You wake to a gurgling sound, or a splash of water reaching your mask, and find the inside of your CPAP hose wet. This is rainout, and almost everyone meets it eventually — usually the first cold week of the year, or the first night with the AC on. It is not a leak, not a broken humidifier, and not a sign anything is wrong with the machine. It is condensation, and once you understand the cause the fixes are obvious and mostly free.
What rainout is
Your humidifier deliberately warms and adds moisture to the air so the therapy does not dry out your nose and throat. That warm, moist air then has to travel the length of the hose to your mask. Along the way it cools toward room temperature — and warm air holds more moisture than cool air, so as it cools, the excess moisture condenses into liquid water on the inside of the tube. It is exactly how dew forms on grass overnight, or how a cold drink “sweats” in a warm room: the same physics, in a smaller tube. The water then pools at the low point of the hose, or trickles toward whichever end is lower — sometimes, unpleasantly, toward your face.
Why it is worse in winter, AC rooms, and coastal climates
Rainout is driven by the temperature gap between the humidified air inside the tube and the room around it. The bigger that gap, the more condensation:
- Winter. A cold bedroom pulls heat out of the hose quickly, so the air inside cools and condenses fast. This is why rainout often appears for the first time when the weather turns.
- Air-conditioned rooms. The same effect year-round — and an AC vent blowing directly across the tubing is, in effect, a rainout machine, chilling the hose along its whole length.
- Humid and coastal climates. Here the air the humidifier draws in already carries more moisture, so it has more to give up, and even a modest temperature drop produces water. In coastal Indian cities the combination of high humidity and overnight AC is a classic rainout setup.
If your rainout appeared when the season changed or when you started running the AC, this is exactly why.
The fixes, in order
Work down this list — the early items solve the large majority of cases on their own:
- Add or use a heated tube. This is the proper, complete fix. A heated tube keeps the air warm along its entire length to the mask, so it never cools enough for the moisture to condense. It also lets you keep full humidification for comfort instead of trading it away. The clinical role of heated tubing is covered in heated tubing on CPAP.
- Lower the humidifier level by a notch. Less moisture in the air means less available to condense. Drop it one step at a time until the rainout stops but your nose and throat are still comfortable — the goal is the lowest setting that keeps you from drying out, not switching humidity off entirely. A machine with finely stepped humidity control makes this easy to dial in: the Home Medix HM-CV-20, for example, offers five humidification levels, so you can trim the moisture down precisely rather than choosing between “on” and “off.”
- Insulate or cover the hose. A fabric hose cover (cheap, widely available) — or simply routing the tube under your blanket — keeps the room from cooling the air inside it. This is the best low-cost option if a heated tube is not available for your machine.
- Position the machine below your head. Set the CPAP lower than the mattress so that any water that does form drains back down toward the humidifier chamber instead of running up the hose to your mask.
- Move the AC airflow. Make sure no vent, cooler, or fan is blowing directly across the tubing; redirecting it is sometimes the entire fix.
Hose water vs nose or mouth water
One distinction worth making, because it changes the fix. Water in the tube is rainout, solved by the steps above. A wet feeling in the nose or mouth, or a water taste, is usually a different issue — an overfilled humidifier chamber slurping water up the line, or condensate forming right at the mask. Check the chamber fill line first if the water seems to be arriving at your face rather than pooling in the hose. If you are on oxygen rather than CPAP, the equivalent condensation problem in the oxygen line and its fixes are described in concentrator night-use considerations and does an oxygen concentrator make the room hot or humid.
Don’t over-correct
The instinct when rainout strikes is to switch humidification off completely. That stops the water but usually trades it for a dry, sore nose, nosebleeds, or a raw throat — and dryness is itself a common reason people abandon CPAP. The better path is to reduce the temperature gap (heated tube, hose cover, placement) so you can keep the humidity you need without the condensation. Turning humidity off should be the last resort, not the first move.
Takeaway
Rainout is condensation — warm humidified air cooling and dropping its moisture as water inside the hose — and it is physics, not a malfunction. It worsens whenever the room is much cooler than the air in the tube, which is why winter, AC rooms, and humid coasts trigger it. Close the temperature gap: a heated tube is the real fix, supported by a lower humidity setting, a hose cover, sensible machine placement, and keeping cold airflow off the tubing. Avoid the trap of simply turning humidity off and drying yourself out instead.
This is general information, not medical advice. If reducing humidity to control rainout leaves your airway dry or irritated, raise it with your provider rather than tolerating either problem.