I have an Aprilaire 700 humidifier installed on my furnace. I used it for two seasons without issue, but when I turned it on this season it wouldn't run. I can't get water to flow in at the highest setting, and it won't run in test mode either. The green light blinks, but nothing happens. Playing around with my multimeter, I found that I have voltage coming in from my furnace, and there is also voltage on the wire from the humidifier. If I manually short the two leads to the humdifier, the solenoid opens, fan runs, and it appears to work correctly.

It seems to me that the part of the control board that closes the circuit to the humidifier has gone bad. It doesn't look like there is anything I can do to fix it, which sucks because it looks like that's an $85-$100 replacement part.

Is there anything else I should check?
Is there anyway to test the control board
Is there something I can try to fix on the control board?
What could have caused it to go bad? I didn't see any physical signs of problems on it. I don't want to have to replace this thing every 2-3 years at that price.

Control board do not have replaceable parts. You would have to replace the whole board if this is your problem. However, I would start by jumping the two wires at the humidistat and see if the humidifier works when blower on furnace is running.

Yes, I get the same results when I jump the two wires at the humidistat that lead to the humidifier. The solenoid opens and the unit appears to work like it is supposed to.

I wanted to post the resolution on this one for others that have the same problem. I called Aprilaire and spoke with technical support about my problem. We walked through my diagnostic procedure and he had me verify that the outdoor temperature sensor was giving the correct reading (you have to disconnect it from the humidistat to do this). When I reported back that the temperature control was working correctly, they also concluded that my humidistat controller was bad.

The thing that really frustrates me is that although the product has a 5 year warranty and is only 2 years old, Aprilaire will not replace it because I installed it myself and did not have a licensed HVAC contractor install it.

My opinion: I got a substandard product that did not meet my expectations. Aprilaire won't stand behind their product, and they weren't willing to do anything to make me a happy customer. If someone's looking for a humidifier, I don't recommend Aprilaire. They certainly won't be getting any future business from me.

Sorry but here is your problem.

Aprilaire will not replace it because I installed it myself and did not have a licensed HVAC contractor install it.

I understand what you are saying but many manufactures have been burnt by homeowners doing there own work and electrical wiring. Read the warranty and see what it says. If it does not mention what they told you I would call them back and bring it to there attention.

I, today, have the very same issue. I called Aprilaire, they told me because I installed it, they won't warranty it. However, my paperwork supplied with the 600A is dated 2005, and there is no mention about if I don't use a HVAC licensed contractor to install their item, it has no warranty.
Well then, where do all the Web sites that sell stuff get their products? From the manufacturers! Therefore, if the products being sold on web pages by sellers don't disclaim that the item has no warranty if not followed to the letter (installed by a HVAC contractor), then I'm lost.

Simple cut and dry, fraud, thievery and total BS!

Well file a lawsuit. I cannot blame you.

Mine is doing the same thing. There is obviously a design flaw with that humidistat regardless of who installs it.

I am trying to replace an aprilaire 760 model with the new 700. The first one I tried was a 700m but it was defective. I have since returned and had to move to the 700 with automatic humidistat (other w out of stock). Before I go through installation I want to check the operation of the unit. Can I expect that the fan will operate by just plugging in the unit and touching the two brown wires coming from the unit together or does the unit need to be fully installed? This used to work with the 760 models. I do not want to cause issues with the new unit if this is not possible. Thanks.