Reusing an R-22 Line Set for an R-410A Roof-Mounted Heat Pump
A 3-ton R-410A heat pump is being installed on a roof 22 ft above the indoor coil. The existing line set from an R-22 system is 75 ft of 3/8 in. liquid and 3/4 in. suction, with four 90° elbows and one 45° elbow. The new manufacturer's installation manual allows up to 100 ft equivalent length and 50 ft vertical separation, and specifies 3/8 in. liquid and 3/4 in. suction for this capacity. The installer plans to reuse the line set, braze the new connections without nitrogen, pressure test to 150 psig with nitrogen, and evacuate to 500 microns. The suction line is insulated with 1/2 in. wall elastomeric insulation. The roof penetration is not sealed, and the line set is supported every 10 ft.
1. Calculate the equivalent length and determine whether the existing line set meets the manufacturer's limits. What other line-set issues matter beyond length and diameter?
Read the worked answer
Using the manufacturer's equivalent-length table, or common allowances of about 5 ft per 90° elbow and 2 ft per 45° elbow:
- Straight pipe: 75 ft
- Four 90° elbows: 4 × 5 = 20 ft
- One 45° elbow: 2 ft
- Total equivalent length: 75 + 20 + 2 = 97 ft
That is within the 100 ft limit, and the 22 ft vertical rise is within the 50 ft limit. The 3/8 in. liquid and 3/4 in. suction sizes match the manual for this capacity. However, length and diameter are not the only issues:
- The line set previously carried R-22 and mineral oil. R-410A uses POE oil, which is hygroscopic and can be contaminated by residual mineral oil, acid, or moisture. The manufacturer may require replacement or an approved flush.
- The existing line set may be kinked, contaminated, or have hidden joints.
- The additional length may require adding refrigerant per the manual.
- The roof penetration and support spacing must be corrected.
Recalculate with the exact table in the manual, because equivalent-length allowances vary by manufacturer.
2. Critique the proposed brazing, pressure-testing, and evacuation procedure. What changes are required?
Read the worked answer
The proposed procedure has three significant errors:
- Brazing without nitrogen purge: air inside the tubing oxidizes during brazing and forms carbon scale and copper oxide. That debris travels through the system, can plug the TXV screen or filter drier, and contaminates the compressor oil. Dry nitrogen must flow through the line set during brazing.
- Pressure testing at only 150 psig: R-410A systems are typically tested at 500–550 psig with dry nitrogen, following the manufacturer's limit and isolating any component with a lower rating. A 150 psig test will not reveal leaks that appear at operating pressure.
- Evacuation alone: evacuate to 500 microns or below and hold the vacuum to confirm no moisture or leaks. If moisture is suspected from the old line set, use triple evacuation with nitrogen breaks and install a new filter drier.
Also leak-check all braze joints, replace the filter drier, and confirm the line set is clean and dry before charging. Never pressure-test with oxygen or refrigerant.
3. What suction-riser oil-return details would you require for the 22-ft vertical rise?
Read the worked answer
Oil return must be addressed because refrigerant carries only a small amount of oil up a vertical suction riser, especially at low load. Requirements:
- Follow the manufacturer's piping diagram. A common practice is an oil-return trap at the base of the riser and additional traps at intervals recommended by the manufacturer (often every 20 ft of vertical rise).
- Install an inverted trap at the top of the riser to prevent oil and liquid refrigerant from draining back into the evaporator during the off cycle.
- Slope horizontal suction lines in the direction recommended for oil return; for a heat pump, confirm the requirement for both heating and cooling modes.
- Avoid double trapping or creating an oil-logged section.
- Support the line set per code and manufacturer spacing; seal the roof penetration and protect the insulation.
After startup, verify suction pressure, superheat, and compressor oil level. Poor oil return can cause compressor bearing wear or seizure even when pressures initially look acceptable.