Cloudy or Milky Hot Water? When It Is Air and When It Is Not
Cloudy or milky hot water is almost always tiny air bubbles, and it is harmless. Fill a clear glass and leave it on the side: if it clears from the bottom upwards within a minute or two, that is dissolved air coming out of the water, and it is especially common on unvented cylinders and combi boilers, where water is heated under mains pressure. If the glass stays cloudy, clears from the top down, or leaves white flakes or grit at the bottom, it is not air. That is scale from the cylinder or the immersion heater, and in hard London water it usually means an element or coil is furred up and working harder than it should. Air with spitting taps, banging pipes or a hot supply that keeps cutting out is a third case, and that one needs looking at.
The glass test
Run the hot tap for a few seconds, then fill a clear glass and put it down where you can see it against a dark background. Watch it for two minutes.
Air bubbles rise, so milky water caused by air clears from the bottom of the glass first and the top last, leaving clear water and nothing in the bottom. Particles sink, so cloudiness caused by scale or sediment clears from the top down, or does not clear at all, and leaves something in the base of the glass. Run the same test on the cold tap. If the cold is milky too, the air is in the mains supply, which is common after work in the street and clears on its own.
- Clears bottom up within a minute or two, nothing left behind: dissolved air. Harmless.
- Clears top down, or leaves white flakes or grit: scale or sediment from the cylinder.
- Cold is milky as well: air in the mains, usually after water company work.
- Air with spluttering, banging or the hot cutting out: an airlock or air being drawn in.
Why hot water looks milkier than cold
All tap water carries dissolved air. Cold water under pressure holds more of it than hot water at atmospheric pressure. When water is heated in a pressurised cylinder or a combi and then leaves the tap, two things happen at once: the pressure drops and it is warm, so the air comes out of solution as millions of tiny bubbles. The water looks white until they reach the surface.
That is why it is noticeably more common on unvented cylinders than on old tank fed systems, and why it tends to be worse in winter, when the incoming mains water is colder and holds more air. It is also normal for a few days after an unvented cylinder has been fitted, drained or serviced. None of this is a fault. The water is safe, and the hot water in a cylinder is not intended for drinking in any case.
White flakes and grit are scale
London sits on chalk, so the water is hard and carries dissolved calcium. Heat drives it out as limescale, which builds on the hottest surfaces in the system: the immersion heater element and the coil inside the cylinder. Over time bits of that crust break away, and they come out of the hot taps as white flakes, a gritty sediment in the bottom of the glass, or water that stays hazy.
On its own, a few flakes are an inconvenience. The problem is what they say about the element or coil. Scale is an insulator, so a furred up element runs hotter than it should and fails sooner, and a scaled coil takes longer to heat the cylinder. If the flakes arrive with a cylinder that rumbles or kettles as it heats, or hot water that takes longer than it used to, that is the same scale seen from a different angle. Our guide to hard water and hot water cylinders in London covers what scale does and what can be done about it.
When air is a problem
Milky water on its own is fine. Air that arrives in slugs, with the hot tap spitting and spluttering, pipes banging, or the flow stopping and starting, is different. On a tank fed system that usually means an airlock, often after a drain down or because the loft tank has run low, and our guide to an airlock in hot water pipes explains how to clear it and what to check first.
If it keeps coming back on a tank fed system, air is being drawn in somewhere. The common causes are a loft tank ball valve that is not keeping up, a partly blocked cold feed to the cylinder, or a heating pump pulling air in through the open vent. On an unvented cylinder, persistent spitting at every hot tap is unusual and worth reporting, along with anything else you have noticed, such as water in the tundish or pressure that rises after the cylinder heats.
Milky water from a combi boiler
Combis heat mains water as it passes through, so dissolved air comes out at the tap in exactly the same way and it is equally harmless. A combi that produces milky water and also flicks between hot and cold, or cuts out, has a separate problem, usually low flow, a sticking diverter valve or a scaled plate heat exchanger. That is covered in our guide to no hot water from a combi boiler.
When to call us
You do not need anyone for milky water that clears from the bottom up. It is worth a visit if the hot water is leaving flakes or grit, if the cylinder is noisy or slow to heat, if air keeps spitting through the hot taps, or if the cloudiness comes with any other change, such as a dripping tundish or water that smells or looks discoloured. On a scaled cylinder we test the element, check the cylinder is heating fully, and replace the element or descale where that is worth doing. On a tank fed system with recurring air, we trace where it is getting in.
Hot water cylinders are our core work and we have 20 engineers across London. Our rate is on the pricing page and agreed before we attend. If you call, a note of whether your cylinder has loft tanks or a tundish, and what the glass test showed, saves time.
Questions we get asked
- Why is my hot water cloudy?
- Usually because dissolved air is coming out of the water as it leaves the tap, which is common on unvented cylinders and combis. If a glass clears from the bottom up within a minute or two, it is air and it is harmless.
- Is milky hot water safe?
- Yes, if it is air. The bubbles are the same air that is dissolved in all tap water. Water from a hot water cylinder is not meant for drinking in any case, so use the cold kitchen tap for drinking and cooking.
- Why is there white stuff in my hot water?
- White flakes or grit are limescale that has broken away from the immersion element or the cylinder coil. London water is hard, so it is common, but regular flakes suggest the element or coil is heavily scaled and should be checked.
- Why did my hot water go cloudy after a new cylinder was fitted?
- A newly filled unvented cylinder carries a lot of dissolved air, and it comes out at the taps for the first few days. It should settle down within a week. If it does not, or the taps spit air, ask the installer to check the system.
- Should I be worried if the cold water is cloudy too?
- Not usually. Milky water from the cold tap is air in the mains supply, often after work in the street, and it clears on its own. If it lasts more than a day or is discoloured rather than white, contact your water company.