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.