R-410A Retrofit With Existing Line Set and High-Static Ductwork
An installer is replacing a failed 3-ton R-22 split heat pump with a 3-ton R-410A unit in a 1,900 ft² house. The existing line set is 3/8 in. liquid and 3/4 in. suction, 35 ft total, with a 20 ft vertical rise to the indoor coil. The new unit’s installation manual permits a 3/8 in. liquid line and a 3/4 in. suction line for this capacity up to 50 ft equivalent length and 25 ft rise, provided the existing line set is cleaned with an approved flushing agent and a new filter drier is installed. The existing duct system has a return static of 0.55 in. w.c., a supply static of 0.35 in. w.c., and a 0.35 in. w.c. pressure drop across a dirty 1 in. filter. The air handler is rated at 0.50 in. w.c. external static for 1,200 cfm. The homeowner reports the old system was noisy and had poor airflow.
1. Can the existing line set be reused as-is? State the decision and the verification and cleaning steps you would require before connecting the new R-410A unit.
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Reuse is conditionally acceptable, not automatic. Verify four things first:
- The actual line sizes match the new manual: 3/8 in. liquid and 3/4 in. suction.
- The equivalent length and vertical rise are within limits: 35 ft and 20 ft are below 50 ft and 25 ft.
- The line set is not kinked, corroded, reduced at a fitting, or inaccessible for inspection.
- The old mineral oil residue can be removed. R-410A uses POE oil, which can carry residue and sludge to the metering device.
Procedure: recover the old charge, remove the old filter drier, flush with an approved agent if the manual permits, purge with nitrogen during all brazing, install a new bidirectional filter drier, pressure-test with dry nitrogen, and evacuate to 500 microns with a decay test. If any section is contaminated or inaccessible, replace the line set. Because the rise is 20 ft, also follow the manufacturer’s oil-return requirements, such as an inverted trap at the indoor coil if specified.
2. Using the static-pressure readings, explain why airflow is poor and recommend specific corrections.
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The measured external static is 0.55 in. w.c. return plus 0.35 in. w.c. supply, or 0.90 in. w.c., and the filter adds another 0.35 in. w.c. That is about 1.25 in. w.c. total, well above the air handler’s 0.50 in. w.c. rating. Even if the filter drop is included in the return reading, 0.90 in. w.c. is still far above rating. The blower cannot deliver 1,200 cfm against that restriction, so airflow falls, the system gets noisy, and capacity and delta T suffer.
The filter alone consumes 0.35 in. w.c. If supply is 0.35, only 0.15 in. w.c. remains for the return under a 0.50 in. w.c. budget, but the return is 0.55 in. w.c. That is a 0.40 in. w.c. excess before the filter is even counted.
Corrections: replace the dirty 1 in. filter with a low-pressure-drop media filter, enlarge or add a return grille and return duct, reduce flex duct length and sharp turns, seal duct leakage, and verify 350–450 cfm/ton. Do not simply increase blower speed to overcome a duct restriction; that raises noise and motor load and may not solve the root cause.
3. List the commissioning checks you would perform before and after start-up, including charge verification.
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Before start-up: confirm the nitrogen pressure test holds at the manufacturer’s test pressure, confirm evacuation to 500 microns and a passing decay test, verify electrical supply voltage, breaker size, ground, and control wiring, and set the blower for the required airflow. Measure static pressure with a clean filter and correct any excess before charging.
After start-up: weigh in the charge per nameplate plus line-set adjustment, or use the manufacturer’s charging chart. Do not rely on a generic superheat or subcooling target. Record indoor and outdoor temperatures, suction and liquid pressures, superheat, subcooling, delta T, and compressor amps. Verify the reversing valve shifts, defrost initiates and terminates, auxiliary heat is locked out above the balance point, condensate drains, and the outdoor unit drains defrost water away from the foundation. Finally, confirm the homeowner’s noise complaint is resolved at the new airflow.