Amana Heatpump 15-17 Delta T

[DNMechanical]

My house was built in 2008, has Amana 13 SEER Heatpump, R22 piston metering, 3.5 ton. The house is cooling and heating ok. Avg summer bill is about $130-140, highest was $210 which is not bad for living in TX. But my only concern is I only get 15-17 degree drop across the Evap coil. I charged according to superheat,subcool, indoor wetbulb and outdoor drybulb. To my knowledge my charge was nearly perfect. Today, I took out the piston and reference to GOODMAN piston size chard and found out the piston was wrong size. it used .73 and chart said .82. I installed a .82 that came with condensor. Now i got 17-18 temp drop... no matter what I do, i just can not get 20 degrees drop... is this normal???

[superd77]

Yes. 20 is abnormal.

[DNMechanical]

20 is abnormal? can you please elaborate?

[HVAC/Stud]

Do you have the correct air flowM

[skippedover]

HVAC is a science, not a guessing game. You should always size equipment to the house load at design temperature. The duct system and supply outlets should all be sized according to the airflow needed for the house load. The air handler should be sized to deliver the necessary CFM at your design static and your condenser should be selected to match the air handler. Once all of this is accomplished, then a proper refrigerant charge can be determined using the appropriate charging criteria for superheat or subcooling.

In days gone by, we'd expect to see 20°-25°F TD across the indoor coil. Today we're looking for 15°-18°F TD across the indoor coil. If the system is designed and installed according to the science, then the results will be consistent and repetitive. Guessing is eliminated and the customer can be guaranteed a comfortable home.

Anything less is cutting corners, which leads to guessing, which leads to unpredictable results.

[mark beiser]

20 is abnormal? can you please elaborate?

While a 20º split is within what is normally considered the acceptable range, it is at the higher end of it, and will typically only be seen if the indoor humidity is a little low, or the system is just a little light on airflow, but not critically so.

[BaldLoonie]

I don't think you are out of line. SO many factors. Humidity & air flow especially. As Mark said, lower humidity and a bit light on airflow increases the drop. 73 piston on a 3.5 ton was likely starving the coil so your better performance out of the bigger coil makes sense. I hate pistons, TXV gives you maximum performance, especially under lower loads and lower outdoor temps. A cool evening with a bit of a load, for example, can lead to a fairly high superheat and part of the coil not doing you much good.