A sun-warmed hose tested near 130 degrees after 15 minutes on rocks, and the Arizona Burn Center says garden hose water can reach 180 on extremely hot days. Nobody publishes a standard figure for exposed pipe, but standing water outdoors can scald. Run the tap until it cools before anyone is in the spray.
What has been measured and reported
ABC15 ran its own test in Phoenix: "We left it full of water, on the rocks, for 15 minutes. When we turned it on, the first spurt of water was nearly 130 degrees." The same report says Dr. Kevin Foster of the Arizona Burn Center told it that garden hose water can reach 180 degrees on extremely hot days.
Coverage of a Valley baby burned by hose water reports that when it is 115 degrees outside, the water sitting in a hose under the sun could be as hot as 150 degrees, according to Capt. Larry Subervi. That is an estimate from a fire captain, not a lab reading, so treat 130 to 180 as the risk range rather than a single number.
Two limits on those figures. They are hoses, not the copper or PVC lines in your wall, and water inside the house and in buried supply lines runs cooler. Why a cold tap can still turn warm in July is covered in why a cold tap runs hot in a Phoenix summer. The second limit points the other way: a bare pipe on a sunny wall that holds standing water follows the same physics as a hose. City of Phoenix Water Services says exposed pipes, "especially those made of plastic material such as polyvinyl chloride (PVC), are more susceptible to extreme temperatures, leaks and breaks."
What those temperatures do to skin
The U.S. Consumer Product Safety Commission's time-temperature burn chart, reproduced in A. O. Smith's Technical Bulletin 34, lists exposure times under full immersion:
| Water temperature | First-degree burn | Permanent burn (2nd and 3rd degree) |
|---|---|---|
| 122 F | 1 minute | 5 minutes |
| 131 F | 5 seconds | 25 seconds |
| 140 F | 2 seconds | 5 seconds |
| 149 F | 1 second | 2 seconds |
| 154 F | instantaneous | 1 second |
Put the hose numbers on that chart. Nearly 130 degrees is already in the range where a first-degree burn takes seconds. At 149, the chart gives a permanent burn in 2 seconds. Dr. Foster's own wording is blunter: "Anything that's 140 to 150 degrees can cause a second-degree burn in a second or less." In that case, the baby was sitting on the ground when his mother turned on the faucet to fill a small pool, and the extremely hot water came out of the sprinkler head attached to the hose.
What to do with that information
- Run the water until it cools before anyone is in the spray. The danger is the first water out. It is the water that has been sitting in the hose or the exposed section. Test it with your hand, away from kids and pets, before aiming it anywhere near them.
- Never fill a kiddie pool or start a sprinkler with children nearby on a hot day without running it first.
- Coil the hose in shade, and drain it when you are done. A hose that drains does not hold a scalding slug of water.
- Keep pets and children out of the spray path until you have felt the water temperature.
What it means for the pipes themselves
Heat has its own cost on exposed lines. Phoenix's tips advise that if you have exposed pipes, "consider relocating them, insulating them properly or cover them with dirt when practical," and add that "It's best to consult a professional plumber for these services." Sun is hard on rubber washers and plastic parts, so an exposed hose bib may need new parts sooner; see how to replace an outdoor faucet and why an outdoor faucet leaks.
One spec worth knowing: Arrowhead Brass rates its hot and cold wall hydrants for a "Maximum pressure: 125 PSI. Max temperature: 140° F." If you run hot water to an outdoor faucet, for example for a dog wash, the hardware has to be rated for hot water, and a plumber can advise how to control the temperature at the spout. Our guide to Arizona summer heat and water heaters and pipes covers the equipment side.
When a plumber helps
HQ Plumbing and Air relocates or insulates exposed lines, replaces worn hose bibs and adds vacuum breakers as part of residential plumbing, with a firm price before work starts. For anything leaking inside a wall or under the slab, call right away.