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Hi all, I'm about to install a replacement whole house humidifier on my forced air heating and I had a question about the power supply. The humidifier is a Honeywell HE240 and it comes with a plugin 24V transformer (also it states 0.416A and 10VA, so sure if one of those is the amps?). The thing is I know the last humidifier was wired to a transformer from inside the furnace which is listed on it's housing as a 25V, however in measuring it seems more like 27). My question is can I just use this instead as it would be a much nicer install that way. The old humidifier was a drum/foam element job, whereas this one is the mesh with water system (not sure the name for that; "bypass"?). So the old load was a motor and the new is a solenoid. But in my mind I'm thinking that likely this would be a standard kind of setup; power consumption wise, however I really wanted to consult before recklessly hooking up to the old supply. Thanks.
 

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Is the one inside the furnace just for a humidifier, or does it also run the furnace. if just for a humidifier, then no problem using it. Your new humidifier actually can use up to a 30 volt power source.
 

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not tryin to be disagreeable but most modern furnaces have a 35-40 va 24v transformer and can handle your solenoid valve. the circuit board may have a HUM terminal but you need to check it when the heat and fan runs to see if it puts out 24 or 110 volts. older furnaces had smaller transformers and using the add on transformer was necessary. if possible use it but it is not always necessary. some of the exotic mod and communicating tstat furnaces you have to add the extra transformer so it depends on the model and brand also. with them if in doubt read the manufacturers install instructions.
 

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The transformer is only a ratio of input to output. The lower the input, the lower the output unless it regulated, but I’ve never seen a regulated transformer in HVAC. If regulation is needed for a electronic control card then it’s done on the card itself.
Were I live now the voltage is 125 volts in the winter and drops to 120 in the summer when everybody has the AC on. I used to live in an area where the voltage was 110, but that was a long time ago and manufactures have to be able to have product that will be flexible so I don’t get excited anymore over 5 volts.
 
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