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Old 01-02-2009, 11:23 PM   #1
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2 Air Handlers 1 Thermostat


I have a coal fired boiler paired with an oil fired boiler as an emergency back-up (as when my wife lets the coal fire burn out when I'm not around). My house is large (about 4000 sq. ft) with two air handlers with hot water coils. (2 zones) I would like to run these two air handlers simultaneously with one thermostat if possible as the heat load is nearly balanced between the two zones, I'd like to reduce heating cycles on the boiler by always heating both zones at the same time. (Or somehow syncronize the two thermostats...) My wife would be comfortable if the house was 85 degrees, so a slight over-run in one zone isn't an issue. I've tried several different wiring configurations and there must be something major that I'm missing. Independantly the units run fine with their given thermostats, but when I try to "parallel" the second unit into one thermostat, the second unit fan won't kick on. I've heard that there are twinning kits out there, but I thought that I wouldn't need that because I'm not dealing with 2 stage heating or cooling , just fans circuits. I'm fairly certain I'm missing something with the control inputs on the control board in the airhandler. I can't find any schematics of the control boards, just block diagrams. I originally thought it would be as simple as paralleling the W2 and C connections between the two air handler control boards... but that was obviously incorrect... that's why I'm here. Any suggestions would be fantastic.
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Old 01-03-2009, 07:18 AM   #2
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2 Air Handlers 1 Thermostat


both AHUs have their own tranformer so your basically closing R/G for the fans from the single each stat setups.should be able to stack the Rs then the Gs into the single controlling point to run both at the same time........no......yes?might have to add isolating relays to close the R/G on each AHU,and add a TR to close the relays from the closure call R/W on the single controlling stat.will check back latter
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Old 01-03-2009, 11:18 AM   #3
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2 Air Handlers 1 Thermostat


Biggles- Thanks for the reply... I was doing some thinking overnight and looked up AHU Twinning kits and found one on Grainger.com for $287.00 (ouch). (http://www.grainger.com/Grainger/items/6MD94) After looking more closely at the outputs, it was easy to see that they are isolating via relays the two AHU's 24VAC circuits and providing a new 24VAC line for the controlling thermostat. (Which is precisely what you said...) I was able to replicate what they were doing with a few Honeywell R8222 relays that I had laying around (at a price of about $18) and a few left over contacts from a 24VAC circulator control. Granted I'm not running any outdoor units or 2 stage heating or cooling, so it's much more simple than the Johnson Controls item, but it works great now!

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Old 01-03-2009, 05:30 PM   #4
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2 Air Handlers 1 Thermostat


they call that surviving $$$......... happy new year
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