Aprilaire 6504 vs. 6404

[MrGibbage]

Are these controllers really designed to work with the specific Aprilaire thermostats? It looks like they take the usual 4-wires, so why wouldn't any thermostat work? And when you read sites like this, you get even more confused
http://www.iaqsource.com/product.php/april...mp;category=655

quote: "The 6504 works only with the Model 8570 thermostat. The Aprilaire Zoned Comfort Control system can work with virtually all low voltage mechanical or electronic thermostats."

What is the difference between the 6504 and the 6404? Besides the supposed "works with this thermostat or that thermostat".

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[ASIHome]

My understanding is that the 6504 is a non-expandable 4-zone system that uses the 8570 stat and the 6404 is an expandable (up to 12 zones) that uses the 8870 stat.

http://www.asihome.com/ASIshop/index.php?&...acturers_id=225

 

[MrGibbage]

The funny thing is, the installation manual for the 6404 lists (suggests?) several aprilaire thermostats that may be used, and the 8870 isn't one of them.

 

[ASIHome]

So I guess my statement should read:

My understanding is that the 6504 is a non-expandable 4-zone system that uses the 8570 stat and the 6404 is an expandable (up to 12 zones) that uses multiple Aprilaire stats. I wonder if the manual was written before the 8870 came out?

 

[MrGibbage]

Which leads to the next problem/question: Why does the 6504 cost more than the 6404? Only because it *CAN* use the 8570 thermostat? I feel like I'm missing something...