Quick Answer:
If you see air spit from the hot tap but not the cold, it usually means air is trapped in the hot-water side of the supply — often in the water heater, a recent repair, or a section of hot piping. Open the hot faucet fully to purge the air and test hot-only versus cold-only to narrow the source. If the problem repeats across fixtures or across floors, have a plumber inspect the heater and venting so the supply isn’t losing pressure.
Why This Happens
Hot-only air bursts happen when air gets into the hot-water loop but not the cold. Common causes include:
- Refilling the water heater after a shutdown or outage, which lets trapped air rise into hot lines.
- Recent plumbing work on the hot side that introduced air. See Air spitting after plumbing repair for a related situation to check.
- One-way valves, check valves, or a faulty mixing valve letting air collect on the hot side.
- Differences in piping runs: upstairs hot runs may be higher and trap air more easily than downstairs runs.
Step-by-Step What to Do
1. Open the affected faucet fully to purge air
Turn the hot faucet all the way on and let it run for several minutes. Use a bucket if you worry about splashing. This is often enough to push the air out and restore steady flow.
2. Test hot-only versus cold-only
Run the hot only, then the cold only, then both together. If the air appears only with the hot side open, you’ve isolated it to the hot supply or heater. If both show air, the source may be upstream on the main supply.
3. Note timing: outages and heater refills
If spitting started right after a power outage, water service interruption, or after the heater was drained/refilled, that points to trapped air in the heater or hot piping. Allow time after a refill for air to be purged through taps.
4. Check upstairs vs downstairs
Test fixtures on different floors. If upstairs hot taps spit air but downstairs do not, the problem is likely localized to the upstairs run or a higher point where air pools. If multiple floors show the same behavior, the issue is more central and should be inspected.
5. Repeat purge and observe
If air stops after purging and does not return, monitor the system for a day. If the air reappears when you first run hot water or when a shower starts, that pattern can help a plumber trace the problem — for example, see why **Air spurts when shower starts**.
What Not to Do
- Don’t keep cycling the main shutoff valve repeatedly hoping to clear air — that can cause pressure swings and make the problem worse.
- Don’t dismantle cartridges, mixers, or valves unless you’re trained — risking damage or leaks. Leave cartridge work to a professional.
- If air returns after purging or affects multiple floors, don’t attempt major system changes yourself; let a plumber inspect venting, the heater, and supply components.
When to Call a Professional
- If air keeps returning after you purge several fixtures.
- If spitting affects multiple bathrooms or floors, or if the bursts coincide with pressure drops.
- If you find any leaks, banging pipes, or the water heater shows unusual noise after refilling.
- If you’re uncomfortable working around hot water or turning valves you don’t recognize.
A licensed plumber can trace the source safely with pressure tests, heater checks, and inspection of valves and vents.
Safety Notes
- Hot water can scald. When purging, start with lower temperatures if possible and stand back until the water stabilizes.
- Use a bucket and avoid directing hot spray at skin or electrical devices.
- If you must work near the heater, turn power or gas off per manufacturer instructions before touching components — if unsure, call a pro.
Common Homeowner Questions
- Why only the hot side? Because the air is trapped on the hot loop (heater, valves, or higher piping) and not in the cold supply.
- Will the air go away on its own? Often yes after a few purges or runs, but recurring air needs inspection.
- Is this urgent? Not usually an emergency, but persistent or widespread spitting should be evaluated to avoid pressure or valve damage.
For more related articles, see the Air Spitting From Faucets hub.