I have a aprilaire 700 I just installed on return plenum. Have a problem with the wiring, the instruction say to connect the R&C wires from humidistat to the same on furnace power strip. Is the C

SOURCE: Humidifier control .

if humidifier was wired into thermostat circuit for power instead of seperate transformer , this can happen. you need to see if you voltage going to the solenoid valve. if you do , and it isn't opening, the valve is bad . this is assuming that it is using a # 50 relay to come on only when the fan runs . . make sure the water valve is open

SOURCE: Lous noise in the furnace when aprilaire 700 on

I have had the same problem , off and on, on my two 700 units for over 4 years. It is caused by water hammer which results when a water solenoid valve closes quickly under pressure. Refer to your owners manual or get a copy of it off the internet. Note the areas where they tell you to remove and clean the inline water strainer and the inline orifice restrictor. These are VERY small components and can clog up easily so they need to be cleaned per the manual. While cleaning them won't totally eliminate the problem it has helped mine with the noise. You really need to have a qualified person, like a plumber, install a small, water hammer arrestor in the inlet line. The trick is to be sure it ends up parallel to the water line flow and not perpendicular to it.... a common mistake. If you are not qualified to work on solenoids and piping, don't do it......it will end up costing you more to repair your mistakes. Good luck

Carl

SOURCE: Aprilaire 550 doesn't seem to be getting power

See if you have a screw marked EAC This to power an Electronic Air Cleaner unlike the HUM it also gets 120VAC only when the fan comes on. Not sure what make of furnace you have but if your heat is working OK don'tn change the control board the can be very expensive. there are other ways to wire humidifiers without using the HUN screw

SOURCE: Solenoid won't open--voltage drops when connected to humidistat

The coil may shorting out the 24vac supply to the solenoid. Recommend replacing soleniod.
Normally you should be able to read the 24vac across the solenoid when energized.
The other option is to get a 120vac to 24vac transformer and test the solenoid. This way you will know if the solenoid is the cause or the 24vac source is the problem.

SOURCE: Water only runs with humidistat in test/reset mode

you need to make sure your humidity level in the house isnt higher then what your humidifier will provide at any givin outdoor temp. this chart is in the owners manual. you cant remove the wires going to the ODS or it will give an error unless you have a resistor to put in its place. Anyway if the humidifier works in test mode according to AA it works so you may have higher humidity than you think, the highest your gonna get is 45% as the temp outside drops it will allow less humidity so it doesnt cause window condinsation (property damage) here is a copy of the chart. the #s on the left are the dial setting
TABLE 2 – % Relative Humidity Guide
Outdoor Temperature (°F)
-10°F 0°F 10°F 20°F 30°F 40°F
1 10% 10% 10% 15% 20% 25%
2 10% 10% 15% 20% 25% 30%
3 10% 15% 20% 25% 30% 35%
4 15% 20% 25% 30% 35% 40%
5 20% 25% 30% 35% 40% 45%
6 25% 30% 35% 40% 45% 45%
7 30% 35% 40% 45% 45% 45%