1st stage vs 2nd stage cooling


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Old 06-20-09, 11:15 AM
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1st stage vs 2nd stage cooling

Quick question - we just finished wiring up everything for central air. The condenser isn't here yet, but we were checking voltages on the tstat wire at the condenser location.

Both Y1 and Y2 were live with 25vac, so I'm assuming the 4 degree difference in temp required high stage. So that's the question, when you are in high stage, are both Y1 and Y2 energized, or should it be that in high stage, on Y2 is, and in low stage, only Y1?

Thanks!
 
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Old 06-20-09, 01:31 PM
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yes both stages will call and most of the time second stage will stay on till it has shut off.
 
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Old 06-20-09, 04:53 PM
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Thanks. So in what conditions will the 1st stage call only? A minor temp difference?
 
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Old 06-20-09, 06:30 PM
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ya or a time i think some brands do like 10 minutes some thermostats can change the temp difference that will kick on 2nd stage some of the newest stuff i think also tie in humidity to decide high or low
 
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Old 06-20-09, 06:53 PM
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Thanks. I don't have a humidistat on my system, so I don't think humidity would be the reason on mine. I'm just trying to test out all the wiring from the electrical work today. So just trying to get just the first stage running to confirm all is correct.
 
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Old 06-22-09, 05:59 AM
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It all depends on your t-stat. A honeywell VisionPRO IAQ will only use first stage until it has reached 90% capacity. It calculates the capacity of the first stage based on run times and # of cycles.
 
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Old 06-22-09, 06:15 AM
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I'm using a Honeywell RTH7600D.
 
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Old 06-22-09, 06:33 AM
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There will be a temperature differential between the first and second stages on the thermostat. I don't know the spec for that model but it's usually in the neighborhood of 1.5 degrees. In HVAC speak it's called the interstage differential. So power up the stat and lower the setpoint one degree at a time until only the frst stage energizes.
 
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Old 06-22-09, 06:42 AM
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So if room temp is say 76, and I set the tstat to 75, it should energize the first stage only (Y1). But if I set the desired temp to 74, or 73, both Y2 should become energized, with Y1 still energized as well.

Sound right?
 
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Old 06-22-09, 07:16 AM
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Yes. According the wiring diagram for the outdoor unit, 1st stage is starting the compressor and 2nd stage is operating the compressor solenoid which then loads the compressor up to full capacity. In the opposite direction, as the temperature is satisfied, the 2nd stage de-energizes which unloads the compressor and, when the setpoint is satisfied, 1st stage will shut off the compressor.
 
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Old 06-22-09, 06:00 PM
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Thanks. Here's another question - does the air handler perform any differently, like the compressor, when called by stage 1 vs stage 2? Just wondering about the wiring. For the second stage, the wiring goes through a bundle of wires that comes off the circuit board, but the first stage is just pigtailed with the wire from the tstat and the wire on the air handler for the tstat.
 
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Old 06-22-09, 06:45 PM
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The indoor fan motor is variable speed.
 
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Old 06-24-09, 09:51 AM
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Originally Posted by StevenG
Thanks. Here's another question - does the air handler perform any differently, like the compressor, when called by stage 1 vs stage 2? Just wondering about the wiring. For the second stage, the wiring goes through a bundle of wires that comes off the circuit board, but the first stage is just pigtailed with the wire from the tstat and the wire on the air handler for the tstat.
There should be two separate wires for the air handler from the t-stat. There should be a Y1 and Y2 (or YLo and Y) wire. When Y1 is powered, the air handler will run the blower in 1st stage to match the 1st stage requirements of the condenser. When Y2 energizes, the air handler ramps up to full blower speed for the 2nd stage of the condenser.

Your T-stat must have terminals for both Y1 and Y2 for it to work properly.
 
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Old 06-24-09, 12:24 PM
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Correct. The tstat has a Y and Y2. They are connected to the corresponding wires on the tstat wire bundle on the air handler. Then, from the air handler, there is a wire for Y/Y2, which is used for Y2 since I have a 2 stage condenser, and then Y1 is pig tailed with the Y1 wire from the tstat and the air handler.

Just found out my condenser is in too, so we'll be hooking it up friday.

I was really just curious if only one of the Y1 and Y2 wires were energized at a time, or if both were when calling for high stage. Sounds like both should be, which tells me everything was wired correctly.
 
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Old 06-24-09, 11:13 PM
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Originally Posted by badtlc
It all depends on your t-stat. A honeywell VisionPRO IAQ will only use first stage until it has reached 90% capacity. It calculates the capacity of the first stage based on run times and # of cycles.
Sounds like something worth owning, makes more sense to me than a slight delta-t. Other opinions?
 
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Old 06-25-09, 06:22 AM
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Originally Posted by PoorJohn
Sounds like something worth owning, makes more sense to me than a slight delta-t. Other opinions?
Be warned it is expensive. I'm sure Honeywells other than the IAQ use the same method but i LOVE my IAQ t-stat. It was worth the expense.
 
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Old 06-25-09, 06:34 AM
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Supposedly a Honeywell with the Smart Response technology learns how long it takes your system to reach a set point. Not sure how much truth there is. I know it uses it to reach a set temp by the time you set in a program as well.
 
 

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