Direct answer: When a bathtub runs hot for a moment and then turns cold, the usual cause is either the bathtub mixing valve not holding the hot side open or the house simply running out of hot water. Check whether the problem happens only at the tub or at other fixtures too before you touch parts.
Most likely: If sinks and showers still stay hot but the tub does not, the bathtub valve cartridge or anti-scald setting is the strongest suspect.
Start with the easy split: tub-only problem or whole-house problem. That one check saves a lot of wasted time. Reality check: a bathtub fills fast, so it can expose a weak hot-water supply sooner than a sink. Common wrong move: cranking the handle harder and assuming the heater is bad when the tub valve is actually crossing hot and cold inside the wall.
Don’t start with: Do not start by replacing the water heater or buying a random bathtub cartridge just because the temperature swings.
Bathroom sink or another shower can hold hot water, but the tub starts hot and fades to lukewarm or cold.
Start here: Go straight to the bathtub valve checks. A worn bathtub cartridge or mis-set anti-scald stop is more likely than a heater problem.
The tub goes cold, and a nearby sink also loses heat around the same time.
Start here: Check hot-water supply first. The tub may just be using up a weak or undersized hot-water source faster than smaller fixtures.
The tub is stable until a toilet flushes, a sink runs, or a washer starts, then the tub turns cold.
Start here: That points to pressure balance or mixing trouble at the bathtub valve, especially on a single-handle setup.
You get a brief warm burst, but the handle will not deliver sustained hot water.
Start here: Look for an anti-scald limit set too low, a sticking bathtub cartridge, or a hot-side shutoff not fully open behind an access panel if your tub has one.
A failing cartridge can start in one position, then drift or cross-mix hot and cold as water pressure changes during the fill.
Quick check: Run the tub, then test a nearby sink. If the sink stays hot while the tub cools off, the bathtub cartridge moves to the top of the list.
On many single-handle tub valves, the limit stop physically prevents the handle from reaching full hot.
Quick check: If the handle stops early and the water never gets truly hot, remove the trim handle and inspect the limit setting before buying parts.
A tub draws a lot more hot water than a sink, so a weak water heater, long pipe run, or partially closed hot-side valve shows up here first.
Quick check: Fill the tub while checking another hot fixture. If both cool off together, the issue is upstream of the tub valve.
When another fixture opens, the valve may over-correct and shove the mix cold, even though hot water is still available.
Quick check: Have someone flush a toilet or open a sink while the tub runs. If the tub temperature swings sharply, the balancing section inside the bathtub valve is suspect.
This is the cleanest first split. If the whole house loses hot water, opening the tub trim will not fix it.
Next move: If other fixtures stay hot while the tub goes cold, keep working on the bathtub valve path. If other fixtures also lose heat, stop chasing tub parts and treat this as a hot-water supply issue.
What to conclude: A tub-only failure usually points to the bathtub valve cartridge, anti-scald setting, or balancing section. A whole-house drop points upstream to the water heater, hot-water delivery, or a partially closed supply valve.
A low anti-scald setting can mimic a bad cartridge, and it is a much simpler fix.
Next move: If the tub now reaches and holds a normal hot temperature, the limit stop was set too conservatively. If the handle now reaches full travel but the tub still turns cold, the cartridge or balancing section is more likely.
What to conclude: A handle that cannot physically reach the hot side will never deliver full hot water. If full travel is available and the symptom remains, the problem is inside the valve body components.
A sticking pressure-balance section often shows itself when another fixture changes pressure in the house.
Next move: If the tub barely changes and quickly recovers, the balancing section is probably not your main problem. If the tub snaps cold or stays unstable when other water is used, the bathtub cartridge or pressure-balance section is likely sticking.
Once you have ruled out whole-house hot-water loss and a simple limit-stop setting, the cartridge is the most common repair path.
Next move: If the new or cleaned cartridge restores steady hot water at the tub, reassemble and move to final verification. If a confirmed matching cartridge does not fix a tub-only temperature swing, the valve body may have an internal balancing issue or supply problem that needs a plumber.
You want to confirm the fix under real use, not just with a quick handle turn.
A good result: If the tub now fills with steady hot water, the repair is done.
If not: If the symptom remains, stop buying parts blindly. The next move is either whole-house hot-water diagnosis or professional valve-body service.
What to conclude: A stable final test confirms you fixed the actual cause. If not, the remaining causes are less DIY-friendly and more expensive to guess at.
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Most often, either the tub valve is not holding the hot mix correctly or the house is running out of hot water under the tub's higher flow rate. If other fixtures stay hot while the tub goes cold, the bathtub cartridge or anti-scald setting is the better bet.
Yes. A worn or sticking bathtub cartridge can cross-mix hot and cold, drift under flow, or react badly to pressure changes. That is one of the most common tub-only causes.
That usually points to a pressure-balance problem in the bathtub valve. A normal valve may shift a little, but it should not dump the temperature hard to cold and stay there.
Sometimes, but not usually if a nearby sink can hold hot water at the same time. A bathtub uses a lot of hot water fast, so it can expose a weak supply first, but a true tub-only problem usually lives in the tub valve.
No. Check the handle travel and anti-scald limit first. It is a simpler, cheaper fix, and a low limit setting can look a lot like a bad cartridge.
If other fixtures also cool off, move upstream and diagnose the hot-water supply. If the tub is still the only fixture acting up, the remaining suspects are the valve body, a separate pressure-balance section, or an in-wall supply issue that is better handled by a plumber.