Quick Answer:
If only the hot side at the shower has weak flow, first compare the hot water flow at another fixture (a nearby sink). If the sink has normal hot pressure but the shower is weak, the problem is usually in the shower’s hot-side valve, cartridge or its ports. You can often clear or diagnose the issue yourself, but don’t force components; call a plumber if pressure stays low or you’re unsure.
Why This Happens
Hot-side low flow at a shower can come from a few common causes:
- Mineral build-up or debris blocking the cartridge ports or valve passages.
- A partially closed or damaged internal hot-side valve that restricts flow.
- An issue in the shower mixing cartridge (wear or internal blockage) that limits hot inlet flow.
- Less commonly, a problem in the hot water supply line to the fixture or a failing pressure-balancing device.
Step-by-Step What to Do
1. Confirm it’s only the shower
Run hot water at a nearby sink and at other fixtures. Watch the flow for 30–60 seconds. If the sink’s hot water is normal but the shower is weak, the issue is localized to the shower valve or cartridge.
2. Turn off water supply to the shower
- If there are local shutoffs for the shower, use them. Otherwise, turn off the house’s main while you work on the valve.
- Open a nearby hot tap to relieve pressure before removing parts.
3. Remove the handle and escutcheon to access the cartridge
Carefully remove the handle and trim plate following the manufacturer’s instructions. Keep screws and small parts organized.
4. Inspect the hot-side valve and cartridge ports
- Look for mineral scale, rubber debris from worn seals, or sediment in the hot inlet port of the cartridge or valve body.
- Gently flush the valve body holes with clean water or use a soft brush. Avoid pushing debris deeper into the plumbing.
- If the cartridge is removable, pull it out to inspect its hot inlet side for blockages or damage.
5. Test flow before reassembly
With the cartridge removed and the water back on briefly, test hot flow into the valve body (use a rag or container). If hot flow is now normal, the cartridge or ports were likely the problem. If flow remains low, the restriction may be upstream or in the valve housing.
6. Clean, replace, or reassemble
- Light scale can sometimes be cleaned with a vinegar soak or a scale remover recommended by the valve maker—follow product instructions and rinse thoroughly.
- If parts look damaged or cleaning doesn’t restore flow, replace the cartridge or valve assembly with the correct part for your model.
- Reassemble the trim, restore water, and test the shower hot flow carefully.
What Not to Do
- Don’t force hot-side valves open beyond their stops — this can damage stops or internal parts and make the problem worse.
- Don’t use excessive tools or sharp picks inside valve ports; they can scratch sealing surfaces or push debris into the plumbing.
- If hot pressure stays low after basic checks and cleaning, don’t keep disassembling unfamiliar parts — a licensed plumber is appropriate.
When to Call a Professional
- You’ve cleaned and inspected the cartridge and ports but hot flow is still low.
- There’s corrosion, damaged valve parts, or unclear source of the restriction.
- You’re uncomfortable shutting off the main water, removing the cartridge, or handling potentially hot water lines.
- If the problem may involve the hot water heater or a hidden supply line issue.
Safety Notes
- Turn off the water supply and relieve pressure before working on valves.
- Be careful of hot water — test temperature with your hand at a safe distance before touching components.
- Wear gloves and eye protection if using cleaners or dislodging debris.
- Use the correct replacement cartridge or parts for your valve model; incorrect parts can leak or fail.
If you want reading on related issues, these can help: Low pressure only on cold side at shower and Shower pressure weak in morning only.
Common Homeowner Questions
- Why does only hot water at the shower lose pressure? — Usually a local restriction in the shower valve, cartridge, or port, not the household hot supply.
- Can I fix it by cleaning myself? — Yes, cleaning or replacing the cartridge often fixes it, but only if you are comfortable disassembling the trim and cartridge.
- Will my water heater cause this? — Rarely; if hot flow is normal at other fixtures the heater is unlikely the cause.