Quick Answer:
If hot water pressure falls only when two or more hot taps are open, the problem is on the hot side of the system — often the water heater, a hot-side mixing valve, or a partial blockage. Start by comparing hot and cold at the same fixture and watch how the water heater responds when the pressure drops.
Why This Happens
Cold and hot water normally share the same supply pressure coming into the house. If pressure remains normal on the cold side but falls only on hot taps, the likely causes are inside the hot system: a clogged outlet or internal part of the water heater, a failing check valve or mixing valve, or a restriction in hot-side pipes. Plumbing fixtures that mix hot and cold can hide the difference unless you compare them directly. For a focused look at a situation where hot behaves oddly while cold is stable, see the note on Pressure unstable on hot side only.
Step-by-Step What to Do
1. Compare hot vs cold at the same fixture
- Close other taps. At a single faucet, open the hot side only and note the flow. Then open the cold side only and compare flow or pressure by feel.
- If you have a simple flow cup or can time filling a container, that gives a quick comparison: equal fill times suggest similar pressure; much slower hot flow points to a hot-side restriction.
2. Observe the heater while recreating the drop
- With someone else opening multiple hot taps, watch the water heater for signs: indicator lights, pilot behavior, burner cycling, unusual noises, or warning lights on a tankless unit. Note whether the heater reduces output, cycles off, or shows error codes.
- Do not try to open or service gas or electrical components yourself. Just observe and record behavior to tell a technician.
3. Check accessible valves and mixing devices
- Make sure the hot inlet valve to the water heater and any visible shutoffs are fully open.
- Check thermostatic mixing valves, anti-scald devices, and shower cartridges — these can restrict hot flow when multiple fixtures demand hot water.
4. Test at more than one fixture
- Open a hot tap in a different room. If the drop happens house-wide, it points to the heater or a main hot supply. If it’s limited to one area, the issue is local to those pipes or fittings.
5. Use simple measurements if available
- If you own a pressure gauge that attaches to a hose bib, you can compare static and running pressure on the nearest accessible hot and cold outlets. This gives clearer evidence of a hot-side restriction.
What Not to Do
- Do not replace cold-side plumbing for a hot-side-only problem. That wastes money and won’t fix a hot-side restriction or heater issue.
- Do not open gas lines, attempt major water heater repairs, or remove electrical covers on the heater if you are not qualified.
- Do not ignore gas smells or persistent leaks — leave the area and call emergency services or a professional.
When to Call a Professional
Call a plumber if:
- The hot pressure drop is house-wide or recurs after simple checks.
- The water heater shows error codes, unusual noises, or won’t produce steady hot water during the test.
- You find corrosion, leaks, or evidence of a failing check valve or mixing valve.
If tests show a clear difference between hot and cold pressure, or you want a diagnosis and repair rather than trial-and-error work, bring in a pro. For related diagnostics on uneven supply, see the note on Pressure uneven between hot and cold.
Safety Notes
- If you smell gas, evacuate the building and call emergency services — do not operate electrical switches or the heater.
- Turn off water at the main if you encounter a major leak. Secure the water heater area and avoid electrical contact with standing water.
- Limit your checks to what is visible and safe: observation, opening/closing accessible valves, and simple flow comparisons. Leave gas, electrical, and tank internals to qualified technicians.
Common Homeowner Questions
- Why does cold stay strong while hot drops?
Because the restriction or failure is on the hot side — inside the heater, a hot-only valve, or hot piping. - Can I fix a hot-side blockage myself?
Minor cartridge or aerator cleanings are OK; anything involving the water heater, gas, or internal valves should be done by a pro. - Is a new water heater likely needed?
Not always. A technician will check valves, connections, and internal components before recommending replacement.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Pressure Fluctuates Only on Hot Water.