You mean including when the furnace/blower is not operating? It should be tied into the blower switch/relay so that it operates only then. If it's not, rewire it so that it is.

Also, is it running constantly because the humidity is too low? We had one of those in our old house and it ran pretty much all the time the furnace did in the winter, because the air was that dry.

I have the same problem on my Aprilaire 550, water constantly flows to humidifier even furnace blower is not running. if I dial down the control, water stops but not flows when furnace is on.

Replaced humidity control(manual type) but didn't help.
what should I do next, replace solenoid?? or need pro to rewire it??

Finally I found the problem and fixed it. My humidifier's transformer was hard-wired into junction box of furnace and to humidifier directly, NO relay sensor, of cause water will keep flowing to humidifier unless control stat is turned down.
That was installed by "professional contractor' from Air Group in NJ in the first place years ago, I hadn't had it working for years because of saddle valve blocked. This year water supply was re-routed with a real valve.
Can we trust any contractors in these days??

That'll do it, all right. Saddle valves are the spawn of the devil. Ball valves are the way, the truth and the light.

Can we trust contractors? A good question. Unfortunately it seems with many if we want a good, really perfect job, we have to do it ourselves, because I guess most people are willing to settle for an ok job and pay less, because the price is what drives most of us.

On the other hand, some diy jobs can be pretty disastrous, too.

Check the wiring see where your humidifier is plugged in make sure it's plugged into the furnace correctly and in the right spot

Didn't see you already figured it out I'm glad you did