Why is my cold water not cold after an Arizona repipe?

A repipe that goes overhead puts long runs of tubing in the attic. Water standing in a cold branch that crosses an attic sits in hot air for hours with little airflow to carry the heat away, and by late afternoon it has warmed toward the temperature of the air around it.

In Phoenix that air is extreme. KJZZ reports that Phoenix typically sees 21 days a year above 110°F, but 70 days passed that mark in 2024, and an attic under a roof in that sun runs hotter than the air outside. It also stays warm long after sunset, so the lines don't get much time to cool overnight.

Water that sits in that space, even for a few hours, takes on the heat. The water in a pipe is only cold when it's moving. Once you stop using it, it starts to warm toward the air around it. If your old lines ran through a slab or inside cooler walls, that may be why the cold water felt colder before.

The hot line starts hot from the water heater and then sits in the same attic. With no insulation and no cool air around it, it holds its temperature or gains a little. The result is a tap that runs very hot quickly. Two more things can add to it:

• The water heater setting. Check the thermostat. Our guide on what temperature a water heater should be explains the usual setting and the scald risk of running higher.
• Heat soaked into the cold line. When cold water is already warm, you turn the cold tap up and get a hotter mix at the faucet. That makes a shower feel much hotter than the same setting used to.

Two quick checks separate attic heat from a real fault:

1. 1Time of day. Test the cold tap in the early morning, then again in the late afternoon. If it's fine at 7 AM and warm at 5 PM, that's attic heat. A plumbing fault wouldn't follow the sun.
2. 2How long it takes to clear. Run the cold tap and time how long it takes to turn cold. Water that clears within a minute or two was standing in a warm branch. If it takes much longer, or never gets cold, the run is long or the whole cold side is heat-soaked.

For an actual number, run the cold tap into a glass and read it with a kitchen thermometer, then write down the reading at 7 AM and again at 5 PM. Two numbers give a plumber something concrete to work with, and they show whether the problem is mild or severe. Expect it to be mostly a summer issue, since the attic is hottest then. Most of the time it's exactly that. The water is safe. It just isn't cold.

Is warm PEX in the attic damaging anything? Not in any way that matters here. PEX is the same tubing used for hot water lines, so an attic that warms a cold line is no threat to it. The one thing PEX doesn't like is sunlight. The Plastics Pipe Institute says the long-term performance of PEX "will be damaged by excessive UV radiation from sunlight," and that PEX is not designed for outdoor use, so exposed tubing should be shielded. This is a comfort problem, not a safety one.

Start with what you can do today, then talk to the plumber about routing.

1. 1Run the cold tap for a bit. The warm water standing in the pipe clears out and cooler water follows. It uses water, so it's a stopgap. Catch the first gallon or two in a bucket or pitcher and use it on plants or for filling a pet's bowl once it's cooled, so the stopgap isn't a total waste.
2. 2Fix the thermostat. If the hot side is too hot, turn the water heater down to a safe setting.
3. 3Insulate the lines you can reach. Foam sleeves slow the heat exchange, so the hot side holds its heat longer. They won't keep a cold line cold, because a wrapped line still warms up eventually, just later. See whether you should insulate hot water pipes.
4. 4Ask about rerouting. The most effective fix is moving the lines out of the hottest air. A common approach is to run cold lines low in the joist bays, with attic insulation pulled over them, and away from the roof deck. That's a real job, so ask for a route drawing before any work, showing which runs sit below insulation and which above.

Keep the insulation dry and sealed so the cold line doesn't sweat. ENERGY STAR says demand recirculating pumps can solve the long wait for hot water at a distant fixture, but a pump doesn't cool a warm cold line. Our page on whether a recirculation pump is worth it covers what it does.

No, not necessarily. Overhead PEX is a common way to repipe a house, and it avoids opening slabs. But a good plan accounts for the heat, with the cold lines kept low and covered. If yours are pressed against the roof deck or run through a hot corner, that's fair to raise with whoever did the work. Our overview of how Arizona heat affects water heaters and pipes is a good next read, and if leaks pushed you toward a repipe in the first place, repair or repipe when copper keeps getting pinhole leaks covers that decision.

If you're still shopping for a repipe, use this to your advantage. Before you sign, ask for these four things in writing:

1. 1A route drawing showing where each cold and hot line runs.
2. 2Which runs sit below the attic insulation and which sit above it.
3. 3Whether the cold lines will be kept low and away from the roof deck.
4. 4How any exposed tubing will be shielded from sunlight.

A plumber who has done overhead repipes in the Valley has answers to those. If the answers are vague, that tells you something about the plan, and it's much easier to change a drawing than a finished attic.

Overhead routing is popular because it avoids cutting into your slab. The route within the attic is what matters for water temperature, so ask your plumber where in the attic the cold lines will sit, and why.

Planning a repipe of your own? Ask up front how the cold lines will be routed. HQ gives free estimates and upfront pricing.