Can You Drink Hot Tap Water? Why the Cold Tap Is the Safe One

Why hot tap water is not treated as drinking water

In the UK the kitchen cold tap is the one that is required to be fed directly from the mains so that it delivers fresh, wholesome water. Nothing is guaranteed about the hot tap in the same way, for three reasons.

• It has been stored. Water in a hot water cylinder can sit for hours or days, and parts of a cylinder, particularly the bottom, can sit at lukewarm temperatures where bacteria are happier than in cold mains water.
• It may not come from the mains at all. On an older tank fed system, the hot water starts life in a cold water storage tank in the loft, which is open to the roof space unless it has a proper lid and screened vents.
• Hot water dissolves more. Heat increases how much metal water takes up from copper pipes, brass fittings, old solder and, in older properties, lead. Boiling it afterwards does not remove any of that.

Which hot water system you have makes a difference

A vented cylinder, the traditional copper tank in an airing cupboard fed from a tank in the loft, is the one where the advice matters most. That loft tank can collect dust, insulation fibres and the occasional insect, and it is often decades old. Our guide to unvented vs vented cylinder types shows how to tell which you have in a minute.

An unvented cylinder, a Megaflo or similar, is fed straight from the mains and sealed, so nothing sits open to the loft. The water is still stored and still heated, so the usual advice stands, but the loft tank problem is gone. Our page on Megaflo explained covers how these systems work.

A combi boiler heats mains water as you use it, with no storage at all. That is the cleanest hot water of the three, but it still passes through the boiler's heat exchanger and has picked up more from the pipes on the way, so drink and cook from the cold tap anyway.

Is your kitchen cold tap really on the mains?

This is the check that matters more than the hot tap question. In most homes the kitchen cold tap is fed from the rising main, but in some older London houses that have been altered over the years we still find a kitchen cold tap fed from the loft tank, or a filter tap or boiling water tap connected to the wrong supply.

A simple test: turn the stopcock off, then open the kitchen cold tap. If it stops within a few seconds, it is on the mains. If it keeps running for minutes, it is being fed from a tank and should be corrected. Our guide to where is my stopcock shows where to find it. The cold tap should also run cold; if the cold tap runs warm for a long time, read our page on warm water from the cold tap, because cold water that has been sitting next to hot pipes is not ideal either.

If you do find a kitchen cold tap fed from a loft tank, stop using it for drinking until it has been reconnected to the mains. It is usually a straightforward change.

Older homes, lead pipes and first draw water

Many London homes built before about 1970 still have some lead pipework, either the supply pipe from the boundary or short runs inside. Lead dissolves more readily into hot water and into water that has been standing in the pipe overnight. If you think you may have lead pipes, use only the cold tap for drinking, and run it for a short while first thing in the morning or after you have been away until it runs noticeably colder. Your water supplier can advise on testing and on replacing a lead supply pipe.

Artificially softened water is a separate case. If you have a water softener, at least one tap, normally the kitchen cold, should be left unsoftened for drinking. We explain why in our article on hard water and hot water cylinders in London.

Kettles, cooking and baby feeds

It is tempting to fill the kettle or a saucepan from the hot tap to save time. Do not. The small time saving is not worth putting stored, metal rich water into food, and boiling does nothing about dissolved metals. Always fill kettles and pans from the cold tap.

For baby formula, use fresh water from the cold kitchen tap, boiled as the feed instructions say. Never use water from the hot tap, and do not use softened water.

When hot water looks, smells or tastes wrong

You are not meant to drink it, but hot water is still a good indicator of what is going on inside your cylinder. If it changes, it is telling you something.

• Brown or orange water from the hot tap only usually points to corrosion inside an old cylinder or its pipework. See our page on brown water from the hot tap.
• A smell of rotten eggs from the hot water only is usually a reaction inside the cylinder rather than the mains. Our guide to hot water smells like rotten eggs explains the causes and the fix.
• Milky water that clears from the bottom up is almost always air, and harmless. Cloudy hot water that does not clear is worth checking.
• A metallic or musty taste from the hot side, or bits in the water, can mean a loft tank that needs attention or a cylinder near the end of its life.

Keeping stored hot water safe to wash in

Even though you are not drinking it, you are showering in it, and that is the reason stored hot water has to be kept hot enough. A cylinder held at around 60 degrees keeps bacteria such as Legionella under control; one that sits lukewarm for weeks, for example in an empty flat, does not. Our guide on what temperature should a hot water cylinder be set at explains the numbers and how to check yours.

If you have an old tank fed system with a tired loft tank, or you are not sure what your kitchen tap is connected to, we can check it and tell you plainly what, if anything, needs doing. That ranges from reconnecting one tap to replacing the loft tank or moving to a sealed mains fed system through an unvented cylinder installation. We are 20 engineers across London, our rate is set out on the pricing page before anyone attends, and you can call 020 4652 5111 or send an enquiry.