Quick Answer:
When upstairs water is weak but downstairs is acceptable, the problem is often a combination of a flow restriction in the riser piping and a pressure-reducing valve (PRV) that can’t supply enough flow. Measure pressure upstairs and downstairs during the same flow event, check for partially closed stops, and if the difference persists, evaluate PRV sizing and riser restrictions. If the PRV is the bottleneck, replacement or upsizing is typically needed.
Why This Happens
- PRVs reduce incoming pressure to protect plumbing. Some models limit flow as well as pressure; when the upstairs run requires more flow, the PRV can’t keep up.
- Riser piping or fittings may be partially restricted by deposits, a closed or partially closed valve, or an undersized section that chokes flow to the upper floor.
- Because the downstairs fixtures are nearer the PRV or on a separate branch, they can appear normal while the upstairs suffers.
- Small, easy-to-check problems often sit at the fixture stops; bigger issues involve PRV capacity or riser restrictions and need measurement to confirm.
- If you’re trying to compare locations, use the same flowing fixture to create a true comparison — that confirms whether the issue is flow capacity, not just static pressure differences.
- For reference articles, see the note about upstairs low pressure and related PRV issues in our other material.
Step-by-Step What to Do
1. Measure pressures during the same flow event
- Attach a pressure gauge to a hose bib or the laundry box downstairs and another upstairs if you have two gauges. If you only have one gauge, test one location then the other while running the same fixture for a repeatable load.
- Run a realistic demand (for example, a bathtub fill or two showers) while watching both gauges or noting readings in sequence. Record both static and flowing pressures.
- If downstairs shows adequate flowing pressure while upstairs is several psi lower under the same flow, that proves a flow-restriction or PRV capacity issue.
2. Check for partially closed stops and obvious restrictions upstairs
- Check fixture shutoffs and any local isolation valves serving the upstairs branch. Open them fully and exercise them a few times.
- Look for inline strainers or check valves on the riser or at the PRV that could be clogged with debris.
- Clear aerators and showerheads to avoid misdiagnosing a fixture restriction as a system problem.
3. Evaluate the PRV and riser piping for sizing and restrictions
- Check the PRV model and its rated flow capacity. A PRV sized to reduce pressure but with low flow capacity will cause a noticeable drop when upper-floor demand increases.
- Inspect riser piping size and layout: long, narrow, or heavily elbowed runs reduce available flow. Look for undersized replacement sections or partially closed balancing valves.
- If possible, note the pressure upstream of the PRV and the downstream flowing pressure to see how much pressure the PRV is holding back during demand. This helps identify whether the issue is a pressure drop across the PRV or in the riser.
- If you see a large difference and no local closed valves, consider documentation on a related issue like a pressure drop after PRV for guidance on what to check next.
4. Replace or upsize the PRV if confirmed as the bottleneck
- If testing shows the PRV cannot provide required flow even with the riser clear, replacement with a higher-capacity model or an alternate arrangement (pressure-reducing station, parallel PRVs, or larger feed piping) may be needed.
- Follow manufacturer data for flow (gpm) at the target downstream pressure. Don’t simply crank the PRV higher — that can cause damage and violate local codes.
- Have a plumber size the new PRV and confirm pipe sizes and any required permit or inspection steps before work begins.
What Not to Do
- Don’t assume upstairs-only is “normal” — it can be restriction + PRV capacity, not just a quirk of the system.
- Don’t fully open the PRV without understanding supply pressure and code limits; raising downstream pressure beyond code can stress fixtures and the water heater relief valve.
- Don’t remove the PRV or disconnect sections of piping without shutting off the main and relieving pressure — that’s unsafe and can flood the house.
- Don’t ignore simple shutoff checks; jumping straight to PRV replacement without measuring can waste time and money.
When to Call a Professional
- Call a licensed plumber if you confirm a PRV flow limitation or if riser piping needs re-piping or access work.
- Get help if you see corrosion, leaks, or if replacing the PRV requires soldering, brazing, or cutting into the main feed; these typically need a pro and may require permits.
- If the problem is intermittent or tied to municipal supply fluctuations, a pro can test supply-side pressure and advise on a solution that meets code and fixture protection needs.
Safety Notes
- Always shut off the main water and relieve system pressure before working on valves or the PRV.
- Wear eye protection when working on pressurized fittings. Expect some water to drain when removing a PRV or opening a valve.
- Electrical equipment near work areas should be covered or turned off; water and electricity are a dangerous mix.
- If you are not comfortable with plumbing tools or gas/compound soldering, hire a professional. Incorrect installation can cause leaks, property damage, or code violations.
Common Homeowner Questions
- Q: Why is only the upstairs weak when downstairs is fine?
A: The upstairs is fed through the riser and may be limited by an undersized or restricted riser plus a PRV that can’t deliver required flow under demand. - Q: Can I just adjust the PRV higher to fix it?
A: Not without checking supply pressure and code limits; raising pressure can cause other problems and should be done carefully or by a plumber. - Q: Will replacing the PRV always fix the problem?
A: Often it will if the PRV is the bottleneck, but you must confirm there are no riser restrictions or undersized piping first.