Quick Answer:
If your tub water swings between hot and cold while filling, the most common reasons are changes in water pressure elsewhere in the house, a mixing/anti-scald valve reacting to those changes, or a water heater supply problem after recent plumbing work. Start by checking other fixtures and isolating hot-only flow; most issues are pressure-related and fixable without replacing the tub spout.
Why This Happens
Several supply and valve behaviors make a steady mix of hot and cold hard to maintain while filling a tub:
- Shared supply lines: the tub shares hot and cold branches with sinks, toilets, and appliances. When one of those runs or cycles, it changes pressure and flow to the tub valve.
- Pressure-balance or thermostatic mixing valves: these safety valves react to sudden changes in hot or cold pressure and will temporarily shift the output temperature to prevent scalding, which looks like the water going hot then cold.
- Partial blockages or partially closed supply valves reduce flow and can make the mixing valve overcompensate.
- If you recently had the water turned off and back on, you might be seeing water heater issues with trapped air, debris, or burner/electric recovery problems that change hot supply. You may also see symptoms from low water pressure if the system hasn’t fully stabilized after a shutoff.
Step-by-Step What to Do
1. Check other fixtures
Run the hot and cold taps in other rooms while someone watches the tub. Note if the tub temperature shifts when a toilet flushes, the washing machine fills, or a faucet is used. This tells you if the problem is caused by shared supply and pressure changes.
2. Test hot-only vs mixed
Test the tub control in two ways:
- Turn the control to full hot (no cold). If the temperature stays steady, the hot supply and heater are likely stable.
- Return to a mixed setting. If the temperature now fluctuates, the mixing valve or pressure changes from the cold side are the likely cause.
3. Check anti-scald / mixing valve settings
Many tub valves have an adjustable temperature limit or internal cartridge. Locate the valve behind the access panel, and:
- Look for a set screw or stop that limits handle travel and temperature. Adjust only a small amount and test.
- If you aren’t comfortable opening the valve, note the model and consult the manufacturer instructions or a plumber rather than forcing parts.
4. Consider pressure balance and supply issues
Pressure-balance valves hold the mix steady when one side drops. If they’re worn or if supply pressure is uneven, they can overreact. Things to check:
- Are any supply shut-off valves partially closed? Open them fully and test again.
- Have any appliances or valves recently been added or serviced? Changes to the plumbing can alter pressure distribution.
- Look for signs of low supply pressure that match the temperature swings—if pressure is low, the mixing valve may not function correctly.
5. Check the water heater only as needed
If the hot side seems to cut out (hot turns to cold entirely), check the heater for obvious problems: pilot light for gas, power for electric, and whether you recently had the water shut off and back on. If you suspect a heater problem, don’t attempt risky repairs yourself; this is a situation to consult the heater’s manual or a professional.
What Not to Do
- Do not raise the water heater temperature excessively to “fix” the mixing problem — that increases scald risk and utility costs.
- Do not bypass or remove safety devices such as thermostatic or pressure-balance cartridges to force more hot water flow.
- Do not ignore the scald risk — fluctuating temperatures can suddenly expose users to dangerously hot water.
When to Call a Professional
- Temperature swings continue after you check other fixtures and adjust valves.
- You find leaks, a failed cartridge, or you need to replace a mixing valve but aren’t comfortable doing it.
- The water heater shows signs of failure (no hot water at other taps, unusual noises, or gas/electrical issues).
- Household members are vulnerable (children, elderly, disabled) and you need a reliable, safe solution quickly.
Safety Notes
- Always test water temperature with the back of your hand or a thermometer before bathing someone who can’t react quickly.
- A common recommended water heater setpoint for safety and comfort is about 120°F (49°C); higher settings increase scald risk. Adjustments should be cautious and follow local codes.
- If you must open valve access panels, shut off the water supply first and follow simple lockout steps; if unsure, call a plumber.
Common Homeowner Questions
- Why does the tub only go cold after a toilet flush? A toilet flush briefly drops cold pressure; a pressure-balance valve may compensate by reducing hot flow, causing temperature swings.
- Will a new cartridge fix the problem? Often yes, if the cartridge is worn or clogged; if the issue is system pressure, the cartridge alone might not solve it.
- Should I check the heater or pressure first? Check fixtures and pressure behavior first; if hot supply cuts out entirely, then inspect the heater or call a technician.