Quick Answer:
If your shower temperature shifts when the washing machine runs, it’s usually a water-pressure issue: the washer’s fill cycle changes the flow to the hot or cold side and a single-handle shower valve may not keep the mix steady. Run a simple test to confirm, reduce simultaneous demand when you can, and call a plumber if the swings keep happening under normal use.
Why This Happens
Washing machines fill intermittently with either hot, cold, or a mix. When the washer valve opens, it changes water pressure and flow in the same household plumbing line that feeds the shower. If the shower uses a pressure-balancing valve or a thermostatic cartridge that doesn’t react well to sudden pressure drops, the mix of hot and cold water can swing, making the shower suddenly hotter or colder.
Seasonal or recent-work factors can make the problem more noticeable. For example, colder supply water or recent changes to piping can change how the valve behaves. See Shower temperature unstable in winter and Temp swings after plumbing repair for related situations to compare.
Step-by-Step What to Do
Step 1 — Prepare and notice baseline behavior
- Set the shower to a comfortable temperature and leave it on steady flow.
- Note how stable the temperature is for a minute or two with no other appliances running.
Step 2 — Run the shower alone, then start the washer fill and observe
- With the shower running, start the washer’s fill cycle (choose a normal wash setting, not a drain or spin-only cycle).
- Watch and feel the shower for changes in temperature or pressure while the washer fills.
- Note whether the swing is toward hotter or colder water and how long it lasts.
Step 3 — Compare results and reduce simultaneous demand if needed
- If the temperature is steady when the shower runs alone but swings when the washer fills, the washer is creating the pressure change. Reduce simultaneous use by avoiding running both at once when possible.
- Stagger laundry cycles or run the washer during times you don’t need a long shower.
Step 4 — Consider the shower valve and next checks
- Pressure-balancing valves can struggle if the hot or cold pressure drops quickly; a worn or low-quality cartridge may not respond properly.
- Check whether other fixtures (toilets, other taps) cause similar swings; this helps identify whether the issue is isolated to the shower valve or is a wider supply problem.
- If your water heater or plumbing was recently worked on, keep an eye out for new symptoms and document when swings occur.
What Not to Do
- Don’t keep running the washer and shower together repeatedly to test—repeated, large temperature swings can cause scalding or strain on the system.
- Don’t ignore the problem if it happens during normal everyday use; persistent swings are a sign a professional should look at the system.
- Don’t attempt complex valve repairs or cartridge replacements unless you’re confident in plumbing work; improper installation can make the problem worse.
When to Call a Professional
- Call a plumber if the temperature swings happen during normal household use, not just during a careful test.
- Get help if the valve is old, if you smell gas or see leaks, or if other fixtures show large pressure or temperature changes.
- A licensed plumber can test supply pressures, inspect or replace pressure-balancing or thermostatic cartridges, and recommend fixes like dedicated lines or pressure regulators when needed.
Safety Notes
- Sudden temperature swings can cause scalding. If the shower becomes unexpectedly hot, step out immediately and adjust the water heater temperature until the issue is resolved.
- Avoid prolonged testing that forces repeated hot bursts; that increases scald risk and can waste water and energy.
- If you suspect leaking, loose fittings, or any gas appliance issues, stop use and contact a professional right away.
Common Homeowner Questions
- Why does the shower get hotter when the washer runs?
Because the washer can reduce cold-water pressure (or hot, depending on its cycle), upsetting the balance and causing a temporary increase in the other side. - Can I fix this by adjusting the water heater?
Lowering the heater temperature can reduce scald risk but won’t stop pressure-related swings; it’s a safety step, not a fix for the cause. - Will replacing the shower valve help?
Often yes—replacing a worn pressure-balancing or thermostatic cartridge with a good-quality part usually improves stability, but have a plumber diagnose first if the supply pressures are inconsistent.