Why does hot water look cloudy then clear in a glass?

Cloudy or milky-looking hot water that becomes clear from the bottom of a glass upwards is usually full of tiny air bubbles. The bubbles rise and escape, leaving the water clear. That is normally harmless and does not automatically mean the boiler is contaminating the water.

The direction in which the cloudiness clears matters. Use a clean glass, compare the hot and cold taps and contact the water supplier if the appearance persists, has an unusual smell or taste, or contains colour or sediment.

The short answer

The Drinking Water Inspectorate says white or cloudy water is normally caused by trapped air. Fill a clear glass and leave it still. If the water clears from the bottom upwards within a few minutes, tiny bubbles are rising to the surface.

If the cloudiness settles downwards, leaves particles, has a colour, or does not clear, do not assume it is air. Save a sample and ask the water supplier for advice.

Why hot water can look cloudier than cold water

Water can hold dissolved gases. Heating and pressure changes allow some of that gas to form very small bubbles, rather like the bubbles that appear when a fizzy drink is opened, although tap water is not carbonated in the same way.

Hot water may therefore look white immediately after leaving the tap and then become clear as the bubbles escape. A tap aerator can add more air, and pressure changes in household pipework can make the effect more noticeable.

How to do the glass test

Use a clean, clear glass. Run the tap normally, fill the glass and place it on a level surface. Watch it from the side for several minutes without stirring it.

If a clear layer appears at the bottom and moves upwards, air is the likely cause. The bubbles rise, so the lowest part becomes clear first. The Drinking Water Inspectorate says this pattern is not something to be concerned about, although repeated or persistent episodes should still be reported.

Take care with very hot water. There is no need to use water hot enough to scald, and children should not carry out the test.

Compare the hot and cold taps

Fill one glass from the cold kitchen tap and another from the hot tap. If only the hot water is cloudy, the cause is more likely to be within the property’s hot-water system, hot pipework or tap. If both are affected, network work or the incoming supply may be involved.

Try another tap as well. One affected outlet can point to a local aerator or tap fitting. Every outlet being affected suggests a wider cause. Do not remove fixed plumbing or open the boiler to investigate.

Recent water-main work can introduce air

The Drinking Water Inspectorate notes that cloudy water often follows a burst main, planned network work or the restoration of a supply. Changes in flow and pressure can carry air through the local mains.

This is usually short-lived. Check the water company’s incident information and ask whether neighbours have noticed the same thing. If the problem is new, affects cold water too or lasts longer than a day, contact the supplier.

Internal plumbing can also cause it

The Inspectorate lists partly closed stop taps, tap aerators and cold pipes warmed by nearby heating or hot-water pipes as possible household causes. Water UK also advises that trapped air in hot water may merit advice from a WaterSafe plumber or Gas Safe heating engineer.

Observe rather than dismantle. Note the affected taps, time of day, how long the glass takes to clear and whether recent plumbing work took place. That gives a plumber or supplier a useful starting point.

Is cloudy water caused by limescale?

Air bubbles and chalky material can both make water appear white, but they behave differently. Air normally clears from the bottom upwards without leaving sediment. Chalk or other particles may settle, remain suspended or leave a deposit.

Hard-water scale around a tap or kettle does not prove that every cloudy glass is limescale. If a sample leaves visible material, photograph it and ask the water supplier to identify the likely cause.

Could the boiler be responsible?

A combi boiler heats mains water as it passes through, so temperature and pressure changes can make dissolved air easier to see. That does not by itself indicate an internal boiler fault.

Arrange professional advice if only hot outlets are repeatedly affected and the change began alongside unstable hot-water temperature, unusual boiler noises, low flow, a leak or a fault code. Keep the boiler casing closed and do not try to flush or dismantle internal components.

When the water needs further investigation

Contact the water supplier if the cloudiness does not clear, lasts more than 24 hours, affects neighbours or comes with an unusual taste or smell. Also report brown, orange, black, blue or green water, an oily appearance or visible particles rather than treating it as harmless air.

The Inspectorate recommends keeping a sample of discoloured water in a clean sealed glass container in the fridge, with the date and time noted, so the water company can inspect it if needed.

Safe checks at a glance

• Watch a clean glass and note whether it clears from the bottom upwards.
• Compare hot and cold water and more than one tap.
• Check for colour, particles, smell or taste.
• Note recent mains or household plumbing work.
• Contact the water supplier if the change persists or is not clearly air.
• Keep the boiler and sealed plumbing components closed.

Related Incredaboil guidance

If the appearance is accompanied by poor hot-water performance, read our guide to hot water taking longer after the boiler has been off. Treat water-quality concerns separately from boiler repair until the evidence points to the appliance.