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Old 09-03-13, 10:12 AM   #1
mejunkhound
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Default small compressor with oversized evap/cond

Replaced the 4T scroll compressor in my 4T Rheem air-air unit with a 2T compressor.

Mollier diagram says I should be able to boost COP from 3.5 or so to over 4 at 45 F, also put on lower speed evap and cond fans.

As soon as the weather turns cold, will see how well it works and post the results, noticeably lower noise levels seen immediately.

The 4T unit experienced severe frosting here in PNW with the 45F and 95% humidity days, hope to keep the evap temp above 32 F with having a 2x oversized evap.

PS: no need to worry about lack of BTUs at 20F and below, system switches over to GSHP at 40F (may adjust that also) and also have wood furnace backup.

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Old 11-09-13, 03:25 PM   #2
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That's all the newer high SEER units do. Large coils with small compressors to get head pressure down.
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Old 11-09-13, 04:42 PM   #3
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Unless the compressor was failing/has failed and needed to be replaced anyways (in which case you'll need to find out why it failed), it's more economical to do the "Davuluri Treatment" by changing the refrigerant and expansion valve. Use ES12a (and a R134a TXV) for a R22 compressor and ES22a/R433b (and a R22 TXV) for a R410a compressor. Tune the exact charge by aiming for 10-15F of subcooling.
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Old 11-10-13, 08:31 AM   #4
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Yep, comp had failed (3 screws holding lower bearing in place came loose, copeland scroll, covered in detail in another thread)

Since the condensor is also oversized (HP mode), I also overcharged for best subcool readings.

Running great, do not even notice that duct temps are only 95 F now (vs. 110F before). Also added a relay to drop the blower to lower speed when in the 2T mode.

Left the evap fan at full airflow, as even with 41F air temp, the evap does not get pulled below 32F which eliminates the unit ever going into defrost mode.

note: at 41F, my outdoor tstat automatically switches over to my $500 5T GSHP, covered in one of the sub-threads in the legendary homemade heat pump thread.

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