The water smells or tastes wrong.

Fill a glass from the cold faucet, take it into another room, and smell it there. Then do the same from the hot. Half of this diagnosis is working out whether the smell is in the water, in the drain, or only on one side, and the kitchen sink confuses all three because the drain smells and the faucet does not.

Then ask whether it is one faucet, the hot side only, or everything.

What each one usually means

Rotten eggs, hot water only. Bacteria reacting with the magnesium anode rod in the water heater. Common, fixable, and not the same thing as a gas smell, which is in the air rather than in the water. The fixes are flushing and sanitising the tank, or replacing the anode with a different type. Which sits alongside the rest of keeping a tank alive.

Rotten eggs, hot and cold. More likely the source or the well rather than the heater. On a well, that is a testing conversation.

Chlorine or swimming pool. Normal municipal disinfection, sometimes stronger after work on the mains or seasonally. Filling a jug and leaving it in the fridge uncovered for a few hours removes most of it.

Metallic. Iron, manganese, or copper, often with staining to match. Common in older galvanized or copper pipework, and usually worse at the first draw of the day.

Earthy or musty, cold water. Frequently harmless algae by-products in the source water, seasonal in surface water systems. Worth reporting, since the utility will know if it is widespread.

Sweet or oily. Uncommon and worth a call.

A smell only at one sink, and only when the water runs. That is the drain, not the water. Fill a glass and take it to another room to confirm. Sewer smells have their own causes and fixes.

Narrowing it down in five minutes

1. Cold versus hot. Hot only means the water heater in almost every case.
2. One faucet versus all. One faucet means that fixture or its branch.
3. First draw versus after running. A smell that clears after thirty seconds points at the pipework near the faucet. A smell that persists points upstream.
4. In the glass versus at the sink. Take it to another room.
5. Since when. Sudden, after work on the mains, or after the supply was off, is usually disturbance. Which usually comes with discoloration too.

What to do about the common ones

Water heater, rotten eggs: flush the tank, and ask a plumber about an anode rod change. A powered anode is the durable answer where the problem recurs.

Chlorine: a jug in the fridge, or a carbon filter at the faucet.

Metallic with staining: report it, ask the utility what your service line is made of, and consider whether the house pipework is the cause.

One faucet only: clean or replace the aerator, and check the flexible supply hoses, which can impart a taste as they age.

Where a filter helps and where it does not

A carbon filter at the faucet or in a jug genuinely improves taste and smell from chlorine and many organics. It is not treatment for a contamination event, it does not replace boiling under a boil water notice, and a cartridge left in past its life is worse than none.

For anything beyond taste, the sequence is: find out what is actually in the water before buying equipment to remove it. Treatment sold before a test is sold on a guess.

On a well

Everything above applies, with one difference: nobody is monitoring it but you. A change in taste, smell, or appearance is a reason to test rather than a reason to flush, and testing after any flooding, any work, or any nearby problem is ordinary practice. The rest of living with a well.

The short version

Glass of cold and a glass of hot, smelled in another room. Hot only is almost always the water heater’s anode rod. A drain smell at one sink is not a water problem. Fuel or solvent means call the utility immediately and do not drink it. Test before buying treatment.