You really need the airflow matched to the capacity of the outdoor unit. Dry climate 400-450 CFM per ton of cooling. Humid climate 350-400 CFM. Low cool is about 75% of high and when you have a coil meant for 100% and move what's said to be about 67% refrigerant through it, it is already at a disadvantage dehumidifying on low. Run 100% airflow and low cool and really could have some issues.
If inside isn't variable speed I wouldn't waste your money on anything but a 13-14 SEER single stage A/C.
You need a variable-speed blower to move the right amount of airflow for each first and second stage. Can it be done without a VS blower? With some creative wiring and controls, maybe, but the unit won't operate at it's advertised SEER or perform as well and why would you even consider a two-stage unit if you don't want the benefit of comfort and efficiency?
Also, the unit will only have a 5 year parts warranty. No manufacturer will give you the full 10 year warranty if the outdoor unit is pieced to an old indoor unit.
Aren't there stats that can stage ( select speed ) the blower per Y1/Y2 ?
Any stat with a Y1 and Y2 terminal can do it. If PSC, you would just need relays to select the right blower speed or directly wire terminals to an ECM. There just won't be any profile for controlling dehum and actual CFM may not be exactly what's needed for each stage.
There are ecm constant torque motors that can provide the proper airflow for a 2 stage unit. Most standard PSC will not allow for proper control.
Variable speed motor ideology is flawed, in that it can cause duct sweating. Lowering air speed to reduce humidity can cause issues if your ducting system is not well insulated and tightly sealed. I’ve witnessed even R8 insulated and sealed ducts dripping like crazy in a crawl space. I’ve seen flex lines saturated internally with water. Poke a hole in them and watch water drain line a faucet.
It all depends on where you’re ducts are located and if you’re in a high humidity area. I’ve seen uninsulated ducts inside walls sweating because over time a house can become loose and allow more infiltration. This will leave water stains on the drywall.
Longer runtime is more ideal for humidity removal, and or a ducted dehumidifier. Staging provides lower capacity and longer run cycles.
My upstairs system maintains below 50%RH. I have a 2 stage with variable speed motor, but it is not configured to reduce air speed on “dehum” demand. It will however reduce speed per “staging” requirements, just like a constant torque can do. The constant torque is far less expensive to replace. I also have a ducted dehumidifier connected to this system.
FYI outdoor RH is 76%.
Last edited by mgenius33; 07-29-2021 at 09:22 AM.
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Originally Posted by VTP99
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