The smell usually shows up only in the hot water, and often at one or two faucets first. It's worst in the morning, or after the house has sat empty a few days — a vacation is the classic trigger, because the bacteria get warm, still water to work in undisturbed.
Before you price a new water heater: this is a chemistry problem, not a dying tank, and the fix usually costs little or nothing.
The bacteria involved are sulfate-reducing bacteria. They occur naturally in many water supplies, and the type that colonizes water heaters is generally considered harmless — the gas they produce is the problem. They feed on hydrogen, and a tank heater hands them a steady supply.
Every tank heater has an anode rod, a sacrificial metal rod that corrodes so the tank lining doesn't have to. Most rods are magnesium, and magnesium gives off hydrogen as it corrodes. The bacteria eat the hydrogen and release hydrogen sulfide gas, which dissolves into the hot water and comes out at your faucet. Cold water never passes through the tank — which is exactly why the cold tap smells fine.
A few conditions make it worse. Well water is the most common setting for it. Water softeners can encourage it, because softening strips out the trace of chlorine that keeps bacteria in check. Low usage and a low thermostat setting help them too.
Run the cold tap and sniff. If the cold water smells too, the heater isn't your problem — the source water is, and the fix belongs at the well or the treatment system, not the tank.
Also rule out the drain. Fill a glass at the tap, carry it across the room, and sniff there. If the glass smells, it's the water. If the smell only hangs around the sink, it's biofilm in the drain — scrubbing the drain solves it.
Bacteria settle into the sediment at the bottom of the tank, so a full drain and flush knocks their numbers way down — sometimes that's all it takes.
Shut the heater down and let it cool. Gas off at the valve, or breaker off for an electric heater. Give it at least two hours — the water can scald.
Drain the tank through a hose. Run a garden hose from the drain valve to a floor drain or downhill spot outside, open a hot tap to break the vacuum, and empty the tank.
Flush in bursts. Open the cold supply for 30 seconds at a time to stir the sediment loose, then let it drain. Repeat until the water runs clear.
Refill, then restore power. Close the valve, refill completely, and only then turn the gas or breaker back on. The full procedure is in this guide to how to drain a water heater.
Sustained heat kills these bacteria, and your heater can deliver it. Turn the thermostat up to 140°F and leave it there for several hours — overnight is fine. Then run each hot tap briefly to push treated water through the lines, and turn the thermostat back down to its normal setting, typically 120°F. If the smell fades and then creeps back within a couple of weeks, that's proof it's bacteria — go to Fix 4.
Disinfecting the tank kills the colony outright instead of thinning it. The usual approach: shut off the power or gas and the cold supply, drain several gallons, and introduce unscented household bleach or hydrogen peroxide into the tank — most often through the anode rod port or the hot-water outlet. Refill, let the treated water sit a few hours, run it briefly through the hot lines, then drain and flush until the treatment smell is gone.
Because you're opening the tank and dosing drinking water, plenty of homeowners hand this one to a plumber — a reasonable call. If you do it yourself, follow the manufacturer's guidance for your model, don't improvise the amounts, and never mix bleach with vinegar or any other cleaner: the combination produces toxic gas.
If the smell keeps returning, stop treating the bacteria and take away their food: swap the magnesium rod for an aluminum-zinc alloy rod. The zinc works against the bacteria, and the alloy gives off far less hydrogen. This is the fix plumbers reach for when flushing and heat only buy a few weeks.
There's a stronger option: a powered anode rod. It runs on a small electric current instead of corroding, so it produces no hydrogen at all, never needs replacing, and usually ends the smell permanently. Background on the rod types is in this explainer on what an anode rod is.
One practical note: a rod that's never been changed can be seized tight, and freeing it sometimes takes an impact wrench. If it won't budge with a breaker bar and a 1-1/16 inch socket, hand the job off rather than force it.
| Fix | Typical Cost | How Long It Tends to Last | Best For |
|---|---|---|---|
| Flush the tank | Free if you own a hose | Weeks to months | The first thing to try |
| Heat soak at 140°F | Free | Days to weeks | Confirming it's bacteria |
| Chlorinate the tank | A few dollars in supplies, or a service call | Months | A smell that came back after flushing |
| Aluminum-zinc rod | Commonly $20–$60 for the rod | Years | The recurring case |
| Powered anode rod | Commonly $150–$250 | Permanent while powered | Chronic cases, softened well water |
Flushing once and stopping there. If the smell returns within a month, the rod is feeding the bacteria faster than flushing removes them. Move on to the rod swap.
Treating the whole house instead of the tank. Chemicals dumped into a softener or the main line never concentrate where the bacteria live: the colony is in the tank. Treat the tank.
Replacing the heater and keeping the same setup. A new heater ships with a magnesium rod too. Change nothing else, and the smell typically returns within the year — on a brand-new tank.
Cold water that smells means the source is the problem. On a well, sulfur can come from the groundwater itself or bacteria in the well, and the answer is water testing and treatment, not heater work. And if the heater is also rumbling, leaking, or running out of hot water fast, the smell may be one item on a longer list — this water heater failure checklist covers the signs that the tank itself is on its way out.
Work in order: confirm it's the hot water only, flush the tank, heat-soak if you want proof, and swap the rod when the smell keeps coming back. Most households solve this for the price of a garden hose they already own.
The bacteria that cause it are generally regarded as harmless, and the gas at household levels is a nuisance rather than a health hazard. If you're on a well, though, only a water test can tell you everything that's in it — a sulfur smell is a good reason to get one.
Because the reaction happens in the tank. Cold water runs straight to your faucet, while hot water sits in a warm tank with a corroding anode rod — exactly the conditions these bacteria want.
In most cases where the rod is the food source, yes. The zinc works against the bacteria and the alloy releases far less hydrogen than magnesium. A powered rod is the surest version: no corrosion, no hydrogen, nothing for the bacteria to eat.
Ask where the smell lives: at the faucet while hot water runs means water; strongest near the heater or gas line with nothing running means gas. If it's the second, leave the house and call the gas company or 911 from outside.