Note when the temperature drops and how long it stays cold. A gradual decline late in a long shower points in a different direction from repeated hot-cold cycling.
If it cools near the end of a long shower
A storage water heater may be running out of its hot-water reserve, especially after laundry, dishwashing, or another shower. Tank size and recovery rate matter.
If it changes when another fixture opens
Pressure and the balance between hot and cold supplies can shift. A worn pressure-balancing or thermostatic shower valve can also react poorly to those changes.
With tankless heaters, watch the flow rate
Some units need a minimum flow to remain firing. Very low-flow showerheads or changing household demand can cause cycling; heater error codes can add useful evidence.
If only one shower does it
The mixing cartridge, valve screens, or temperature limit may be involved. Compare a nearby sink and another shower if available.
If the whole house is affected
The search moves toward the water heater, fuel supply, electric elements, sediment, or equipment sizing. Do not open burner, gas, or high-voltage compartments.
“It turns cold exactly when another fixture runs” or “it lasts twelve minutes and then needs recovery time” gives a technician much more diagnostic value than “the shower goes cold.”
Sources
- U.S. Department of Energy — Water Heating — Water-heating efficiency, temperature, equipment, and household hot-water use.