When cold pressure is normal but hot pressure is low, the restriction is probably on the hot-water side: a partly closed valve, sediment, a mixing cartridge or aerator, a restricted pipe, or the water heater itself. Comparing several fixtures helps show whether the issue is local or house-wide.
Compare several faucets and showers
If only one sink is affected, start with its aerator, supply lines, and cartridge. If every fixture has low hot flow, the cause is more likely near the water heater or main hot-water distribution.
Check accessible valves that may be partly closed
A water-heater outlet valve or isolation valve moved during service can be left partly closed. Do not force old corroded valves or work on hot, pressurized connections.
Sediment can restrict flow over time
Scale and debris can accumulate in aerators, cartridges, piping, and some water heaters. If the problem developed gradually throughout the home, hard-water history and heater maintenance are useful clues.
A noisy, leaking, or abnormally hot water heater needs service
Do not open a gas or electric water heater to search for an internal restriction. If there is leakage, relief-valve discharge, gas odor, damaged wiring, or unsafe temperature changes, stop using it and arrange service.
Sources
- U.S. Environmental Protection Agency WaterSense — Home Maintenance — Household maintenance related to water, leaks, toilets, pressure, and efficient use.