Commissioning a New Ductless Mini-Split in a Bedroom Addition
A contractor installs a single-zone ductless mini-split (12,000 Btu/h) in a 300 sq ft bedroom addition. The line set is 25 ft of 1/4 in. and 3/8 in. copper with a 15 ft vertical rise, and the outdoor unit sits on a wall bracket above a deck. The manufacturer's pre-charge covers 25 ft of line set. The installer pressure-tests with nitrogen to 550 psig for 30 minutes, holds vacuum to 500 microns for 15 minutes, then releases the factory charge and opens the service valves. The homeowner reports the unit cools but the room feels humid and the outdoor unit is noisy at night. The installer did not record the pressure-test duration or vacuum decay.
1. Evaluate the pressure test and evacuation procedure against good practice, and identify what is missing.
Read the worked answer
A 30-minute nitrogen pressure hold at 550 psig is short; good practice is a longer hold, often a minimum of one hour and preferably longer for larger or longer line sets, with the pressure recorded at start and end. A 15-minute vacuum hold at 500 microns is also brief; the standard is to evacuate to 500 microns or below and hold with the pump isolated for at least 15-30 minutes, verifying the vacuum does not rise above 500 microns, which would indicate moisture or a leak. The installer failed to record the pressure-test duration, starting and ending pressures, and vacuum decay, so the commissioning record is incomplete. Without documented decay data, moisture or a small leak cannot be ruled out, and the factory charge release may have masked a problem.
2. Explain why the room feels humid despite cooling, and what commissioning measurements would confirm the cause.
Read the worked answer
A ductless head that is oversized for a 300 sq ft bedroom, or one running at high fan speed, will satisfy sensible load quickly and short-cycle, removing less moisture. At 12,000 Btu/h for 300 sq ft, the unit is likely oversized, so runtime is short and latent removal is limited. High fan speed also raises coil temperature and reduces dehumidification. The installer should measure supply and return dry-bulb and wet-bulb temperatures, calculate the actual sensible and latent capacity, and check indoor relative humidity. Confirm the unit is set to a fan speed that supports dehumidification, such as auto or low, and consider a dry mode if available. If humidity remains above 55% RH, a correctly sized unit or a dedicated dehumidifier may be needed. Document the measurements.
3. Address the nighttime noise complaint and the vertical-rise installation details.
Read the worked answer
Noise at night may come from the wall bracket transmitting vibration into the structure, from refrigerant flow noise in the line set, or from the outdoor fan cycling at low ambient. Check that the bracket uses vibration isolators and that the unit is level and not contacting the siding. Verify the line set is supported and not vibrating against framing, and that the vertical rise is within the manufacturer's limit; a 15 ft rise is usually acceptable but must be confirmed against the installation manual, and an oil trap may be required above certain rises. Confirm the drain line slopes properly and does not gurgle. Record the final bracket torque, line set support spacing, and any accessories added. Explain to the homeowner that nighttime noise often reflects mounting and line-set issues rather than a defective unit.