Laundry room with a tank-type water heater beside the washer
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Shower temperature swings hot and cold (when a toilet flushes or on its own)

Sudden swings at one shower are usually the shower valve, not the heater. A pressure-balancing or thermostatic cartridge is supposed to hold temperature when pressure changes elsewhere in the house; scale or wear stops it from reacting. If other hot fixtures are steady, work on the valve. If every fixture drifts, look at the heater.

Single source ~60 min moderateBrands: All

Before you start

  • Shut off water to the valve (stops or main) before pulling a cartridge
  • Reset the cartridge rotational limit stop after replacement so the hottest position is not a scald risk

Diagnostic steps

Unlock all 4 steps with expected values

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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Shower temperature swings hot and cold (when a toilet flushes or on its own) · OnCall Tech