How to Check Your Hot Water Temperature at the Tap

Why this matters

Hot water burns faster than most people expect, and the people it burns worst are the ones least able to pull away: young children, older adults, and anyone with reduced feeling in their hands or feet. The setting on the water heater is often a guess, because many dials are marked with letters or words rather than degrees, and the temperature at the tap can differ from the setting by several degrees either way. A ten-minute check with a kitchen thermometer tells you what is actually coming out of the faucet, which is the only number that matters to the person standing under it.

Before you start: what to stay away from

You are only touching a faucet and a thermometer. Do not remove covers from the water heater, do not touch the temperature and pressure relief valve (the lever valve near the top of a tank with a pipe running down toward the floor), and do not touch the vent pipe on a gas heater, which gets hot. Keep children out of the room while the hot tap is running.

Stop and call if any of these appear: water or steam coming from the relief valve's discharge pipe, a hissing or loud rumbling from the tank, water at the tap that spits steam or is too hot to stand near, or a gas odor near the heater. For a gas odor, leave the house without touching switches and call the gas utility or 911 from outside. For the others, keep clear and call a plumber. If you already know how, turn the heater off first (gas control to OFF, or the electric heater's breaker off), then close the cold water valve into it; both stay off until the plumber has found the cause. See related: When to Stop and Call a Licensed Pro Around the House.

Step 1: Check your thermometer first

Use an instant-read kitchen thermometer, the probe type with a dial or a digital display. A meat thermometer that reads a range up to about 200 F works. A candy thermometer is fine too.

Fill a glass with ice, add cold water, stir, and wait a minute. Put the probe in the middle of the glass, away from the sides. It should read 32 F. If it reads 34 F, your thermometer reads 2 degrees high, and you will subtract 2 from every reading later. Write the offset down.

Why bother. Inexpensive thermometers are often off by a few degrees, and a few degrees is the whole question here. A reading of 122 F from a thermometer that reads 3 degrees high is really 119 F, which is a different answer.

Step 2: Pick the hot tap closest to the water heater

Water cools as it travels through the pipes, so the tap nearest the heater gives the reading closest to what the heater is actually putting out. In many houses that is a laundry sink, a basement bathroom or the kitchen. If you are not sure, follow the hot water pipe from the top of the heater as far as you can see it.

Then pick a second tap: the one the most vulnerable person in the house actually uses. That is usually a bathtub or a shower.

Step 3: Run the hot water for a couple of minutes

Turn the hot water on fully, with the cold off, and let it run. Two minutes is usually enough at a nearby tap; a far tap upstairs may take longer. You are waiting for water that has been sitting in the pipes to clear out and for the temperature to stop rising.

Do not test right after a long shower or a dishwasher cycle, when the tank may be partway through reheating and read low. Do not test right after the heater has done a lot of short reheats either (several quick hand-washes in a row), because tank heaters can stack hotter water at the top than the setting and read high on the first draw. A good time is after an hour or two of little use.

Step 4: Hold the thermometer in the stream

Hold the thermometer by its handle and put the probe directly in the stream of water, not in a cup underneath it, which cools as it fills. Keep your hand out of the water. Watch the reading climb and wait until it holds steady for about 15 seconds. Write down that number, then apply the offset from Step 1.

If the stream is too hard to hold the probe in, fill a pre-warmed mug (run hot water into it first, dump it, then fill it) and read it immediately. This reads a degree or two low, so note that you used a mug.

Step 5: Compare with about 120 F

A setting of about 120 F at the tap is the figure commonly recommended for scald safety, including by the U.S. Consumer Product Safety Commission. Burn research widely cited for adult skin puts the time to a serious burn at roughly five minutes at 120 F, about 30 seconds at 130 F, and about 5 seconds at 140 F. Children's thinner skin burns in a fraction of those times. So the risk climbs steeply over a narrow range, and that is why a reading of 130 F is not "a bit warm" but a meaningfully different exposure from 120 F.

Plumbing codes approach the same problem at the fixture. The model plumbing codes, as adopted by your jurisdiction, generally require new showers and tubs to have a control that limits the water to 120 F at the outlet, using valves listed to the ASSE 1016, 1069 or 1070 standards. Those limits apply to new and replaced fixtures under your local code; an older bathroom may not have one. A limiting valve at the shower does not protect the kitchen sink.

Step 6: Decide what to do with the number

• About 115 to 122 F at the near tap: you are in the commonly recommended range. Write the date and reading in your records file and check again in a year or after any water heater work.
• Above about 122 F at either tap: worth changing, and more so the higher it reads, especially with young children or older adults in the house. See below for who should change it.
• Below about 115 F, or water that runs warm and then goes cold quickly: this is a different problem. It can be a setting, a failing part inside the heater, sediment, or a mixing valve set low. Note how long the hot water lasts and call a plumber.

Why changing the setting is often the plumber's job

Electric tank heaters usually have one or two thermostats behind metal access panels on the side of the tank. Behind those panels are live 240-volt terminals, and the thermostat is set with a screwdriver inches away from them. With the breaker on, that is a shock hazard; even with it off, you need to know which breaker, prove it off, and refit the insulation and covers correctly. That is work for a plumber or an electrician.

Gas tank heaters usually have a dial on the front of the gas control. Many are marked "warm", "hot" and letters instead of degrees, so you cannot set a temperature, only move a marking. Your heater's manual says whether you may adjust it and by how much. If you do, move it one small mark at a time, wait several hours, and retest at the tap. Do not touch the gas control otherwise, and never if you smell gas.

Tankless heaters often have a display that shows a set temperature, and the manual may let the owner change it. The display is the setting, not the tap reading, so still check at the tap.

Some homes have a mixing valve near the heater, a valve that blends cold water into the hot line. If yours does, the tap temperature is set there, not at the heater. Adjusting it belongs to a plumber.

See related: Reading the Label on Your Water Heater, and Living with Your New Water Heater.

A worked example: a house with a toddler and a grandparent

A family moved into a house with a gas tank water heater in the basement. A two-year-old and a grandparent in her eighties live with them. The dial was set at a mark one notch past "hot," with no degrees shown.

Thermometer check. In ice water it read 33 F, so it reads 1 degree high. Offset: subtract 1.

Near tap, the basement laundry sink, about 6 feet of pipe from the heater. After two minutes the reading held at 136 F. Corrected: 136 minus 1 is 135 F.

Far tap, the upstairs tub. After three minutes it held at 131 F. Corrected: 131 minus 1 is 130 F. The 5-degree drop from basement to tub is heat lost in the run of pipe, which is normal.

Reading the result. Both taps are well above 120 F: the near tap by 15 degrees, the tub by 10. At the tub, 130 F is the temperature where the cited time to a serious adult burn is about half a minute, and the toddler's bath water is filled from that tub. That is the case for changing it.

The fork. Lowering the tank setting is the simple answer, but the grandparent's age matters in a second way. Legionella, the bacteria behind Legionnaires' disease, grows best in warm water, roughly in the range of the mid-70s to about 113 F, and older adults and people with weakened immune systems are the most at risk. Some health guidance favors storing water hotter in the tank, around 140 F, and blending it down to a safe temperature at the taps with a mixing valve. Others accept 120 F storage for a typical household. Which is right for this family is a question for their plumber and, for the medical side, the grandparent's doctor.

What they did. They called a plumber, who recommended a mixing valve at the heater for this household: the tank stays hot and the house gets about 120 F. After the install, the family retested the next afternoon. Near tap: 120 F read, so 119 F corrected. Tub: 116 F corrected. Both inside the range, and the tub, which loses heat on the way up, now reads a few degrees cooler than the near tap as expected. They wrote both readings and the date in their records file.

What would have changed the plan. With no one in the house at elevated Legionella risk, simply lowering the gas dial a notch and retesting would have been reasonable, if the manual allowed it. With an electric heater, either option would have been a pro visit, because the setting lives behind the live access panels.

How to know the check worked

• Your thermometer was checked in ice water, and you applied the offset.
• You read the near tap and the tap the most vulnerable person uses, and wrote both.
• The near tap reads higher than or about the same as the far tap. If the far tap reads clearly hotter than the near one, retest; something about the timing, the thermometer or the plumbing is unusual, and a plumber should hear about it.
• After any change, you retested after several hours, not right away. A tank takes time to settle at a new setting.

References

• U.S. Consumer Product Safety Commission guidance on water heater temperature and scald prevention
• International Plumbing Code and Uniform Plumbing Code, as adopted by your jurisdiction, provisions on shower and tub temperature limiting; ASSE 1016, 1069 and 1070 valve standards
• CDC guidance on Legionella and building water systems
• See related: Reading the Label on Your Water Heater
• See related: When to Stop and Call a Licensed Pro Around the House