Quick Answer:
If your shower temperature started swinging after a repair, the most common cause is loose debris or a disturbed cartridge/valve. Start with simple checks you can do yourself: remove and clean the showerhead, flush the valve briefly, and test changes. If those steps don’t fix it, call a pro.
Why This Happens
Plumbing work can dislodge mineral scale, pipe paste, or bits of solder that travel to the shower valve. Those particles can lodge in the cartridge or restrict flow, causing sudden temperature changes when the flow balance shifts. Similar symptoms also appear after supply interruptions — see Temperature shifts after city outage for that scenario. Repairs can also loosen a cartridge or change the way the mixing valve seats, so small movements create swings in water temperature.
Step-by-Step What to Do
1. Turn off the water to the shower
- Find the local shut-off for that fixture or turn off the main water if needed. This reduces the chance of spraying when you remove parts.
2. Remove and clean the showerhead
- Unscrew the showerhead and look for debris inside. Flush the head with water and rinse away mineral grain or solder bits.
- Run water through the arm for a few seconds to push out loose material before reinstalling the head.
3. Flush the valve briefly
- With the cartridge or trim removed (if comfortable doing so), open the water briefly to flush any debris from the valve body. Do this only for a few seconds to clear particles.
- Collect water with a cup or towel so it doesn’t spray the wall or floor.
4. Inspect and gently clean the cartridge or mixing element
- If you can remove the cartridge easily, look for grit or scoring. Rinse it and the valve bore. Do not use harsh tools that could scratch sealing surfaces.
5. Reassemble and test slowly
- Put parts back together, turn water on, and test with slow temperature changes. Watch for consistent hot-to-cold balance.
- If short bursts of hot or cold continue, note whether the swings happen under certain flow conditions (low flow, both handles, etc.).
What Not to Do
- Do not assume repairs are unrelated; debris often lodges in cartridges after work.
- Don’t use metal picks or wire brushes on cartridge sealing surfaces — they can damage seals and make leaks or fluctuations worse.
- Don’t run the valve wide-open if you suspect hot-water scalding; test slowly to avoid burns.
- Don’t replace parts at random without diagnosing — replacing the wrong component can be unnecessary and costly.
When to Call a Professional
- After the basic cleaning and flushing steps, if temperature swings continue, especially under normal flow, call a plumber.
- If you see leaks when the trim is reinstalled or the cartridge is hard to remove, stop and call a pro to avoid damage.
- If the issue appears limited to the hot supply, for example, Temp fluctuates only on hot side, it often points to the water heater or a one-sided valve problem that a technician should evaluate.
Safety Notes
- Turn off water before removing the showerhead or trim. If you’re unsure where shut-offs are, turn off the main supply.
- Wear gloves to avoid cuts from sharp debris and to protect your skin from hot water.
- Do not use an open flame or heat near plumbing work. Avoid electrical contact around wet areas.
- If you’re uncomfortable disassembling valves or if tools required exceed your skill level, stop and call a licensed plumber.
Common Homeowner Questions
- Why did this start right after the repair? Small particles from the repair often get dislodged and travel to the valve, causing temporary problems.
- How long should I try cleaning before calling a plumber? If cleaning and flushing don’t stop swings after one short test and reassembly, call a professional the same day or within a couple of days.
- Can the water heater cause sudden shower temperature swings? Yes — issues with the water heater, like a faulty dip tube or thermostat, can cause inconsistent hot supply and should be inspected by a technician.