Vent, Valve & Volt
Valve: Plumbing

Shower goes hot or cold when someone flushes

A shower that jumps hot or cold when a toilet flushes is usually a pressure-balancing valve that has stopped doing its job, or a valve that never had one. A few questions sort out which, and which cartridge your valve takes.

Also searched as: shower temperature fluctuates, shower water temp changes when toilet flushed, shower goes cold when toilet flushes, shower goes cold then hot again.

Before you start. Test the water with your hand or a thermometer before anyone steps in, especially young children. CPSC recommends setting water heaters to 120F to prevent scalds.
A hand turning a single-lever shower handle while the shower runs
Hot and cold water enter a shower valve from each side. A spool inside the pressure-balance cartridge slides to keep their pressures matched when another fixture draws water, so the mix stays steady. pressure-balance cartridge hot in cold in spool slides to match hot and cold pressure mixed water up to the showerhead a stuck spool is why the shower swings when a toilet flushes
What a pressure-balance cartridge does
Every question and answer on this page, in order

Question 1: When does the temperature change?

Have someone flush a toilet while you hold your hand in the shower stream, then try again with nothing else running.

Question 2: How big is the swing?

Question 3: Is your shower valve a pressure-balancing type?

Pressure-balancing valves include Moen Posi-Temp and Moentrol, Delta Monitor, and Kohler Rite-Temp. The brand is often on the handle or the trim plate.

Question 4: What brand is the shower valve?

Look for a logo on the handle, the trim plate or the center cap. If there is none, a photo of the handle helps a plumber or a parts counter tell.

Possible results

Need the words for the phone call? Build a what-to-tell-the-plumber checklist, or read when to call a plumber.