Quick Answer:
If only upstairs hot fixtures have low pressure while cold taps are fine, the problem is usually on the hot side: a partially closed valve, a clogged branch line, a failing mixing valve, or the water heater’s supply/recirculation issue. Start by comparing hot vs cold pressure at the same fixture and watching the heater when the pressure drops to see if the heater is reacting.
Why This Happens
- Branch plumbing: Upstairs hot water often runs through a separate branch or manifold. Sediment, scale, or a partially closed valve in that branch can reduce only the hot flow.
- Water heater behavior: If a tankless or recirculation pump is cycling or losing flow, hot pressure can fall while cold stays steady. Observe whether the heater cycles or shows errors during the drop—this is a strong clue. See Pressure drops only when heater running for the specific pattern to look for.
- Mixing or pressure-balancing valves: A failing mixing valve or thermostatic valve can restrict hot flow to upstairs fixtures while leaving cold side normal.
- Time-based demand: Peak usage at certain times can show up more on the hot lines once water has traveled through the heater; for more on that pattern, check Hot pressure varies by time of day.
Step-by-Step What to Do
1. Confirm the scope at one fixture
- At an affected upstairs sink or shower, turn the hot on fully, then switch to cold fully. Compare how strong each feels. If cold is strong and hot is weak, the issue is on the hot side between that fixture and the heater.
2. Check other upstairs fixtures
- Test multiple hot fixtures upstairs. If all upstairs hot fixtures are weak, suspect a shared branch or the heater. If only one fixture is weak, focus on that fixture’s valve, aerator, or shower cartridge.
3. Inspect shutoff valves and accessible fittings
- Locate isolation valves near the heater and at the upstairs manifold. Make sure they are fully open. Slowly open any partially closed valves and note if pressure restores.
4. Watch the heater while you recreate the problem
- With someone at the upstairs tap, observe the heater when pressure drops: listen for cycling, check display lights or error codes, and note if turning the hot tap off makes the heater stop or change. This helps determine if the heater itself is limiting flow.
5. Check for flow restrictions
- Remove and inspect faucet aerators and showerheads for sediment. If cleaning them makes no difference upstairs, suspect a clogged line or valve further back.
6. Inspect mixing valves and recirculation systems
- If you have a thermostatic mixing valve, anti-scald device, or recirculation pump, check for correct operation. A stuck mixing valve can choke hot flow upstairs while cold remains fine.
7. Narrow down by isolating the heater
- Temporarily open a hot tap at a ground-floor fixture and compare pressure to upstairs. If ground-floor hot is normal but upstairs is weak, the issue is in the riser or upstairs manifold. If all hot is weak when the heater runs, the heater or its connections may be the cause.
8. Call a pro for diagnostics if needed
- If the above steps point to a blocked pipe inside walls, a failing heater component, or a recirculation problem you can’t access, hire a plumber to inspect and pressure-test the hot-side plumbing and the heater.
What Not to Do
- Do not replace cold-side plumbing for a hot-side-only problem.
- Don’t force fittings or valves without shutting off the supply—this can cause leaks or damage.
- Avoid guessing at major repairs (like ripping out walls) until you’ve confirmed the problem is in the hot branch or heater.
When to Call a Professional
- There is no improvement after basic checks (valves, aerators, visible fittings).
- You suspect a blocked or damaged pipe inside the wall or attic.
- The heater shows error codes, won’t stay on, or you’re not comfortable working on gas or electrical connections.
Safety Notes
- Hot water can scald. Test with your hand at low volume and adjust temperature before prolonged testing.
- If you need to work near the heater, turn off power or gas according to the manufacturer’s instructions first.
- Do not cut into pipes or remove plumbing components unless you know how to shut off and drain the system; uncontrolled water can cause damage and injury.
Common Homeowner Questions
- Why does cold stay strong while hot is weak? Because the supply to the hot side includes the heater and any hot-only branches that can become clogged or have closed valves.
- Could the water heater be the problem? Yes. A failing tankless unit, blocked inlet, or faulty recirculation pump can reduce hot pressure upstairs.
- Is this expensive to fix? It depends—cleaning aerators or opening a valve is inexpensive; repairing internal piping or a heater component can cost more and may require a plumber.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Pressure Fluctuates Only on Hot Water.