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Help with wiring an Aprilaire 700M to a Trane XR90 and Venstar T7900 thermostat

Help with wiring an Aprilaire 700M to a Trane XR90 and Venstar T7900 thermostat

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  #1  
Old 02-08-16, 09:49 AM
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Help with wiring an Aprilaire 700M to a Trane XR90 and Venstar T7900 thermostat

Hoping that someone here can help me out. I'm trying to finish up installing an Aprilaire 700M whole-house humidifier system with my existing furnace. I want to control the humidifier directly from a new thermostat, and have it provide humidification independent of the furnace's call for heat. The furnace is a Trane XR90, and the new thermostat is a Venstar T7900. I've already installed the humidifier on the Supply side above the A/C coil, but have not wired it up yet (hence this post). Water is being supplied by a hot water line with a T-valve to the humidifier. I'd like to finish up the install, but I'm unsure of how I need to wire it based on what I have and what I want to do. Here are my questions, and my thanks for anyone who is willing and able to help me out with this:

1. Biggest question is the wiring circuit. I've included pics of the Venstar thermostat's connections and the Trane XR90's control board. The Aprilaire 700M just has 2 wires coming out of it, and does come with a 24V transformer that can be used if needed. There is an 8-wire cable going between the existing thermostat and the furnace, so I should have enough wiring to make connections between the thermostat and the humidifier, but may have to extend from the furnace's control area to the humidifier with some of the unused wires to complete the circuit. What I don't know is what needs to be wired to what. I think the W3/Aux connection on the Venstar T7900 thermostat needs to go to one of the 700M wires, but don't know if the humidifier's other wire needs to go to the furnace's blower motor and if the transformer is needed in any part of the circuit. Also don't know if I need some type of fan relay to go between the furnace's blower motor and either the thermostat or humidifier. In regard to the XR90's control board, I'm pretty sure that nothing is plugged into the HUM-H or HUM-N blades on the bottom of the board (but will have to take a better pic of the board to validate). I'm including a generic pic of the board here for reference now. Ultimately, I'd like to get the entire circuit figured out before I start making any changes (for obvious reasons).

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Name:  Trane XR90 Control Board - General.jpg
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2. I can add an electrical outlet to power the 700M humidifier, but I've also read that it can be hooked up to the furnace's control board - as long as the furnace has the extra amperage to support it. I'm thinking it would be easier/safer to just run the electrical outlet, but wanted to get some opinions before doing so. There is a junction box that contains a switch to turn on/off the furnace, but I have not pulled the cover to look at the wiring to determine if I can or should tie the humidifier in there. I'm thinking there could be a BK/3 and a WH/1 wire to tie the humidifiers 120V power in, but don't know if that means I cut the end of the plug off or do something else. Also including a pic of the XR90 wiring diagram for control board (the portion obscured by the watermark is for Note 4, which does reference a humidifier or EAC connection there.

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Name:  Trane XR90 Wiring Diagram.jpg
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3. The Trane XR90 furnace has a Skuttle Make-Up Air system installed on it. When we were inspecting the house, a former building inspector for our township said that we should block it so that cold air would not be brought into the furnace and house. I went ahead and did that on his recommendation, and itís been that way for over 9 years. I finally did some research on it, and it looks like things would be better off if I unblocked it and allowed it to work again. I didnít know if it would be a factor one way or another for the humidifier install. What are your thoughts and recommendation on it?

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Once again, I'll be extremely grateful for any help somehere can offer on this. I'd really like to get this buttoned up, as I have several more projects I need to get moving on.

Thanks in advance,

Todd
 
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  #2  
Old 02-08-16, 11:12 PM
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Welcome to the forums.

I just love a product with little to no technical information available. I haven't used this stat and know very little about it other than it's needlessly complicated to run.

It looks like the stat can control the humidifier directly. If that is the case then the following wiring diagram will be what you need.......

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Old 02-09-16, 06:45 AM
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Thanks for the reply, PJ! I might have made a bad call on choosing the thermostat. I thought about getting the Nest, but wasn't as crazy about it's interface. I definitely needed one that could directly control the humidifier, and initially liked the features/interface of the Venstar but maybe it will end up being something I hate when I start to program it.
 
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Old 02-11-16, 09:10 AM
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PJ, did you have any thoughts on my last question in the post regarding the Skuttle Make-Up Air system on the furnace? It's currently plugged off, but didn't know if I should open it back up before finishing the humidifier install.

Thanks

Todd
 
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Old 02-15-16, 05:51 AM
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PJ, just wanted to send a huge thank-you your way for helping me with wiring diagram. I wired it up this weekend, and everything worked perfectly! Thanks again for taking the time to help me out!

All the best,

Todd
 
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Old 02-15-16, 10:10 AM
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Thanks for stopping back and letting me know.

Sorry I missed your question about the Skuttle unit. I don't really know how to answer that question. Some of those units are helpful and some are not.
 
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