Two days ago, my plumber had capped existing hot and cold waters lines in one of the shower and tub area as part of the remodel process. Since then I did not get hot water anywhere in the house. I rushed to change the water heater today and still no luck. Now I get some warm water in some bath and cold water. The funny part is that even after I shut down the cold water under sink, I get only cold water from hot water line. The water heater is working normally it seems.

Looks like one of these things is happening here

  1. Sediment build up – after installing the new heater today, I got muddy/gray water with lots of air in one of the bath
  2. Bad design, like hot and cold water mixing – I could be wrong as I never had this issue for the past 15 years living in the house
  3. Leak – We had bad weather in Dallas few weeks ago… so it might have cause but the issue is showing up after 3 weeks.
  4. My plumber did something silly two days ago. With my basic knowledge of plumbing, all he did was capping the lines to Tub, and installing a new valve for shower. Could he have cause some blockage?

I highly appreciate if you can shed some light.

  • If the water heater is electric, has the breaker at the main panel been flipped/turned on?
    – DAS
    Mar 13, 2021 at 5:36
  • This happened to me. A shower was being renovated, the new valve was installed but the output was capped. The handles on this (two handle) valve were not installed and so there was no indication that the valve was open. When a hot tap was opened elsewhere cold water flowed through the open shower valve into the hot side. Solution was to close the shower valve. There was no fault in the water heater. Mar 13, 2021 at 12:46

4 Answers 4


The cold water could have a higher pressure, and if a backflow preventer is defect, the cold water will enter the hot water tubes. Thermostatic faucets are prone for that failure. See also Hot water turning cold in pipe at branch line


Thank you all for the help here.

The plumber who installed the rough out did not install the valve within side, thus cold water got inside the hot water line. Yesterday he came back, installed the valve to make things working again.

Thank you all again.

  • Glad you got this resolved, and thanks for coming back to post the solution. Please also click the check mark so others know it's been resolved.
    – FreeMan
    Mar 15, 2021 at 13:15

If your faucet only has the hot water turned on but cold water is coming out, the issue is at the hot water heater. Is the hot water heater on? Is the temperature setting on hot water heater turned all the way down? If it's a gas heater, is pilot out? Is there a shutoff valve on the hot water line that is closed or partially closed?

Air in line is normal at first when changing hot water heater heater, fyi


If there is a shower valve installed and the line is capped on the outlet side of the shower valve, and the shower valve is open, then water can flow from the cold side into the hot side. When a hot valve is opened anywhere in the house, cold water will rush through the shower valve into the hot piping and reduce the temperature of the water in the hot line.

If there is no handle on the shower valve in a shower being renovated, it is not obvious whether the valve is in the open or the closed position. It must be in the closed position to prevent flow from the cold side into the hot side.

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