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HVAC (Heating Ventilating Air Conditioning) voltage and current limits (low voltage)

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Full Member level 6
Aug 18, 2012
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Does anyone know the voltage and current limits (requirements) for low voltage (24Vac) residential thermostats, furnaces, A/C systems etc?

I'm testing a fancy thermostat and it stops working when the 24Vac falls below 18V (seems like it should have more low voltage margin?)

What about contact rating, this device uses hefty TRIACs however they have leakage current of about 1mA, so until loaded they appear on when they are off (with just a DVM load), I'm not sure how sensitive the relay coils the Triacs drive are?

Does anyone have any good links or knowledge of this discipline?

I seem to remember 30 or 32V being a typical value in the US, for the systems you mention.

However 24V appears common recently. So maybe 32V was a sine voltage peak, or maybe it was for doorbells, or older houses, etc.

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I recall current ratings on the transformers being 1/3 or 1/2 A.

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A single appliance draws little current from the transformer. But if one transformer must serve many appliances, then they might cause the voltage to sag.

Can you read the voltage level getting to your thermostat? When it turns on (clicks)? When the control circuit in the furnace/AC turns on? When additional fans, burners, pumps turn on? You need to watch voltage for a minute or two while the units turn on.

Do you have a long run of wire from the transformer to your thermostat? Common bell wire should not cause much voltage drop, but since you speak of 18V present at your thermostat, then that sounds drastic enough to go on a search for the source of the voltage drop.
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I found a specification sheet for a "nest" thermostat.

Voltage range 20 to 30Vac
Contact ratings are all over the place from 1-10A
Transformers from 10 to >60VA

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