Rooftop Bus Air Conditioner Installation Guide | TCHAIN

The rooftop bus air conditioner is a core component of the cooling system on coaches, school buses and tour buses. Correct installation not only affects cooling performance and energy efficiency, but also directly relates to vehicle sealing, driving safety and later maintenance costs. Based on the rooftop AC field installation manual, this article systematically covers the complete workflow from roof preparation, unit hoisting and piping connection to leak detection and commissioning, providing actionable guidance for bus OEM technicians, fleet maintenance teams and overseas distributors.

I. Safety Operation Notes
Before installation, safety rules must be strictly followed: wear work clothes and flat non-slip shoes; no slippers, shorts or vests are allowed on the job. Clean the roof surface before working on it to avoid water, oil or dust affecting sealing or causing slips. Avoid scratching the vehicle body or air conditioner housing during installation, and clean the roof and cabin after completion. Open flames are strictly prohibited on the production line. When using voltages above 220V, check electrical safety and verify that wires and sockets are properly insulated.

II. Roof Preparation and Sealing

1. Dust removal and cleaning
   Blow dust from the roof mounting area with compressed air, then wipe the area where the sponge sealing strip will be pasted with anhydrous alcohol or acetone to ensure the surface is free of oil and impurities.

2. Paste sealing sponge strips and damping strips
   Attach rubber-plastic sponge strips, XPE sponge strips and damping pads according to the product structure. When the evaporator uses a large-frame structure, the sponge strip compression must be no less than 3 mm to ensure sealing performance.

3. Glue application and smoothing
   Apply sealant to the joints around the large frame and smooth it evenly. A layer of sealant at least 10 mm thick should be applied on top of the strip without smoothing, so the fiberglass rooftop unit can press firmly into the adhesive and improve sealing reliability.

4. Roof sealing summary
   The sequence is: dust blowing → cleaning → pasting → gluing → smoothing. The outer surface must not expose the base material, and the EPDM strip top adhesive should not be smoothed, ensuring no leakage path between the roof and the unit.

III. Unit Hoisting and Base Fixing

1. Pre-hoisting inspection
   After the unit reaches the installation station, check whether the housing is damaged, whether the drain hose is crushed or flattened, and whether flanges or pipes are deformed. Verify the system pressure (normally nitrogen charged to 0.1–0.3 MPa). If anything abnormal is found, stop installation and report to the quality control department.

2. Rooftop hoisting
   No one is allowed to stand beneath the condenser or evaporator during hoisting. Lower the unit slowly, align it with the roof mounting holes, ensure both sides of the evaporator and condenser are in a straight line after positioning, and then remove the lifting cables.

3. Base fixing
   Fasten the base and cover plates following the principle of "center first, then both sides, tightening both sides simultaneously" to prevent the unit from twisting under fastening force. Use the specified air pressure (5.5–6.3 MPa) and tightening torque (M10 bolts 38–47 N·m).

IV. Condenser/Evaporator Piping and Wiring

1. Piping connection
   All O-rings and O-shaped gaskets must be coated with refrigeration oil to improve lubrication and sealing. The middle pipes must not rub against cover plate edges. When pipes pass through fiberglass holes, install rubber grommets first and then seal with adhesive to prevent air and water leakage.

2. Wiring connection
   Pressure switch wires and condenser fan wiring harnesses must be firmly plugged in and tied. Ground wires must be reliably tightened. All wire pass-through holes need adhesive sealing. Any harness entering from outside the evaporator chamber must be protected against water and air leakage.

3. Cover plate installation and fin straightening
   Straighten any bent fins on the evaporator and condenser surfaces. Tighten cover plate bolts to the recommended torque: M6 9–12 N·m, M8 20–25 N·m. Evaporator and condenser cover plates should be uniform in height with even gaps; cover gaps ≤10 mm, and the gap between the cover plate and roof skin should be 5–10 mm. No debris or tools may be left inside the cover cavity.

V. In-Cabin Piping, Insulation and Wiring Layout

1. In-cabin piping connection and fixation
   Fix the ducts inside the air channel with spacing less than 500 mm. Rear compartment piping follows the same method as in-cabin piping. Pipes must not rub against self-tapping screws or sharp edges; if they must pass such edges, install rubber sleeves for protection to prevent pipe wear.

2. High/low-pressure pipe joint insulation
   The flange joints of rooftop high- and low-pressure pipes must be insulated with clay-like sealant. Low-pressure joints run cold and insulation prevents condensation; high-pressure joints normally exceed 80°C, and insulation prevents heat from radiating into the air duct and degrading cooling performance.

3. Drain hose and wiring harness fixation
   Drain hose clamps should be spaced ≤500 mm to ensure smooth drainage. In-cabin water pipes and wiring harnesses should stay away from heat sources and moving parts; cables must be kept at least 150 mm from heat sources and high-temperature pipes, with surface temperature below 80°C. If unavoidable, add insulating sleeves and keep cables at least 50 mm from moving parts.

VI. Compressor Installation and Connection

1. Compressor installation
   Install the compressor in the designed position, ensuring the mounting bracket is secure and vibration damping is effective.

2. Compressor piping connection
   Remove the plugs on the compressor stop valves, place asbestos gaskets coated with refrigeration oil, and connect the pipes with bolts. Ensure reliable sealing; the bend radius R near joints should be as large as possible, generally greater than 1 meter; pipes should have some movement allowance and be securely fixed to prevent vibration wear.

3. Compressor and alternator wiring connection
   Fix wiring harnesses after looping them once at plug-in locations; keep harnesses away from heat sources and moving parts; cables must be more than 150 mm from heat sources and high-temperature pipes, use insulating sleeves if unavoidable and keep surface temperature below 80°C; keep cables more than 50 mm from moving parts.

VII. Leak Detection, Vacuum and Refrigerant Charging

1. Nitrogen leak detection (pressure hold)
   Charge dry nitrogen into the system to 1.5–2.5 MPa and check all pipe joints for leaks. If the system cannot reach vacuum (large leak) or cannot hold vacuum (small leak), repeat nitrogen pressurization and leak detection.

2. Vacuum pumping
   When vacuum exceeds 98 kPa (740 mmHg), the refrigeration unit can operate normally. If a 10 m³/h vacuum pump runs for 15 minutes (or a 5 m³/h pump for 30 minutes) without reaching the required vacuum, the piping system has a leak; stop vacuuming and re-check for leaks. If the gauge pressure drop within 10 minutes does not exceed 3.4 kPa (25.4 mmHg), the system can be considered leak-free.

3. Refrigerant charging
   The principle is "liquid into high pressure, gas into low pressure." Before operation, close the compressor high- and low-pressure valves; connect the service manifold to the refrigerant cylinder and purge air from the hoses; record the initial weight and set the charge amount. Open the high-pressure valve first to charge liquid refrigerant into the high side; after pressure balance, close the high-pressure valve. Stand the refrigerant cylinder upright, open the low-pressure valve and charge gaseous refrigerant into the low side (the air conditioner may be running to assist). Once the set charge is reached, close the valves, remove the service manifold, check valve stems for leaks and replace all caps.

VIII. Rain Test and Commissioning

1. Rain test
   After the rooftop unit is installed, a rain test must be carried out, focusing on the joint between the roof and the air conditioner. If leakage is found, take corrective measures immediately to ensure good sealing,

2. Test run and system commissioning
   After confirming that all accessories—piping, wiring, compressor, control panel and refrigerant—are installed, turn on the unit according to the manual. Check in sequence whether the compressor, condenser fan and evaporator fan are running, and whether the evaporator fan high/medium/low speeds and fresh-air damper operation are normal. After running for 10–30 minutes, check whether condensate water flows from the drain hose; if not, inspect whether the drain pipe is blocked or crushed.

IX. Initial Running-In
Commissioning should follow these steps after charging the rated refrigerant:
Step 1: Check the controller and make sure the airflow switch and cooling switch are off.
Step 2: Start the vehicle engine and let it idle (do not accelerate).
Step 3: Turn on the airflow switch; the power indicator lights up, and the fan runs at low, medium or high speed.
Step 4: Turn on the cooling switch; the cooling indicator lights up and the compressor starts.
Step 5: Turn on the temperature control switch and set the desired cabin temperature.
Step 6: Turn on the fresh-air switch so outside air enters the cabin, and verify the fresh-air device works properly.
If everything is normal, run the unit for 20–30 minutes, then perform intermittent operation: compressor on for 2 minutes and off for 2 minutes, repeated 8–10 times. If intermittent operation is normal, commissioning is complete.

X. Conclusion
Rooftop bus air conditioner installation is a system-level project involving mechanical, refrigeration, electrical and sealing disciplines. Following the standard workflow of "preparation → hoisting → connection → leak detection → charging → testing → commissioning"  it can effectively reduce risks such as water leakage, air leakage and refrigerant loss, extend the service life of the air conditioning system, and provide bus operators with a stable and reliable cabin environment.