Low Hot Water Pressure
One weak fixture and every weak hot tap point to different causes.
By Jonathan West · reviewed 2026-10-28 ·how we research these
Compare hot and cold at the same tap first, because that comparison shows where low hot water pressure starts. If one fixture is weak, suspect its aerator or cartridge. If every hot tap is weak while cold stays normal, the restriction sits between the water heater and the taps.
At DIY Repair Guide, we compiled the method below from general plumbing practice. It suits a home where cold water is normal and hot water is weak at one or more taps. It does not suit a home with no hot water at all. That case is covered on the water heater not heating page. If cold is weak too, use the low water pressure in house page.
Compare Hot and Cold at the Same Tap
Open the hot and cold at one fixture and compare how strong each flow is. Then repeat the test at a second hot tap, such as a shower or another sink.
The comparison gives one of three results, and each points to a different part of the plumbing.
- Only one fixture has weak hot water, so the restriction is local to that fixture.
- Every hot tap is weak and cold is normal, so the restriction sits between the water heater and the fixtures.
- Cold is weak too. The cause is on the main supply.
One Fixture Only: Aerator and Cartridge
When only one fixture is weak, the likely cause is a clogged aerator or a worn single-lever or mixing cartridge at that fixture. Cleaning the aerator is the first check.
Clean the aerator first, then run the tap again. If the hot flow stays weak after cleaning, the cartridge at that fixture is the suspect.
A faulty mixing valve can also give weak hot water. Identifying it is a call-a-plumber job, so note which fixture is weak before you call.
Every Hot Tap: Shutoff Valves and Sediment
When every hot tap is weak and cold is normal, the heater-side causes are:
- A heater inlet or outlet shutoff valve that is not fully open. A partly closed valve chokes flow, and a gate or ball valve can sit part-open.
- Sediment in the tank or lines.
- Corroded old galvanized supply pipe or fittings near the heater.
Valve Position and Flushing
Look at the shutoff valve positions only if you can see them without turning them. Turning, closing or draining a heater valve is work on the heater itself, and a licensed plumber handles that. The ball valve vs gate valve page compares the two valve types.
Manufacturers commonly recommend flushing a tank-type water heater about once a year, and the manual for your model gives the exact interval. Draining a pressurized tank involves scalding water, so a licensed plumber does the flush.
If cold water is also weak, the problem is main supply pressure, and the low water pressure in house page applies.
Corroded Old Supply Pipe and Fittings
Old galvanized supply pipe and corroded fittings near the water heater are a heater-side cause of weak hot water at every hot tap.
Tankless Heaters and Scale
Mineral scale inside a tankless heater can restrict flow, which shows up as weak hot water.
Descaling follows the procedure in the manufacturer's manual for the model. Descaling is work on the heater itself, so a licensed plumber handles it.
Electric and gas water heater repairs that involve gas, venting, the burner or live wiring belong to a licensed plumber, a heating, ventilation and air conditioning (HVAC) technician or an electrician.
Mixing Valves and Recirculation Pumps
A faulty mixing valve, a failed recirculation pump check valve, or a crossover between hot and cold lines at a faucet can each give weak hot water.
These three are identify-and-call items. Identifying which one applies needs a plumber, so the steps here stop at naming the suspect.
A crossover is a connection between the hot and cold lines at a faucet. Before you call, write down which taps are weak and whether cold is normal at each one. Comparing hot and cold at each tap gives you that list.
When to Call a Plumber
A plumber visit commonly runs about $100 to $350 depending on the fix (typical US figures, orientation only, not quotes). The plumber cost page gives the detail.
A relief valve that drips is a different symptom, covered on the water heater relief valve leaking page.
What would change this answer: if cold water is also weak, the cause moves to the main supply and the whole-house page applies. If the relief valve is discharging, that is a separate symptom with its own cause.
Call a licensed plumber when the cause sits on the heater. These jobs go to a licensed plumber:
- Closing a heater shutoff valve
- Opening or draining a pressurized tank
- Flushing the tank
- Descaling a tankless heater
- Any repair that involves gas, venting, the burner or live wiring
Water heaters can scald. Closing a heater shutoff valve, or opening or draining a pressurized tank, involves scalding water, gas or live wiring. Call a licensed plumber for any work on the heater itself.
Symptom-to-Cause Table
The table is compiled at DIY Repair Guide from the sections above and gives the first check for each branch.
Based on the comparison and symptom table compiled here, clean the aerator if only one tap is weak, and call a licensed plumber if every hot tap shows low hot water pressure while cold stays normal. If cold is also weak, the whole-house page applies.
| Symptom | Likely cause | First check | DIY or plumber |
|---|---|---|---|
| Weak hot at one fixture, cold normal | Clogged aerator or worn cartridge at that fixture | Clean the aerator | DIY for the aerator. A plumber for the cartridge |
| Weak hot at every hot tap, cold normal | Partly closed heater shutoff, sediment, or corroded old galvanized pipe or fittings near the heater | Note the shutoff position without turning it | Plumber for any heater-side work |
| Weak hot at kitchen sink only | Aerator, cartridge, or crossover at that faucet | Clean the aerator, then compare other hot taps | Plumber for a crossover |
| Weak hot with a tankless heater | Mineral scale inside the unit | Follow the manual's descaling procedure | Plumber for any work on the heater |
| Weak hot, mixing valve or recirculation pump suspected | Faulty mixing valve, failed recirculation pump check valve | Identify the part | Plumber |
| Cold water also weak | Main supply | Whole-house low water pressure page | Plumber for supply work |
| Discharging temperature and pressure (T&P) valve | Thermal expansion in a closed system, or high static pressure | Note whether the discharge pipe drips | Plumber, see the relief valve page |
| No hot water at all | Different symptom | Water heater not heating page | See that page |
Common questions
Why is my hot water coming out very slowly?
Slow hot water can trace to one fixture or to the heater side, and the comparison at the start shows which. If one fixture is slow, the aerator or cartridge at that fixture is the suspect. If every hot tap is slow while cold is normal, the restriction sits between the water heater and the taps. The usual causes are a partly closed shutoff, sediment, or corroded old galvanized pipe near the heater.
Can a hot water heater affect water pressure?
Yes. A partly closed heater inlet or outlet shutoff valve, sediment in the tank or lines, and corroded old galvanized supply pipe or fittings near the heater can all restrict hot water flow. Mineral scale inside a tankless heater can do the same. A discharging relief valve is a separate symptom and is covered on the water heater relief valve leaking page.
How do I increase the water pressure in my hot tap?
Clean the aerator and check the cartridge at a weak fixture. Confirming that the heater shutoff valves are fully open is heater-side work for a plumber.
Why isn't my kitchen sink getting hot water?
If the kitchen sink is the only tap without hot water, the cause is most likely local to that faucet, so start with the aerator and the cartridge there. If no tap in the house gives hot water, the problem is a different symptom, covered on the water heater not heating page.
Why is my kitchen sink only getting low hot water pressure?
Low hot water at one sink points to that fixture, so clean the aerator first. If that does not restore flow, the cartridge at that sink is the suspect. A crossover between the hot and cold lines at a faucet is also a cause, and a plumber needs to identify it.
Why is my hot water pressure low but the cold water is fine?
Weak hot water with normal cold water points to a fixture or to the heater side, depending on how many hot taps are weak. If only one fixture is weak, look at that fixture first. If every hot tap is weak, the restriction sits between the water heater and the taps, and that is heater-side work for a licensed plumber. If cold is also weak, the cause is on the main supply, and the low water pressure in house page applies.