If your showers are getting shorter or the hot water seems to run out faster than it used to, the bathtub test is a simple way to get a rough idea of how well your water heater is performing.
How to Perform the Bathtub Test
Make sure no other hot water is being used in the house (no running dishwashers, washing machines, or other showers).
Set the bathtub faucet to full hot water only.
Start a timer as soon as you turn the water on.
Let the tub fill and note when the water temperature starts to drop or becomes lukewarm.
What the results can tell you:
Filling a standard bathtub with steady hot water usually means the water heater is still meeting typical household demand.
If the water cools before the tub is reasonably full, the unit may be undersized, full of sediment, or having trouble recovering.
Very limited hot water often points to an aging tank, heavy sediment buildup, or a heating element/burner issue.
This test is only a general indicator. Water pressure, incoming water temperature, and recent hot water use can all affect the outcome.
Common Reasons a Water Heater Fails the Test
Sediment has reduced the tank’s usable capacity
A heating element or gas burner is failing
The tank is too small for your household’s needs
The unit is simply old (most traditional tank water heaters last 8–12 years)
Next Steps
If your water heater struggles with the bathtub test or you’ve noticed declining performance, it’s worth having it inspected. A technician can check for sediment, test the heating components, and help you decide whether a repair, flush, or replacement makes the most sense.
Professional Water Heater Service
Jeffrey’s Appliance Service has been repairing gas and electric water heaters in the Spokane Valley and surrounding areas since 1985. Our factory-trained technicians can evaluate your unit and recommend the best solution.