Read the guide: Why does my house shake at night?
How to test your AC unit
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Step 1: Set the thermostat a few degrees below room temperature so the AC runs. Wait about 5 minutes for it to settle.
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Step 2: Place your phone flat, screen up, on a solid part of the outdoor unit's top panel, away from the fan grille. If there's no flat spot clear of the grille, don't test on top.
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Step 3: Tap Start (on iPhone, tap Allow). Step back and keep your hands off the unit while it measures for 30 seconds.
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Step 4: Take the phone off as soon as it's done. The top panel gets hot in the sun, and warm air blows out of the grille.
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Step 5: Note the main frequency. Then save an image of the result to compare after a service visit or next season.
What your result means
Your result lands on one of four levels. This page uses the machines and appliances scale, which is based on RMS velocity in millimeters per second (mm/s).
| Level | Range (RMS velocity) | What to do |
|---|---|---|
| Smooth | Under 0.71 mm/s under 0.028 in/s | Your unit is running smoothly. The hum you hear is normal fan and compressor noise. |
| Normal | 0.71 to 1.8 mm/s 0.028 to 0.071 in/s | Normal for an outdoor unit. Save this result so you can compare it next season. |
| Rough, check it | 1.8 to 4.5 mm/s 0.071 to 0.177 in/s | More shake than it should have. With the power off, clear debris around the unit, check that it sits level on its pad, and look for loose panel screws. If it stays high, book a service visit. |
| Too much vibration | 4.5 mm/s or more 0.177 in/s or more | Too much vibration. If you hear banging, grinding or scraping, turn the system off at the thermostat and call an HVAC technician. Running a damaged fan or compressor can make things worse. |
What causes AC unit vibration
Some AC unit vibration is normal. An outdoor unit has two big moving parts, a fan and a compressor, bolted into a thin metal cabinet. You’ll always feel a hum on the top panel. What you’re checking for is more shake than usual, or a shake that grows over time. The frequency can hint at which part is to blame.
The fan
Many outdoor fan motors turn at 825 or 1,075 RPM, about 14 or 18 Hz. That’s inside your phone’s range. A bent, dirty or loose fan blade shakes the unit once per turn, so a strong main frequency near the fan’s speed points at the fan. Some newer units have variable-speed fans, so their speed changes. The RPM to Hz converter turns the RPM on the motor’s label into Hz.
The compressor
A fixed-speed compressor runs at roughly 3,000 to 3,600 RPM (50 to 60 Hz). That’s faster than your phone can measure. If the result says “The main vibration may be faster than your phone can measure,” the compressor is a likely source. Inverter heat pumps and variable-speed ACs change compressor speed, so their readings vary from test to test.
Stay safe around the unit
The outdoor unit has high voltage and a fan that can start at any time. Before you touch anything, turn the system off at the thermostat. Then pull or switch off the outdoor disconnect, the small box on the wall near the unit. Never reach into the fan grille. Capacitors inside can hold a charge even with the power off, so leave the electrical parts to a technician.
Common causes and fixes
Sunken or tilted pad
Over time, the pad under the unit can settle and tilt. A tilted unit shakes more and can strain its refrigerant lines. Check it with a level. If it’s off, have a technician lift and shim it.
Debris in the fan
Sticks, leaves and grass clippings can reach the fan blades. With the power off, clear the area around the unit and keep plants a couple of feet back. If debris is inside the grille, ask a technician to remove the top.
Loose panels and screws
The metal side panels and the top can rattle if a screw has backed out. With the power off, tighten any loose screws on the outside of the cabinet.
Fan blade or fan motor
A bent blade, a blade loose on its shaft, or worn motor bearings makes the fan wobble and hum. This is a common and fairly quick repair for a technician.
Compressor mounts
The compressor sits on rubber mounts that wear out over the years. Worn mounts let it knock against the frame, especially at start and stop. A technician needs to replace them.
Refrigerant lines touching the house
If the copper lines rub the siding or each other, they carry the hum into your walls. A technician can re-route them or add padded clamps.
What this tool can’t measure
- The compressor’s speed. It’s usually above your phone’s limit.
- Refrigerant and electrical problems. Low refrigerant, a failing capacitor or a worn contactor don’t always cause vibration.
- Noise. A unit can be loud with little vibration. The phone measures shaking, not sound.
- Every spot at once. A thin top panel can buzz more than the frame. Keep your phone in the same spot each time you compare.
- A certified result. The labels use a general machine scale (ISO 10816-1) as a rough guide. It’s a good guide, not a certified measurement.
Frequently asked questions
Why is my outdoor AC unit shaking?
The usual causes are a pad that has settled and tilted, debris hitting the fan, a bent or loose fan blade, loose panels, or worn compressor mounts. A tilted unit or loose panel is often easy to spot. Fan and compressor problems need an HVAC technician.
Is it normal for an air conditioner to vibrate?
A light hum on the top panel is normal, because the fan and compressor both run inside a thin metal cabinet. A reading of Smooth or Normal is what you want. A rising reading over time, or new rattling or banging, is a sign to get it checked.
Why does my AC shake or bang when it turns on or off?
Many compressors rock on their rubber mounts for a moment as they start and stop, so a short shudder can be normal. A loud bang every time, or a hard start that makes the lights dim, is worth a service call. It can point to worn mounts or an electrical part that helps the compressor start.
Can I level my AC pad myself?
It's safer to ask a technician. The refrigerant lines and wiring are attached to the unit, and lifting it can strain or crack them. A technician can lift and shim the unit or replace the pad without damage.
Why can I feel HVAC vibration inside the house?
The refrigerant lines run from the outdoor unit into the house. If they touch the wall or rest on a joist without padding, they carry the hum inside. The indoor air handler or furnace blower can also shake if it's dirty or out of balance. Use the house vibration logger to track when it happens.
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