circuits connected via 24 VAC line, I was wondering if I should protect my furnace ($$$$) from the humidifier ($$) by inserting a SPNO isolation relay-switch that is energized by the furnace’s HUM-COM circuit and that which closes the normally-open switch on humidifier’s 24 volt line.

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    Wait. How are the 120V/240V circuits switched now? Are you sure it isn't like this already? Feb 27, 2019 at 23:22
  • Can you post the wiring diagram for your furnace + photos of the wiring? Feb 28, 2019 at 3:57
  • 1) ThreePhaseEel: Silly Question: How do I attach a wiring diagram. I don't see it button/link anywhere on the page.
    – user97485
    Feb 28, 2019 at 4:25
  • Harper: I believe I understand the reason you are asking me the question. I guess your reasoning goes as: the humidifier's fan running on 120V is triggered when HUM-COM (24 V) circuit between the Furnace and Humidifier is activated. So, if this is the case (observed situation), then the 24V signal (Control) must be triggering the circuit-closure for the fan (120V) and solenoid (24 V). Hence there must be a built-in SPNO isolation relay-switch inside the humidifier. I'll open the humidifier tomorrow and check it out. Thanks for your question. It made me think in the correct way about it.
    – user97485
    Feb 28, 2019 at 4:40
  • Harper: I followed-up on your suggestion and checked the humidifier's internal. Just as you thought/mentioned, there is a relay [Clion hhc68a-2a] inside the humidifier. I guess I am all set -no need for an isolation relay. Appreciate your help!!!
    – user97485
    Mar 1, 2019 at 18:40

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