Quick Answer:
When hot water pressure changes on a predictable daily schedule, the cause is usually demand, the heater’s operating pattern, or a pressure control device reacting at certain times. A quick check is to compare hot and cold pressure at the same fixture and watch how the water heater behaves while the pressure is fluctuating. That will narrow whether the problem is in the heater, the house plumbing, or the supply.
Why This Happens
- Peak demand: Morning and evening water use in the neighborhood or in your home can lower supply pressure and affect hot water more if the heater or recirculation pump struggles to keep up.
- Water heater cycling: Tankless units and older tanks can change output when they cycle on or off, creating patterns of higher and lower hot pressure.
- Pressure regulator or valves: A pressure-reducing valve or thermostatic mixing valve can behave differently as inlet pressure changes during the day.
- Mineral buildup or partial blockage: Narrowing in hot-side piping, valves, or the heater can show time-of-day effects when flow demand changes.
- Recirculation or timed controls: Pumps or control systems that run on schedules will change flow and pressure at set times.
- If you notice a sudden drop during use, see **Hot water pressure drops mid-shower**; if the pressure spikes instead of dipping, see **Hot water pressure surges**.
Step-by-Step What to Do
1. Note the pattern
- Record when the pressure changes (time of day, duration, and frequency).
- Note what else is running in the house (dishwasher, laundry, irrigation) when the changes happen.
2. Compare hot vs cold at the same fixture
- Turn the faucet to full hot and measure how the flow feels or use a bucket and stopwatch to estimate gallons per minute.
- Do the same for cold at the exact same setting. If cold is steady but hot drops, the issue is likely on the hot side.
3. Observe the water heater during fluctuations
- Watch the heater while the pressure changes. For tankless units, note if it cycles on and off quickly. For tanks, listen for the burner starting or stopping.
- Check for error codes or indicator lights on modern heaters that could point to internal faults.
4. Check valves, aerators and filters
- Make sure fixture aerators and showerheads aren’t partially blocked. Remove and run the hot briefly to see if flow improves.
- Ensure shutoff valves to the heater and main valves are fully open.
5. Look at pressure control devices
- If your house has a pressure-reducing valve (PRV) or mixing valve, check its settings and look for signs of intermittent operation or leakage.
- Note whether the recirculation pump (if present) runs on a timer—this can explain time-linked changes.
6. Try simple mitigations
- Run hot water at off-peak times or reduce simultaneous hot-water use to see if the pattern changes.
- Temporarily disable nonessential timed devices (such as irrigation or recirc timers) and retest.
7. If tests point to the heater
- For tank units: drain a small amount to check for heavy sediment and observe if flow improves later (do not fully drain unless you know how).
- For tankless: note whether the unit is undersized for peak demand or has scale buildup causing intermittent flow reduction.
What Not to Do
- Do not replace cold-side plumbing for a hot-side-only problem. That wastes money and won’t fix a heater or hot-side valve issue.
- Do not open gas valves, burners, or electrical panels without proper knowledge—leave that to professionals if you suspect heater faults.
- Do not over-tighten or force old fittings; this can cause leaks or damage and create bigger problems.
When to Call a Professional
- If the hot pressure is poor at every fixture and simple checks didn’t help.
- If the water heater shows error codes, leaks, unusual noises, or you smell gas.
- If you find a failing PRV, mixing valve, or suspect significant scale buildup in a tankless unit.
- When repairs require gas, electrical, or internal heater work—hire a licensed technician for safe, correct service.
Safety Notes
- Turn off electrical power or gas supply to the heater before attempting any internal inspection.
- Be cautious of scalding when testing hot outlets—keep testing flows short and use a thermometer if needed.
- If you smell gas, leave the house and call your gas provider or emergency services immediately.
Common Homeowner Questions
- Why does it only happen at certain times?
Because supply pressure and household demand change through the day, and heater controls or pumps may run on schedules. - Can I fix it myself?
Basic checks like comparing hot vs cold, removing aerators, and observing the heater are safe; repairs to the heater or gas/electrical systems should be done by pros. - How fast should a plumber respond?
If there’s no hot water, leaks, or gas smell, call right away; for intermittent pressure issues, schedule a diagnostic visit during a time when the pattern occurs.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Pressure Fluctuates Only on Hot Water.