Quick Answer:
If household water pressure is calm in the evening but fixtures blast in the morning, the water heater is likely heating water and raising pressure inside a closed system. Use a max-needle gauge overnight to confirm a slow pressure rise. If the morning peak is much higher than the evening static reading, the system is trapping thermal expansion. Repair, charge, or resize the expansion tank and verify the PRV/backflow/check valve setup, then recheck the next morning.
Why This Happens
- Water expands as it heats. In an open system the extra volume flows back into the supply, but in a closed system it raises pressure inside your plumbing.
- Even if nobody runs water, modern heaters cycle for temperature control and maintenance. Those cycles can slowly increase pressure overnight.
- If the expansion tank is undersized, waterlogged, or uncharged, it can’t absorb the extra volume. A stuck or incorrectly installed pressure-reducing valve (PRV), backflow preventer, or check valve will keep the added pressure trapped.
- If you see noisy or sudden morning pressure surges you may also notice problems covered in **Water heater causes system over-pressure** and in cases where toilets run after heating, **Thermal expansion causing toilet fill leaks**.
Step-by-Step What to Do
Step 1 — Fit a max-needle gauge and record evening static pressure
Attach a reliable max-needle pressure gauge to an outside hose bib or a indoor faucet that’s on the cold side. Note the static pressure in the evening before the heater cycles much (for example, before bed).
Step 2 — Leave the gauge on overnight
Keep the gauge in place for the full night. Do not run water in the house after the evening reading so you see pressure changes caused only by heating cycles.
Step 3 — Check the morning peak reading
Read the gauge in the morning. If the morning peak is far higher than the evening static, thermal expansion is accumulating in a closed system and must be addressed.
Step 4 — Inspect the expansion tank
- Tap the tank: a hollow sound at the top and solid at the bottom indicates air cushion. A uniformly solid sound suggests waterlogging.
- Check or measure the tank’s precharge pressure with a standard tire gauge (air valve at the tank). Compare to the system static pressure and the manufacturer’s recommendation. Recharge or replace if wrong.
- Repair, charge, or resize the expansion tank as needed to handle your water heater’s thermal expansion volume.
Step 5 — Verify PRV/backflow/check valve configuration
Confirm the PRV (pressure-reducing valve), backflow preventer, and any check valves are installed correctly and not trapping pressure unintentionally. If a PRV is set too high or a backflow/check valve is leaking closed, the house becomes a closed system with nowhere for expansion to go.
Step 6 — Recheck the next morning
After fixes, repeat the max-needle gauge test and compare evening and morning readings. If the pressure rise is resolved, the steps worked. If not, further diagnosis is needed.
What Not to Do
- Don’t assume pressure can’t rise when no water is used—heater maintenance cycles can build pressure overnight in closed systems.
- Don’t repeatedly open the temperature-and-pressure (T&P) relief valve to relieve system pressure. That can damage the valve and waste hot water; it’s an emergency safety device, not a normal pressure-control method.
- Don’t bypass a PRV, backflow preventer, or other safety device to “fix” pressure—doing so can violate codes and create contamination or safety hazards.
When to Call a Professional
- Call a licensed plumber if you find high overnight pressure that you can’t fix by recharging or replacing the expansion tank.
- Call a pro if you can’t identify whether a PRV, backflow preventer, or check valve is causing the closed-system condition or if these components need adjustment or replacement.
- Call immediately if you see leaks, bulging fittings, or repeated T&P valve discharge—those are signs of dangerous overpressure.
Safety Notes
- High water pressure can burst pipes, damage appliances, and trigger T&P relief discharge—treat repeated overpressure seriously.
- If you suspect dangerous pressure, shut off the cold water inlet to the water heater and turn off power or gas to the heater before calling a professional.
- Do not adjust or replace a T&P relief valve unless you understand the procedure; it’s a safety component. Testing is fine, but replacement should follow manufacturer instructions or a pro’s guidance.
Common Homeowner Questions
- Why did the pressure only spike at night? The heater was cycling and the closed system trapped thermal expansion while you weren’t using water.
- Will a simple recharge of the expansion tank fix it? Often yes, if the tank is only undercharged; if the tank is waterlogged or too small it will need repair or replacement.
- Can I just lower the water heater temperature? Lowering temperature reduces expansion slightly but it’s not a proper fix for an undersized or failed expansion tank or a closed-system configuration issue.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Thermal Expansion Pressure Behavior.