Quick Answer:
If you notice temperature swings only when you open the hot side, the likely causes are a partial restriction in the hot supply path or obstruction at the cartridge hot port. A quick way to check is to compare hot pressure at the shower with hot pressure at a nearby sink; if the shower’s hot flow is noticeably weaker, suspect a restricted shut‑off, a kinked supply line, or debris in the cartridge. Follow the steps below to narrow it down safely.
Why This Happens
When cold flow stays steady and hot swings, the problem is usually local to the hot-water route feeding that fixture rather than the water heater itself. Common problems are:
- Partially closed or failing isolation (hot) stop valve feeding the shower.
- A kinked or crushed flexible supply line between the shutoff and the valve.
- Debris or mineral buildup blocking the hot inlet port of the shower cartridge or internal screens.
- A failing pressure-balancing or thermostatic cartridge that isn’t letting hot water flow consistently.
If the issue appeared right after changing parts, see Temperature unstable after replacing valve for possible causes tied to installation or compatible parts.
Step-by-Step What to Do
1. Prepare safely
- Close other fixtures in the house so you can judge flow from just the shower and one sink.
- Start with a modest hot setting to avoid scalding while you test.
- Have a towel and a bucket nearby in case you need to catch water while you inspect.
2. Compare hot pressure at the shower to a nearby sink
- Run only hot water at the shower and at a nearby sink (one at a time). Use feel or fill a 1‑liter cup and time how long it takes to fill at each point. This gives a rough flow comparison.
- If the sink’s hot flow is much stronger than the shower’s, the restriction is likely in the shower’s hot supply path (valve stop, line, or cartridge port).
3. Check the shower hot shutoff/stop
- Locate the isolation stops for the shower (often behind an access panel or under a nearby tub access). Confirm the hot stop is fully open by turning it gently. Don’t force a stuck valve.
- If access is limited, don’t cut walls—call a plumber to open the access properly.
4. Inspect supply lines where you can
- Look for obvious kinks or crushed sections on flexible supply lines where they run to the valve. Kinks can reduce hot flow dramatically.
- If you find a kink and can reach it without altering plumbing, reposition or replace the flexible line. If the line is within a wall, stop and call a pro.
5. Flush and inspect the cartridge area (only if comfortable)
- Turn off the shower supply and, if needed, the house hot water before disassembling trim. Take photos to remember reassembly order.
- Remove the cartridge following the manufacturer’s guidance. Inspect the hot inlet port and any screens for debris or mineral buildup and rinse them out. Reinstall and test. If you see damage or heavy corrosion, replace the cartridge.
- If you prefer not to remove the cartridge, flushing the valve through the shower supply after closing the house water and briefly opening the supply can dislodge loose debris—only do this if you know how to isolate and open valve ports safely.
While testing and diagnosing, remember that intermittent behavior such as warm periods followed by loss of hot is similar to patterns discussed in Hot water fades then returns.
6. Test after each change
- After each adjustment or cleaning, restore supplies and test the shower hot flow. Proceed one step at a time so you know what fixed (or didn’t fix) the issue.
What Not to Do
- Don’t keep forcing the handle hotter to overcome hot-side fluctuation—this can cause sudden scalding when the hot flow returns at full pressure.
- Don’t attempt major plumbing work (soldering, cutting supply lines, or opening walls) unless you are qualified and have the proper permits.
- If the hot-side-only problem persists after simple checks, don’t delay in hiring a plumber—continuing to use a fixture that can suddenly deliver very hot water is unsafe.
When to Call a Professional
- If the shower’s hot flow is still weaker than the sink after checking stops and lines.
- If you can’t access isolation valves or cartridge without cutting walls, or if valves are stuck and won’t open.
- When you find damage, heavy corrosion, or you’re unsure about reassembling the cartridge and trim correctly.
- If you’re uncomfortable working around hot water and potential scalding risks.
Safety Notes
- Always test with a cautious temperature setting when diagnosing—don’t rely on maximum hot to evaluate flow.
- Shut off the relevant water supply before removing handles or cartridges. If unsure which valve controls the shower, shut off the house main.
- Wear gloves and eye protection when working on valves. Have a towel ready to catch drips when removing cartridges.
- Lower the water heater temperature to a safe setting (120°F / 49°C) if multiple fixtures are affected or if you’re worried about scalding during tests.
Common Homeowner Questions
- Why does only the hot side fluctuate while cold stays steady? — A partial blockage or failing component in the hot supply to that fixture is the most common cause.
- Can I clear the restriction myself? — You can do basic checks (compare flow, open isolation stops, inspect visible lines) and clean a cartridge if you’re comfortable, but stop and call a plumber if you lack access or confidence.
- Could the water heater be causing this? — If many fixtures show hot fluctuation, yes. If only one fixture does, the problem is almost always local to that fixture’s hot supply or valve.