Quick Answer:
If the shower is weak only when you turn it toward hot, first compare the hot flow at a nearby sink. If the sink’s hot flow is normal but the shower’s hot side is weak, the problem is most likely at the shower valve or its hot-side shutoff. Check that the hot-side stop at the valve is fully open and not clogged, remove the showerhead to test raw flow, and inspect the cartridge or mixing mechanism. If those checks don’t fix it, call a plumber.
Related issues include Low pressure only on cold side at shower and Shower pressure weak in morning only.
Why This Happens
When cold-side pressure is fine but hot-side flow is weak, it usually means the restriction is after the water heater — commonly in the valve, a seat, or a supply stop that serves only the shower. Sediment, mineral buildup, or a partially closed/shrunk rubber seat inside the valve can reduce hot flow without affecting other fixtures.
Step-by-Step What to Do
Step 1 — Compare hot pressure at a nearby sink
- Turn on the hot water at a sink near the shower and run it full. Note the flow and compare it to the shower when set fully hot.
- If the sink has normal hot flow and the shower is weak only on hot, the hot supply to the house and the water heater are likely fine.
Step 2 — Remove the showerhead and test raw flow
- Unscrew the showerhead and run the shower on hot. If flow improves, the showerhead or its internal screen is clogged; clean or replace the head.
- If flow stays weak with the head off, the restriction is inside the valve or the shower supply piping.
Step 3 — Verify the hot-side stop at the shower valve
- Locate the stop/shutoff for the shower valve (often behind an access panel or under the tub).
- Make sure the hot-side stop is fully open. If it’s only partially open or stiff from mineral deposits it may restrict flow.
- If the stop is removable, shut off water, remove and inspect for debris or corrosion. Replace or clean if clogged. If you can’t access it safely, skip to calling a plumber.
Step 4 — Inspect the cartridge or mixing valve
- If stops and the showerhead are fine, the cartridge or internal mixing parts may be blocked or worn. Sediment often lodges in the cartridge ports mainly on the hot side.
- Removing and inspecting the cartridge is a common remedy, but follow the valve manufacturer’s steps or use a tutorial for your valve model. If unsure, have a plumber handle it.
Step 5 — Reassemble and test carefully
- After any cleaning or replacement, reassemble and test both hot and cold. Run water long enough to clear any debris.
- Watch for temperature stability and adequate flow before putting the access panel back.
What Not to Do
- Don’t crank the handle hotter to compensate for low hot flow—this can cause sudden temperature swings and scalding when cold flow changes.
- Don’t force or overtighten valve parts or stops; that can damage threads and seats.
- Don’t use aggressive chemicals inside the valve body or piping; they can damage seals and finishes.
- If a hot-side restriction persists after basic checks, don’t keep tinkering—call a plumber to avoid making the problem worse.
When to Call a Professional
Call a licensed plumber if any of the following apply:
- You cannot access the valve stops or cartridge safely.
- Hot flow is weak at the shower but normal at nearby sinks after you checked stops and the head—this points to a valve or internal supply problem that usually needs replacement or service.
- Removing the cartridge or replacing parts is outside your comfort level, or the valve is old and parts aren’t fitting correctly.
- There’s visible corrosion, leaking when you operate stops, or signs of a larger supply blockage in the wall.
Safety Notes
- Turn off the water supply to the valve before removing cartridges or stops. Drain lines by opening the shower first.
- Be careful with hot water—run tests slowly to avoid scalding. Use a thermometer if you need to check outlet temperature.
- If you find damp insulation, rotten wood, or active leaks in walls, stop work and call a professional to avoid mold or structural damage.
- If you’re not sure which parts match your valve, stop and get help—forcing wrong parts can lead to leaks or failure.
Common Homeowner Questions
- Q: Could the water heater be the cause?
A: If nearby sinks have normal hot flow, the heater is unlikely the problem; the issue is probably at the shower valve or supply stop. - Q: Can I fix the cartridge myself?
A: You can if you’re comfortable following the valve manufacturer’s instructions and have the right tools; otherwise hire a plumber. - Q: Is this usually expensive to repair?
A: Simple cleaning or replacing a cartridge is moderate cost; complex in-wall repairs or replacing valves are more costly—get an estimate from a plumber.