R-22 to R-410A Swap With a Mismatched Indoor Coil
A 2.5-ton R-22 condensing unit fails on a 1990s house. Because an exact R-22 replacement is unavailable, the contractor proposes installing a 3-ton R-410A condensing unit. The existing indoor coil nameplate reads 'R-22 only, 2.5 ton, piston 0.071.' The line set is 30 ft of 3/8-inch liquid and 3/4-inch vapor with mineral oil throughout. The plan is to flush the line set with solvent, install a new R-410A TXV on the existing coil, set the new condensing unit on the pad, and charge by the weight printed on the new unit's nameplate. The existing furnace delivers 1,000 CFM at 0.5 in. w.c. external static, and the ductwork was sized for 2.5 tons. The homeowner wants the least expensive path that 'works.'
1. Identify the technical problems with this plan, addressing the coil, the line set, and the charging method.
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Three separate failures are built into the plan.
Coil. A coil stamped 'R-22 only' is not listed for R-410A. R-410A systems run roughly 50–70% higher pressures and require POE oil, and the coil's tubes, headers, and brazed joints may not be rated for it. More practically, a 2.5-ton coil cannot absorb 3 tons of capacity. The condenser will flood, subcooling will climb, suction pressure will run low, superheat will run high, and the system will deliver roughly 2.5 tons while the compressor works harder than necessary.
Line set. The 3/4-inch vapor line is generally acceptable for 3 tons at 30 ft, and 3/8-inch liquid is fine, so sizing is not the main problem. The mineral oil is. Solvent flushing rarely removes all of it, and residual mineral oil plus residual R-22 chlorine in a POE system creates acid and sludge. The line set should be replaced or at minimum blown and triple-evacuated with a new filter drier.
Charging. Weighing in the nameplate charge assumes a matched indoor coil, a 15 ft line set, and design airflow. All three assumptions are false here, so the weigh-in will be wrong before the unit even starts.
2. What is the correct approach, and what must be verified before quoting the job?
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Replace the indoor coil with a coil listed for R-410A and rated at 3 tons, and use a TXV matched to that coil and to the outdoor unit.
Verification steps before quoting:
- Confirm an AHRI-listed match between the new condenser, the new coil, and the existing furnace, and pull the match's rated capacity and airflow.
- Verify the blower can deliver about 1,200 CFM at the actual external static. At 1,000 CFM the system is at 333 CFM/ton, which invites coil icing and low suction. Ductwork sized for 2.5 tons may need a supply trunk or return upgrade.
- Check the line set against the new unit's installation manual for equivalent length and vertical rise; replace if marginal or contaminated.
- Require nitrogen pressure test to the manufacturer's spec, evacuation to 500 microns with a standing decay test, a new bidirectional filter drier, and weighing in charge with an adjustment for actual line length.
- Confirm the electrical supply matches the new unit's nameplate (R-22 units are often 208/230 V single-phase, but verify MCA/MOP and breaker size).
If the budget cannot support a matched coil, the honest answer is a properly matched 2.5-ton R-410A condenser and coil, not a 3-ton condenser bolted to an R-22 coil.
3. If the contractor proceeds as proposed, what startup readings would you expect, and why?
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Expect a system that looks undercharged and overcharged at the same time:
- Low suction pressure and high superheat, because the small evaporator cannot pass enough refrigerant to satisfy a 3-ton compressor. Superheat might run 25–35°F.
- High subcooling, because the condenser is oversized relative to the coil and excess liquid backs up in the condenser, perhaps 18–25°F.
- Low supply-to-return temperature split and a compressor that runs hot, since capacity is capped near the coil's rating while the compressor does more work per delivered ton.
- A compressor drawing near or above RLA in high ambient, with a hot shell and possible overload trips.
The building will be no better off than with a correct 2.5-ton system, and the equipment warranty may be void because the coil is not an approved match. This is the classic mismatch signature: high superheat plus high subcooling with a clean coil and correct airflow points at a capacity/flow mismatch, not at charge.