Many CPAP humidifiers are designed to run on distilled water because it contains almost no dissolved minerals, so it leaves little or no chalky scale on the heater plate and inside the water chamber. Not every machine asks for it — some water tubs are specified for drinking-quality water instead — so the rule that matters most is simple: use the water your CPAP manufacturer specifies, and empty and clean the chamber as the manual says.
Why do CPAP manufacturers recommend distilled water?
A CPAP humidifier gently warms a small reservoir of water so that the air delivered through the mask overnight is moist rather than dry. As the water evaporates, anything dissolved in it is left behind. With tap water, that includes calcium and magnesium — the minerals that make water “hard” — which slowly collect as a white, chalky film on the heater plate and the chamber walls. It is the same scale you find inside a kettle, just in a smaller and more delicate part.
Distilled water has had almost all of those minerals removed, so there is very little left to build up. That is why, for example, the Philips Respironics DreamStation heated humidifier manual tells users to “use only room temperature distilled water in the tank” and not to put chemicals or additives in the water, warning that this may cause airway irritation or damage to the tank.
Do all CPAP machines require distilled water?
No — and it is worth checking rather than assuming. Requirements differ by brand, by model and even by the type of water tub. ResMed's UK shop page for its HumidAir 11 heated humidifier, which uses a cleanable tub, tells users to refill it with drinking-quality (potable) water, while the DreamStation manual quoted above specifies distilled water only. Your own manual is the authority for your machine.
When you read it, look for four things:
- Water type — distilled only, or drinking-quality water allowed.
- Temperature — some manuals specify room-temperature water.
- Fill line — never fill above the maximum mark.
- Cleaning schedule — what to do daily and what to do weekly.
If you no longer have the manual, or you are unsure, ask your equipment provider or your sleep clinic. Their advice for your specific device always comes first.
What happens if you use tap water when the manual says distilled?
The most visible effect is mineral scale. In hard-water areas, a white residue can appear on the heater plate and in the chamber after a few nights, and it gets harder to remove the longer it is left. Scale makes the chamber cloudy and more work to clean, and it is the reason manufacturers that specify distilled water ask for it. If your manual specifies distilled water, check its warranty terms too — damage linked to using a different water may not be covered.
What distilled water does not do is replace cleaning. Hygiene comes from emptying and washing the chamber, whatever water you use.
Is distilled water sterile? Can you skip cleaning the chamber?
Distilled water is pure, not sterile. Once a bottle is opened, or water sits in a warm chamber, it is exposed to air, hands and surfaces. That is why device makers pair their water advice with a cleaning routine:
- The DreamStation manual warns users to empty and clean the water tank daily to prevent mould and bacteria growth.
- ResMed's product pages advise emptying the tub and wiping it with a clean disposable cloth every day, and washing it weekly in warm water with mild dishwashing liquid.
- The US Food and Drug Administration (FDA) notes that most CPAP accessories, including humidifier tanks, can be cleaned with mild soap and water as described in the owner's manual, and advises following the cleaning schedule of the company that made your machine.
In practice: pour away last night's water each morning rather than topping it up, wash and air-dry the chamber as instructed, and refill with fresh water before bed.
How much distilled water does a CPAP user need?
Less than you might think, but it adds up. The DreamStation humidifier, for example, holds 325 ml at its recommended water level. If you filled it every night, that would be roughly 2.3 litres a week and about 10 litres a month. Many people use less, depending on humidity settings, room conditions and mask fit — but it is still a steady demand, and running out of bottled distilled water the night before a trip is a familiar frustration.
| Chamber filled to 325 ml | Water needed |
|---|---|
| Per night | 325 ml |
| Per week | about 2.3 litres |
| Per month | about 10 litres |
| One 4-litre distiller batch | about 12 full refills |
Can you make your own distilled water for CPAP at home?
Yes. A countertop distiller boils water, lets the steam rise away from dissolved minerals, and condenses it back into liquid — the same principle used to make the distilled water sold in shops. The US Centers for Disease Control and Prevention (CDC) lists distillation among home treatments that remove calcium, magnesium, sodium and many other substances. For the full science, see how does a water distiller work?
A Megahome distiller makes about 4 litres per batch. Owners report roughly 4.5–6 hours per batch and TDS meter readings of about 0–6 ppm on the finished water. At a rated 580 W, a batch uses roughly 0.8 kWh per litre; multiply that by your local electricity price, or try our cost calculator.
One important point of honesty: a home distiller is a household appliance, not a medical device, and Megahome's instruction manuals do not address CPAP use. If you are unsure whether home-distilled water suits your machine, ask the device manufacturer or your provider.
A simple routine that works
- Distil during the day. The cooling fan is audible and the unit gives off some warmth, so most owners run it in the kitchen or a utility room rather than the bedroom.
- Let the water cool. Freshly distilled water is warm; let it reach room temperature before filling the humidifier, as some manuals require.
- Store it clean and closed. Keep the water in a clean, covered container, such as the glass collection bottle, and make fresh batches regularly rather than storing water for long periods.
- Fill to the line, empty in the morning. Fill the chamber only to the maximum mark and discard what is left after each night.
- Look after the distiller too. Descale it at least once a month with citric acid, as Megahome's manual states, replace the activated-carbon sachet within 25 days, and clean the unit and bottle before restarting after a few idle days — the CDC notes that bacteria can grow on a distiller's cooling coils when it is not in use. The distiller switches itself off when the batch is done, so no plug-in timer is needed.
What should you look for in a distiller for CPAP water?
Because the steam path is the water path, choose a unit with a stainless-steel boiling chamber and steam dome and a collection bottle you are happy to keep clean; glass is easy to inspect and wash. Honest owner feedback is also worth reading: in our analysis of nearly 5,000 owner reviews, people praise water purity and taste but mention fan noise and long run times, and note that scale left under the boiler is the most common reason the thermal reset switch trips — another reason to descale regularly.
Megahome has made countertop distillers in Tainan, Taiwan, since 1992. Its MH943T-SW and MH943S-SW models hold TÜV Rheinland GS certificate 50514575, and as of September 2026 Megahome distillers have around 5,000 Amazon ratings averaging 4.5–4.7 stars. You can explore the MH-943TWS or find a local dealer on our where to buy page.
Key takeaways
- Many CPAP humidifiers specify distilled water because it leaves little or no mineral scale.
- Not all do: some tubs are designed for drinking-quality water. Your CPAP manual is the authority.
- Distilled water is pure, not sterile — empty the chamber daily and clean it as the manual says.
- A chamber filled to 325 ml every night needs about 10 litres a month; one 4-litre distiller batch covers about 12 fills.
- A home distiller is a household appliance, not a medical device; when in doubt, ask your device maker or provider.
Frequently asked questions
Can I use bottled drinking water instead of distilled water in my CPAP?
Only if your manual allows it. Bottled spring or mineral water contains dissolved minerals that can leave scale, just like tap water. Some humidifier tubs are specified for drinking-quality water; others are specified for distilled water only.
Is boiled tap water the same as distilled water?
No. Boiling kills microbes, but the minerals stay in the pot — and as some water evaporates they become slightly more concentrated. Distillation keeps the steam and leaves the minerals behind.
What should I do if I run out of distilled water while travelling?
Check your manual before you go. Some manuals explain what to do when there is no water — the DreamStation manual, for instance, says the humidifier setting must stay off when the tank is empty. If in doubt, contact your equipment provider.
How long can I keep home-distilled water for my CPAP?
There is no single rule, so keep it simple: store it in a clean, closed container, make fresh batches regularly, and pour away anything that looks or smells unusual.
Is water from a home distiller suitable for my CPAP?
A home distiller produces distilled water, but it is a household appliance, not a medical device, and the water is pure rather than sterile. Follow your CPAP manufacturer's instructions and ask your provider if you are unsure.
Sources
- Philips Respironics – DreamStation Heated Humidifier User Manual
- ResMed – HumidAir 11 heated humidifier
- ResMed – HumidAir Standard Tub for Air11 Devices
- US FDA – Do You Need a Device That Claims to Clean a CPAP Machine?
- US CDC – About Home Water Treatment Systems
- TÜV Rheinland Certipedia – GS certificate 50514575
- Amazon.com – Megahome countertop water distiller (customer ratings)
