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Old 09-30-2010, 11:01 AM   #1
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what is a 2 stage HVAC system?


Curious if this is a matched 2 speed system where the compressor, blower, and cond fan will all operate at a reduced speed thus reduced load? (not sure how this would reduce operation costs).

Maybe I am off base here.

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Old 09-30-2010, 01:20 PM   #2
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what is a 2 stage HVAC system?


Two stage HVAC means the compressor has two levels (call them high and low) of operation.

The cooling action occurs as the Freon spurts forth from the thin pipe entering the evaporator coil and sizzles against the insides of the thicker pipes of the coil itself. (If where the Freon first hits is already cold, the Freon will migrate further down the coil to sizzle and cool things off.) To get the "low" level the compressor delivers a lower rate of flow of Freon to the coil. The amount of cooling power (BTU absorbed) per ounce of liquid Freon vaporizing is the same. Arriving back at the compressor via the fat pipe, the Freon vapor is still compressed in the same fashion to turn it back to liquid for the next trip to the coil via the thin pipe.

If the fan stays at the same speed, the interior air through the duct past the coil won't get as cool. If the fan also switches to low speed, the air will get just as cool as before but there will be less air going past.

I don't know the formula for whether lots of air not cooled so much versus not so much air cooled down a lot will wring out more moisture (humidity). In either case the air upon exiting the coil area will be at 100% relative humidity unless it was quite dry to begin with. The RH goes down as the air warms up by taking heat from the sides of the duct or commingling with room air upon exiting a louver.

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Last edited by AllanJ; 09-30-2010 at 01:30 PM.
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Old 09-30-2010, 01:31 PM   #3
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what is a 2 stage HVAC system?


Thanks Allen. That confirms my thinking on the system. I am still trying to contemplate why this would be advantageous for efficiency? I do understand the reduced refrig flow along with blower matching to achieve a similar ^T across the evap coil but not sure how this would affect the efficiency since there is a constant heat load with any environment.

I guess in a reduced (stage 1) state, possibly at cooler temps, the system would run longer giving more opportunity to control the humidity better? Or is this possibly in an effort to apply a finer control on the room temp swing to achieve a more constant temp?

Is this sort of the other side of the variable speed blower systems for high eff furnaces? Again, more of a comfort issue rather than actual efficiency?
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Old 09-30-2010, 01:44 PM   #4
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what is a 2 stage HVAC system?


The ECM motors used on these systems also use less electricity which helps obtain the efficiency
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Old 09-30-2010, 02:42 PM   #5
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what is a 2 stage HVAC system?


Per my understanding of ECM motors, these are referred to as DC brushless motors but are really just 3ph AC motors being operated with a square wave PWM controller? I could certainly see where that would be beneficial in both eff and noise.

That would make them efficient when operating at lower speeds but the actual power required to change indoor temps would remain to same thus my reason for asking. Just not sure how this would improve eff or of the switch to the variable speed system is where the real savings are? Higher eff of the motors?
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