We just bought a home with this humidifier installed on the furnace. There's no activity when the furnace is running. The water is connected but does not flow. Could this be a transformer problem? We guess it's about eight years old.

Yes - but it doesn't have to work that way. There are several methods you can use to have the humidifier run only when the furnace runs. Look on the back page of the installation instructions. The method I chose was to install a sail switch. This is a switch with a arm attached which has a "sail' attached to it. When the furnace blower runs, the sail catches the blower airflow and activates the switch; allowing the humidifier to operate.
You can also wire in a current sensing relay (figure 6 on the instructions) or connect the humidifier directly into the furnace motor ignition box (if so equipped. My furnace dates to the late 70's and has no place to plug in a humidifier; which is why I installed a sail switch.)
Now however I probably would go with the current sensing relay option as this looks to be the easiest route to follow. Honeywell has their 32001754-001 avaliable for less than $35 from Sears.

Bad selenoid possibly wrong wiring

Hope this helps, check and see if humidistat is set to 35% humidity and that bypass damper on humidifier is in the open position or winter position.(humidistat is usually located on cold air return duct near furnace). Next, replace humidifier media pad, remember to replace media pad (filter) annually, you also may want to clean any lime deposit out of humidifier housing with a solution of warm water and white vinegar before installing your new media filter.Remember to turn power off at side of furnace when cleaning humidifier and replacing media. Before installing new media turn power back on to furnace, set humidistat to the on position, turn furnace thermostat above room temp., and inspect humidifier to see if water is running from top,(you may want to have a bucket under humidifier when checking for water), if you see water running, turn power back off to furnace, properly install new media filter, reconnect humidifier cover & set humidistat to 35%. If you do not see water present, check water supply valve to humidifier and make sure its turned on. If water supply valve is open, then check water supply tubing for blockage.And the last things to check is to make sure all registers through out your home are open and not blocked by any objects, also make sure furnace filter is cleaned, if none of this works refer to installation manual to see if humidifier was sized and installed properly, Good luck.

sounds like you may have it wired to a continuous power source vs one that shuts off with the furnace. The most common wiring is red to C & white to W

Commonly outdoor wood boilers use a hot water coil mounted in the ductwork above your furnace. In this application the Ruud furnace is still using its blower assembly to circulate the warm air from the boiler to your home. If this is the case. Have your heating guy install a #50 relay on the furnaces blower motor to activate your much needed humidifier. If this is not the case and your Ruud furnace is totally independent of the boiler. Still install the #50 relay in the furnace and manualy activate the fan only feature on your thermostat.
Good luck
HVAC Teacher

Thanks for your question,
Sounds to me like you have a bad solenoid valve in your Humidifier, this will need replaced, the solenoid is supposed to open and close when there is a call for humidity only when the furnace is running, then when the proper amount of humidity is sensed by the humidistat, then will it shut off, with the solenoid not working it will let water flow through like an open faucet, the solenoid is the componet that the 1/4in copper tubing connects to at the humidifier, has two wires running from it, you will need to shut the power off before changing this out, also shut the water off at the valve at the other end of the 1/4in copper line, or you might get soaked,please be careful when installing the new solenoid, making sure the water line connection is secure, or you will have leaks, hope this was helpful, please leave a rating, thanks again for your question,
mr.grzz

You want the humidifier to run in heat mode which is the r-w1 terminal on the stat.

yes water should turn off solinoid is bad

The water is only dispensing over the evaporator pad while the furnace is in the heating mode. When the furnace finishes cycling the humidifier will also stop.

Modle 110 has a relay in it see if it is stuck.Hope this helps