Quick Answer:
If cold water pressure is normal but hot water feels weak at the same faucets, the problem is usually on the hot side only. Start by comparing hot and cold at the same fixture, watch how the water heater behaves while pressure drops, and follow a few simple checks before replacing parts.
Why This Happens
- Restricted flow on the hot side: mineral buildup in the water heater, a partially closed hot shutoff valve, or a clogged faucet cartridge can limit hot flow without affecting cold.
- Internal heater issues: a damaged dip tube, sediment plugging the outlet, or an internal valve can reduce hot output intermittently.
- Appliance or system interactions: sometimes the heater refills or cycles in a way that drops pressure briefly — see Pressure loss when water heater refills for how refilling behavior can affect flow.
- Multiple hot outlets in use: if hot pressure falls only when several taps are open, the problem is capacity or distribution on the hot side rather than the cold.
Step-by-Step What to Do
1. Compare hot vs cold at the same fixture
- Turn the cold tap fully on and note flow strength.
- Turn off cold, turn the hot fully on at the same handle or separate lever, and compare. If cold is strong and hot is weak at the same point, the issue is on the hot side.
2. Check simple valves and aerators
- Make sure the hot-side shutoff (usually near the water heater or under the sink) is fully open.
- Remove the faucet aerator or showerhead to check for blockages. Sediment can reduce hot flow more if it accumulates in hot lines and fittings.
3. Observe the water heater during fluctuation
- While someone runs hot water, watch the heater: listen for unusual noises, see if the unit cycles on and off, and note any brief pressure loss when the unit refills or fires.
- If pressure drops coincide with the heater refilling or firing, it points to internal flow restriction or valve behavior; this is related to **Hot side pressure drops when multiple taps open** in how hot-side demand affects pressure.
4. Check for sediment and simple fixes
- Drain a few gallons from the water heater per the owner’s manual to clear loose sediment (only if you’re comfortable doing this). Sediment near the outlet can reduce hot flow.
- Inspect the inlet and outlet valves for mineral crust or partial blockage.
5. Test other hot outlets and fixtures
- Open another hot faucet or shower. If the weak pressure is present at all hot outlets, the issue is likely at the heater or main hot branch. If it’s only at one fixture, the problem is the fixture or its immediate plumbing.
6. Narrow down before buying parts
- Replace or clean faucet cartridges only after confirming the problem is isolated to that fixture.
- If the heater appears to be the cause, take notes on sounds, timing, and which fixtures are affected before contacting a pro.
What Not to Do
- Do not replace cold-side plumbing for a hot-side-only problem — that won’t fix hot pressure and wastes time and money.
- Do not bypass safety devices on the water heater or disable the pressure relief valve.
- Avoid aggressive DIY work on the gas or electrical connections of a water heater unless you are qualified.
When to Call a Professional
- You’ve confirmed cold is normal and you still can’t find the hot-side restriction.
- Draining the heater or light maintenance doesn’t restore flow or you see signs of internal damage, leaks, or heavy sediment.
- The heater cycles abnormally, produces noises, or you suspect the internal plumbing of the heater is failing. A pro can safely inspect, test flow, and replace internal parts if needed.
Safety Notes
- Turn off power (electric) or pilot/gas supply (gas) before doing any service on a water heater.
- Allow hot water to cool before draining lines to avoid scalding.
- If you are unsure about working on a gas appliance or handling wiring, call a licensed technician.
Common Homeowner Questions
- Why does only the hot pressure drop sometimes? Short answer: something is restricting flow on the hot side or the heater is changing flow when it refills or cycles.
- Can a faucet cartridge cause this? Yes — a clogged or failing cartridge on the hot side can make hot flow weak at that fixture only.
- Is a new water heater always the answer? No — many hot-pressure problems are fixable by cleaning valves, flushing sediment, or adjusting shutoffs; replace the heater only if it’s failing or unsafe.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Pressure Fluctuates Only on Hot Water.