Quick Answer:
When the washing machine fills it often takes a big slug of cold water from the same supply line your shower uses. That changes pressure and how the shower mixing valve proportions hot and cold, so the water temperature shifts. A few quick checks can tell you whether this is a plumbing fault or a normal pressure interaction you can work around.
Why This Happens
- Washing machines pull a lot of cold water quickly when filling. If hot and cold lines share pipes or the house has low incoming pressure, that cold draw reduces cold pressure at the shower and the mixing valve adjusts, causing the shower to run hotter (or colder if the hot side is affected).
- Single-handle pressure-balancing valves in showers are designed to react to pressure swings; they can overcompensate if the swing is large or sudden.
- Older or worn mixing valves, partially closed shutoffs, or a washer fill valve that fails to shut off cleanly can make swings worse.
- If the pattern only happens in cold weather or after recent plumbing work, it points to seasonally lower supply pressure or a changed connection—see Shower temperature unstable in winter and Temp swings after plumbing repair for related situations.
Step-by-Step What to Do
1) Run the shower alone
- Start the shower and set it to a comfortable temperature. Note how steady the temperature and flow are when nothing else runs.
- Check both hot and cold handle positions if you have separate handles; with a single handle note the position and feel of resistance.
2) Start the washer to confirm the relationship
- With the shower still running, start the washing machine fill cycle and watch what happens. Do the temperature changes happen every time the washer fills?
- Run the washer fill more than once to be sure the pattern repeats. This confirms the washer is causing the swings rather than intermittent shower valve failure.
3) Stagger usage to test
- Try starting the washer before you turn on the shower, then the reverse. Note differences in severity and timing of the swings.
- If starting the washer before the shower makes the shower steadier, it points to pressure balance limits rather than a complete valve failure.
4) Quick visual checks
- Look for leaking or partially closed shutoff valves under sinks or at the water heater.
- Listen for a washer fill valve that keeps running or cycles oddly—this could be a faulty fill valve or pressure regulator issue.
5) Simple fixes to try first
- Run the washer at a different time than long showers when possible to avoid the problem while you decide on permanent repairs.
- If your home has a pressure-reducing valve (PRV) or water main shutoff that looks adjustable and you’re comfortable, note its setting and consider having a pro check it. Avoid making major adjustments yourself if you’re unsure.
What Not to Do
- Do not routinely run washer and shower together to test; repeated swings can be unsafe.
- Don’t try to bypass or jury-rig a mixing valve with plumbing tape, clamps, or temporary fittings—this can create leaks or scalding risks.
- Don’t ignore repeated large swings. Assuming it’s “normal” without investigating can expose children or older adults to scalds.
When to Call a Professional
- Call a plumber if temperature swings are large or sudden (hot water suddenly scalds or goes icy cold), if the shower temperature won’t steady after the checks above, or if you detect a leak or damaged valve.
- Also call if the washer fill valve seems to run too long or never fully closes, or if you suspect cross-connections or a failing pressure-reducing valve.
- Ask the plumber to check the shower mixing valve (pressure-balance or thermostatic), house water pressure, and the washing machine fill valve as a combined diagnostic.
Safety Notes
- Sudden hot surges can scald. If someone is at risk, stop the flow and check water heater temperature and mixing valves before using the shower again.
- If you need to stop temperature swings immediately, pause the washer fill or avoid running long showers at the same time as laundry until the issue is fixed.
- When testing, protect children and older adults from exposure to sudden temperature changes—supervise closely or keep them out of the bathroom while you test.
Common Homeowner Questions
- Why does the shower get hotter when the washer fills? — Because the washer draws a lot of cold water quickly, changing the pressure balance the shower valve uses to mix hot and cold.
- Can I fix this by myself? — You can run the basic tests and look for obvious leaks or shutoff problems, but a plumber is best for valve replacement or pressure issues.
- Will a new mixing valve stop it? — Often yes for worn valves. If the underlying problem is supply pressure or the washer fill valve, both may need attention.