The GeneralAire Model 800 humidifier is a notoriously terrible, old fashioned, and obsolete humidifier. But even terrible devices deserve their moment in the sun, so here’s a little bit of information about it.
All you’ll see is the front face plate of this humidifier on the plenum (the first part of the metal ductwork just above the furnace). There will be a copper tube attached, which is how water is fed into it. There should be a valve on the other end of this copper tube so that you can turn on and shut off the water flow.
Inside the ductwork there’s a little tray that fills up with water. Then into that little tray go several fiberglass plates. Water fills the tray and wicks up the plates. Then when the furnace blower fan comes on it blows air over the whole thing and water evaporates into the air stream.
The label is removable and that gives access to a small sight glass that lets you (sort of) see into the unit to know what’s going on.
Maintenance involves replacing the plates a couple of times a year, and cleaning the whole thing regularly. To do that you loosen the four screws at the corners of the face plate and pull the whole thing out.
There’s no way to control the humidity level with this type of humidifier, other than turning the water valve off and on. Modern humidifiers always have a humidistat that lets you dial in the humidity level that you want and then operation is automatic. But with this type you just open the water valve in the fall and you close it in the spring. Or, if you think the humidity isn’t right, you open and close the water valve to suit your needs. That’s a huge pain and not especially efficient.
It’s rare that I see one of these humidifiers that’s actually working, and if you have one you should consider it obsolete and defunct. I suggest just turning off the water supply to this type of humidifier, and replacing it if you want humidification.