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Add on humidifiers are fairly simple to control. They should come with a humidistat which is normally located near your thermostat. The humidistat usually controls a water valve which controls the flow of water to the humidifier. If your humidifier has a auxillary fan, it will control that too. In the simplest setup, 24VAC (normally from an add-on transformer) is fed to the water valve through the humidistat which breaks the connection when the proper humidity is reached. However, the humdifier should only be turned on when the furnace is on (and the blower is blowing warmr air through the plenum/duct or humidifier).
Newer furnaces often provide a 120VAC lead marked 'HUM' which is powered only when the furnace blower is on. If you use this lead to power the humidifer's 24VAC transformer, you can simply wire the 24VAC positive of this transformer to the humidistat and the other side of the humidistat to the water valve. Wire 24VAC common directly to the other water valve lead.
24VAC -------- Humidistat ----
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|----Water Valve----
However, if your furnace does not provide this feature, you need a way to ensure the water valve is turned off when the furnace shuts off. Most humidifiers have optional current sensing relays you can buy. This relay attaches to the COMMON lead of the furnace blower (not the hot leads as variable speed blowers have many). When the blower is on and current flows, the relay engages. Wire this relay in series with the humidistat to offer proper control.
24VAC -------- Humidistat ----|
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| Current
| Relay
|-------| |
|----Water Valve----
Consult the documentation of your humidifier for details on how to properly connect it to yoru HVAC system. If in doubt, ask a professional!
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