Living With Your New Water Heater

Why this matters

A new water heater mostly asks nothing of you. But the first few weeks are when you find out what it is set to, whether the temperature at your taps is safe for the youngest and oldest people in the house, and what the new pipes and parts around it are for. One of those parts, the relief valve and the pipe running down from it, is a safety device that people sometimes cap or plug because it dripped once, and that is the one change to a water heater that can turn it into a bomb.

This card walks through what the installer left on the heater, what each piece does, how to check and set your temperature, and when to call.

What the installer left on it

Most installers leave some combination of a sticker, a tag and a permit card. Yours may look different, but it should answer the same questions.

+--------------------------------------------------+
| INSTALLED: 03/14       BY: shop name + license # |
| TYPE: gas tank, 50 gal       VENT: atmospheric   |
| DIAL SET: about 120 F at the tap (tech measured) |
| EXPANSION TANK: precharge set to house pressure, |
|   reading written here by the tech               |
| RELIEF VALVE PIPE: to floor drain - NEVER CAP    |
| SERVICE: drain/flush per manual, yearly typical  |
+--------------------------------------------------+
| PERMIT NO. ________   FINAL INSPECTION: ________ |
| inspector initials and date once it is finaled   |
+--------------------------------------------------+

Install date and installer. This starts your warranty clocks, both the manufacturer's on the tank and parts and the installer's on the labor. Photograph the sticker and put the photo with your invoice. See related: Your House Records File and What Goes In It.

The temperature the tech set. A good installer measures at a tap, not just at the dial, because the dial markings on many heaters are words or letters rather than degrees.

The permit card. Replacing a water heater needs a permit in most jurisdictions, and the inspector's sign-off goes on the card or into the building department's online record. See related: What a Permit Means for You as the Homeowner.

The manufacturer's own rating plate is separate: model, serial number, capacity, fuel and input. See related: Reading the Label on Your Water Heater.

The relief valve and its pipe: leave both alone

Every tank water heater has a temperature and pressure relief valve, usually called the T&P valve: a brass valve near the top with a lever, and a pipe running from it down toward the floor. If the water in the tank ever gets too hot or the pressure too high, that valve opens and dumps water out of the pipe, so the tank cannot build up enough pressure to burst. Tankless heaters have a relief valve as well.

The pipe is part of the safety device. In substance, the model plumbing codes require it to run, full size and without a valve or a cap, to a safe place where discharge will be seen and will not scald anyone, often ending within about 6 inches of the floor or over a drain (IRC P2804.6.1 in the 2021 edition, as adopted by your jurisdiction; check the current edition and your local amendments). The end is left plain, not threaded, so nobody screws a cap onto it.

• Never cap, plug or tie off the pipe, and never put a valve on it. A blocked relief pipe means the valve cannot do its one job.
• Do not move or shorten it to make room for storage. Keep the end clear and visible.
• If it drips or discharges, do not stop it. Call. Discharge means the valve opened because temperature or pressure went too high, or the valve itself is failing. Either is a technician's call. Keep people away from the end of the pipe, since the water can be scalding.
• If water is spraying from the valve or the tank, keep clear of the hot water, turn the heater off first (gas control to "off," or the breaker for an electric heater, using only the breaker and not touching the heater's wiring), then close the cold water inlet valve on the pipe going into the heater, and call. Leave the heater off until the plumber has found why it happened. See related: Your Home's Shutoffs: Water, Gas and Electric.

The expansion tank's job

You may see a small tank, often gray or blue and about the size of a basketball or larger, on the cold water pipe near the heater. This is a thermal expansion tank.

Water expands when it heats. In many houses the water service has a check valve or a pressure-reducing valve that keeps water from flowing back to the street, so that expanded water has nowhere to go and the house pressure climbs every time the heater fires. The expansion tank has an air cushion inside that absorbs the extra volume. Without it, pressure spikes can make the relief valve drip, stress fixtures and shorten the heater's life.

The tank's air cushion is set to match your house water pressure when it is installed. You do not need to do anything with it. If the relief valve starts dripping weeks or months later, a failed expansion tank is one of the things the technician will check. See related: The Water Pressure in Your House and the Valve That Controls It.

If you have a tankless heater

A tankless (on-demand) heater heats water only as it flows. Two things about it surprise new owners:

It needs a minimum flow to turn on. The heater only fires once water through it passes a threshold flow rate printed in its manual, often a fraction of a gallon per minute. A faucet barely cracked open, especially a low-flow one, may never reach it, so the water stays cold. Open the hot tap a little further.

There is a delay before hot water arrives. Part of that delay is the heater ramping up, a few seconds. Most of it is the cold water already sitting in the pipe between the heater and your tap, which has to run out first. That part was there with your old tank too.

Here is a worked example of that pipe delay. Say the bathroom sink is 40 feet of 1/2-inch copper pipe from the heater, and the faucet runs at 1.2 gallons per minute. A foot of 1/2-inch copper tubing of the common wall thickness holds roughly 0.012 gallon.

So about 24 seconds of the wait is the pipe, before the heater's own few seconds of warm-up are added. As a sanity check from the other direction: 24 seconds at 1.2 gallons per minute is 0.4 minute times 1.2, or 0.48 gallon, the same water that was in the pipe.

What would change it. A kitchen tap closer to the heater waits less, a far bathroom more. A low-flow faucet waits longer because it pushes the same standing water out more slowly. A recirculation system, if your plumber installed one, keeps hot water near the fixtures and shortens the wait at the cost of some energy. If the delay you measure is much longer than the pipe can explain, or the water goes hot, cold and hot again in the middle of a shower, mention it to the installer.

Setting the temperature near 120 F

The U.S. Consumer Product Safety Commission recommends setting water heaters to 120 F to reduce scald risk. Hot water injures faster than most people realize: burn research commonly cited by scald-prevention programs puts a serious burn on adult skin at about 5 seconds at 140 F and about 30 seconds at 130 F, but minutes at 120 F. Young children and older adults burn faster than that.

Measure at the tap, not at the dial. The full method is in its own card (see related: How to Check Your Hot Water Temperature at the Tap), but in short: run the hot water at the tap closest to the heater until it stops getting hotter, then hold a cooking thermometer in the stream.

A worked example. Say your first reading at the kitchen tap is 128 F. On a gas tank heater, the temperature dial on the front of the gas control is meant to be adjusted by the owner; the manual shows which way is cooler. Turn it one mark cooler. Then wait for the tank to cycle: hot water already in the tank does not cool down right away, so test again the next day, after the tank has been used and reheated. Say the second reading is 121 F. That is close enough to 120 F to leave alone, since tap readings move a degree or two from one test to the next.

On an electric tank heater, the thermostats sit behind panels on the side of the tank, next to live wiring, and the insulation and plastic guard have to come off to reach them. Do not open those panels. Ask the installer or an electrician-qualified technician to make the change.

What changes the target. If someone in your home has a weakened immune system, a doctor may advise keeping stored water hotter to limit bacterial growth. The way to have both is a thermostatic mixing valve that stores hot and delivers 120 F at the taps, installed by a plumber. Do not raise the tank setting on your own to solve a different problem, like a dishwasher that does not clean well, without checking whether the dishwasher heats its own water.

The first few weeks: what is normal and what is a call

Normal: a little condensation dripping on a gas heater's burner area during the first cold-water firings, which stops once the tank is up to temperature; the heater firing more often in winter, when the incoming water is colder; a faint odor the first time a gas burner fires, which should clear within minutes.

Call the installer: water around the base of the tank, the relief pipe dripping, hot water that smells of rotten eggs (often a reaction between the water and a part inside the tank that the installer can change), rumbling or popping that keeps getting louder, or hot water that runs out much sooner than your old heater did. See related: Normal and Not Normal Sounds From Your Water Heater.

Leave the house and call the gas utility or 911 from outside: a gas smell near the heater that does not clear, or a CO alarm sounding. Do not touch light switches, the heater's controls or a phone inside. See related: How to Respond If You Smell Gas in Your Home.

Keep the space around it clear. A gas heater needs air to burn and room for its vent; do not stack boxes against it or store paint, gasoline or cleaning products near it. See related: How to Keep the Space Around Your Furnace and Water Heater Safe.

References

• U.S. Consumer Product Safety Commission, water heater temperature and scald prevention guidance (120 F recommendation)
• International Residential Code, Section P2804 (relief valves and discharge piping), P2804.6.1 in the 2021 edition, as adopted by your jurisdiction
• The water heater's installation and owner's manual (temperature adjustment, minimum flow for tankless units, maintenance)
• See related: Your Home's Shutoffs: Water, Gas and Electric; Reading the Label on Your Water Heater; When to Stop and Call a Licensed Pro Around the House