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Old 06-05-2012, 11:05 AM   #1
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2nd stage a/c


My 2 stage a/c is oversized and never (once, actually) has upstaged except during normal use, except during PMs when the tech forces it. 2 stage stat by the way.

There are times when second stage would come in handy, like in late afternoon, when roof heat is radiating into the 2nd floor.

Is there a way utilizing a switch in conjunction with Y1 & Y2 to accomplish this?

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Old 06-05-2012, 11:13 AM   #2
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2nd stage a/c


Post manufacturer and model numbers of current thermostat, outside unit, inside unit, your location, set point temp for thermostat.

You can cheat with a teo stage capable thermostat with the swing and diff. As for your question, the higher stage would only come on if the lower does not satisfy within a given time frame.

Also, to get rid of the sun gain, use UV film in conjunction with keeping the drapes and shades closed during the daylight hours. I it is a second floor that is having problems, you may not have enough return flow, or need a zoned system.

Hard to really tell you, with as little as you gave.

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Old 06-05-2012, 01:17 PM   #3
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2nd stage a/c


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Originally Posted by gregzoll View Post
Post manufacturer and model numbers of current thermostat, outside unit, inside unit, your location, set point temp for thermostat.
Rheem Mod furnace RGFD090.etc., RARL048JEZ a/c, TST411..stat. SP = 74/75. NJ burbs of Philly.

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You can cheat with a teo stage capable thermostat with the swing and diff. As for your question, the higher stage would only come on if the lower does not satisfy within a given time frame.
I use the fast staging setting for both heat and cool. A/C will run 2 - 3 cycles per hour. Wouldn't want to lengthen the off time as temperature swing (especially upstairs) would be unacceptable. My purpose is to (on my demand) provide more cooling (airflow) during peak times.

I was under the impression that 2 stage stats had more intelligence than a simple timer. No?

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Also, to get rid of the sun gain, use UV film in conjunction with keeping the drapes and shades closed during the daylight hours. I it is a second floor that is having problems, you may not have enough return flow, or need a zoned system.

Hard to really tell you, with as little as you gave.
Heat gain through windows not a problem as there is plenty of shade.
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Old 06-05-2012, 01:50 PM   #4
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2nd stage a/c


Most 2 stage A/C stats will have to see an increase in temps after stage 1 is energized to turn on stage 2. No sense bringing on stage 2 if load is being handled by stage 1. Many will also bring on stage 2 if the temp is more than so many degrees out of setpoint (like 3 or so).

Not the same as 2 stage heating with the new multi/variable furnaces which primarily work on timers as you are questioning as they are hooked up single stage most often and the timers cut in automatically through the control board.
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Old 06-05-2012, 09:41 PM   #5
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2nd stage a/c


Two-stage systems are often installed because they feature other benefits, like a higher SEER (efficiency) rating, quieter operation, and sometimes a better warranty. Contractors are normally worried about oversizing a/c units because of the indoor humidity problems this will cause. Two-stage equipment solves this.
I would wonder if running your system on 2nd stage would simply cool your first floor faster, satisfying the thermostat and turning the system off, which would not benefit your second floor at all. Have you tried letting your indoor fan run continuously? This will mix the air between the floors even when the a/c is off. Beyond this, manual dampers or a zoning system would be your best bet.
However, if you are determined to upstage during periods of warmer weather, you could install an outdoor thermostat between the low-voltage Y1 and Y2 wires. Set it at 85 degrees or so. When it is over 85 outside, the 24-volt signal on Y1 will bring on Y2 also. The normal Y2 operation from the thermostat will not be affected. Y2 will "backfeed" into the furnace and kick the blower up to high speed also.
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Old 06-06-2012, 02:04 PM   #6
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2nd stage a/c


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Two-stage systems are often installed because they feature other benefits, like a higher SEER (efficiency) rating, quieter operation, and sometimes a better warranty. Contractors are normally worried about oversizing a/c units because of the indoor humidity problems this will cause. Two-stage equipment solves this.
It didn't solve mine! Went from 4T SS to 4t2S. I originally sized the unit at 3.5T, but contractor 'upsold' me. Cost me 200 CFM.

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I would wonder if running your system on 2nd stage would simply cool your first floor faster, satisfying the thermostat and turning the system off, which would not benefit your second floor at all.
It most likely would. I was thinking that during 'peak' times, I would engage 2nd stage while also lowering the set point one degree--an amount insufficient to bring on 2nd stage immediately. I suppose I could just goose the stat 3 degrees (should jump to 2nd) and allow it to run like that for a period.

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Have you tried letting your indoor fan run continuously? This will mix the air between the floors even when the a/c is off. Beyond this, manual dampers or a zoning system would be your best bet.
However, if you are determined to upstage during periods of warmer weather, you could install an outdoor thermostat between the low-voltage Y1 and Y2 wires. Set it at 85 degrees or so. When it is over 85 outside, the 24-volt signal on Y1 will bring on Y2 also. The normal Y2 operation from the thermostat will not be affected. Y2 will "backfeed" into the furnace and kick the blower up to high speed also.
I tried that once, but was greeted with a hot humid blast right after end of cooling call. I have a programmed circulating stage, but it would not kick in until an hour after cooling call ends. Meanwhile, it would have cycled 2 or 3 times.

Is that connection (outdoor stat) a short between Y1/Y2?

Thanks for directly addressing my (how to) question. Wouldn't a SPST switch conveniently placed indoors accomplish the same thing? Would allow me to choose 2nd stage.

Thanks for your input.
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Old 06-06-2012, 06:36 PM   #7
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2nd stage a/c


Have you seen this thread?Zone Existing System?

If that control can bring on a single stage for one zone and both stages for two zones calling that would be your answer. No need for bypass dampers with the two stage system.
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Old 06-06-2012, 08:45 PM   #8
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2nd stage a/c


Yes, a spst toggle switch between Y1 and Y2 would accomplish the same thing, just manually instead of automatic.

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