I have Aprilaire 400 humidifier installed with Lennox G61MP furnace. The system is set on blower always on for low level circulation when heating/cooling is not ON. When furnace kicks in (heating on and fan at high speed) the humidifier doesn't operate (no water flow). Turning dial on humidistat to TEST doesn't turn it on either. At Humidistat, there is ~ 24 Volt on ODT connection, but no power on R/C or A/A when humidifier should be ON (HEAT ON. FAN on Full). The R/C is connected to HUM control on the furnace control board via a transformer. Since Humidistat is not getting powered when it should be I suspect something is wrong in 'power' going into humidistat. Could it be the transformer? So I removed it (BPC41622 | Aprilaire Humidifier Transformer 4010 - Free Shipping | Bell Parts). When checked with an Ohm meter it shows ~100 Ohm for the in leads and open circuit for the out leads.[I expected an open circuit on both sides, but I may be wrong] There is no other open circuit that be detected (in lead to out lead or in/out lead to the body). If I connect it to 120 volt AC it shows 24 V on the output side. Please help me decide if transformer is the culprit or not. If not what else should I examine?.

 

You take the meter and check the HUM on the furnace for 120v, and 24v at the transformer.

 

My system shuts down whenever I open the front cabinet door. At that time there is no reading at HUM. Should I look into attaching a lead so that I can detect the output of HUM when furnace is in operation. I added some additional information in the original post regarding the transformer, please take a look at that and let me know if it offers any hint.

 

Just tape the door switch down, and with care, take a reading on HUM to neutral. (Make sure the furnace is running in heat.)

 

Thanks for your help.
You won't believe what I discovered after finding out that HUM and transformer connections were live when furnace was on. I followed the wire from the furnace to the humidistate (thinking may be I need to replace the wire going from furnace to humidistat) and discovered that it was connected to ODT sensor and not to R/C and ODT in lead was connected to R/C. Recall I mentioned there was 24 V on ODT in the first post. So I switched the connections at humidistat and the system is working as expected (water flows when furnace is on and doesn't otherwise). I am clueless as to how this system was operating since its installation 7 years ago. BTW, is there a reason to believe that the original connections were correct? (I am a DIY guy, so wants to make sure that even though it is working, what I did is the right thing to do).

 

Glad it was an easy fix for you..

I'm sure it was easy to get two wires mixed up, and wonder why it wasn't tested in the first place?

Hope there was no damage to the humidist?

 

Yeh I am glad it was.
How can one find out? Is there a routine to test the humidist?

 

I'm sure if there was damage, it would not be working, or it may be stuck on.