Quick Answer:
If the shower is weak only when set toward hot, first compare the hot flow at a nearby sink. If the sink’s hot pressure is normal but the shower’s hot flow is weak, check that the shower valve’s hot-side stop is fully open and not clogged. If simple checks don’t fix it, a plumber is the safest next step.
Why This Happens
Low pressure only on the hot side usually comes from a restriction somewhere in the hot supply to the shower or inside the shower valve itself. Common causes:
- Mineral buildup or debris in the shower cartridge or valve port on the hot side.
- A partially closed or clogged shutoff (hot-side stop) at the valve behind the trim plate.
- An issue in the hot supply line to the shower, which may show up in other fixtures on the same line.
If you’re seeing the opposite symptom in another bathroom, check the related guide for **Low pressure only on cold side at shower**. If the problem only appears at certain times of day, consider checking resources like **Shower pressure weak in morning only** for timing-related causes.
Step-by-Step What to Do
1. Compare hot flow at a nearby sink
Turn the shower fully to the hot setting and note how weak the flow is. Then open the hot tap at a nearby sink (same floor and ideally same wall) and compare:
- If the sink’s hot flow is normal, the problem is likely at the shower valve or its connection.
- If the sink’s hot is also weak, you may have a wider hot-supply issue and should inspect other fixtures or the water heater feed.
2. Test flow with the showerhead removed
Remove the showerhead and run hot water from the stub-out.
- If flow improves substantially with the head off, the showerhead or its flow restrictor is the problem—clean or replace the head.
- If flow is still weak with the head off, the restriction is inside the valve or hot supply line.
3. Check the hot-side stop at the valve
Most modern shower valves have small supply stops behind the escutcheon or a removable cap. Steps:
- Remove the trim plate per the manufacturer’s instructions to expose the stops.
- Locate the hot-side stop (usually labeled or on the hot inlet). Try opening it fully. If it won’t move or is hard to turn, it may be clogged.
- If the stop is removable, carefully turn it back and forth to free mineral deposits. If that doesn’t help, note its condition for the plumber.
4. Inspect the cartridge or valve port
If the stops and supply look OK, the cartridge or valve port can be clogged with scale or debris.
- Shut off the main water supply (or local shutoffs if available) before pulling a cartridge.
- Remove the cartridge per the valve instructions and look for scale, grit, or broken pieces blocking the hot inlet.
- Clean gently with a soft brush and vinegar solution if light mineral buildup is present. Reinstall and test.
5. If the restriction persists
If you still have low hot flow after these checks, the restriction may be inside the wall piping or the valve body (hidden debris, corrosion, or a damaged port). At that point, stop and consider professional help.
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 water returns.
- Don’t force or hammer stuck stops, cartridges, or fittings; you can break the valve or piping.
- Don’t use aggressive chemicals inside the valve without guidance—these can damage seals and finishes.
- If a hot-side restriction persists after your checks, don’t keep trying risky fixes yourself—call a plumber.
When to Call a Professional
- Both the sink and shower have weak hot flow — likely a supply or heater issue that needs a pro.
- The cartridge or valve is seized, corroded, or you can’t access the stops safely.
- After removing parts you still have low hot flow, or you see damaged piping or heavy corrosion.
- You’re uncomfortable shutting off the main, removing the cartridge, or working inside the wall cavity.
Safety Notes
- Turn off water at the main or local shutoffs before removing valves or cartridges.
- When testing hot water, run slowly and check temperature to avoid scalding—try with a cup first, not bare hands.
- Keep towels or a bucket handy to catch water when removing trim and cartridges.
- If you smell gas, see major corrosion, or discover unexpected damage, stop and call a professional immediately.
Common Homeowner Questions
- Why is only the hot side weak?
Usually a blockage or partially closed hot-side stop, or mineral buildup in the valve or cartridge. - Can I replace the cartridge myself?
Yes if you’re comfortable shutting off water and following the valve instructions; otherwise call a plumber. - Will adjusting temperature controls fix low pressure?
No—temperature adjustment changes mix, not supply flow. Adjusting won’t remove a physical restriction.