Quick Answer:
If cold static pressure looks normal but the gauge jumps when the water heater finishes a heat cycle, thermal expansion inside a closed system is the likely cause. Take a cold static reading, let the heater run a full heat cycle without drawing water, then note the peak pressure to confirm expansion. If pressure spikes, check the closed-system components and the expansion tank on the cold inlet so it can absorb the change.
Why This Happens
Water expands as it is heated. In an open system that can push water back into the street, that small volume change doesn’t raise household pressure much. In a closed system — one with a pressure-reducing valve, check valve, or a backflow preventer — the expanded water has nowhere to go, so pressure rises inside the plumbing. The expansion tank and its correct location and size are what prevent large pressure spikes. If the tank is missing, undersized, or isolated from the cold inlet, the system can show steady cold pressure but significant peaks after a heat cycle. Problems like Expansion tank undersized problems and other restrictions let pressure climb when the heater runs.
Step-by-Step What to Do
1. Prepare a pressure gauge and safety gear
- Use a simple screw-on pressure gauge on an easily accessible hose bib or the test port at the pressure-reducing valve. Wear eye protection and gloves if you’ll be near the water heater.
2. Take a cold static reading
- Make sure no water is running in the house. Open the hose bib briefly to bleed any trapped air, close it, attach the gauge, and record the cold static pressure shown on the gauge.
3. Trigger a heat cycle without drawing water and watch the peak
- Cause the water heater to call for heat (many gas or electric heaters will cycle automatically; you can run a hot tap briefly to start the cycle, then close the tap so the tank finishes heating with no water flow). Watch the gauge while the heater warms and note the highest pressure reading. If the peak is significantly above the cold reading, thermal expansion is present.
4. Verify closed-system components
- Inspect for a pressure-reducing valve, check valve, or backflow preventer near the main shutoff. These create a closed system and are why expansion raises internal pressure.
- If you’re comfortable, temporarily open an isolation valve (if present and appropriate) to see if pressure drops when the house is allowed to return to the supply. If it does, the closed-system device is trapping expansion.
5. Check the expansion tank and its location
- Ensure an expansion tank is installed and piped on the cold inlet to the water heater so it can absorb the heated water’s rise in pressure. If the tank is on the hot side or isolated behind a valve, it won’t protect the system.
- Check the tank precharge pressure (compare it to your house static pressure) and look for visible corrosion or a waterlogged tank (heavy or cold-to-the-touch center suggests it may be waterlogged).
6. Interpret the results
- If the peak pressure barely moves above the cold reading, expansion is being handled. If it jumps, you have active thermal expansion and should address the expansion tank, PRV, or other closed-system devices. Also see Pressure rises when water heater runs for more context on diagnosis and solutions.
What Not to Do
- Don’t trust one static reading. Expansion problems show up as peaks after heat cycles.
- Don’t remove the temperature-and-pressure relief valve to test or relieve pressure — that can be dangerous and cause scalding or flooding.
- Don’t assume the expansion tank is working just because it’s present; an undersized or waterlogged tank won’t prevent peaks.
When to Call a Professional
- If the pressure spikes above 80 psi, the relief valve is leaking, or you can’t find or access the expansion tank, call a licensed plumber.
- If you’re unsure how to safely isolate valves, test the PRV, or check an expansion tank precharge, get professional help. They can correctly size or replace an expansion tank and adjust or replace closed-system components.
- If you suspect repeated relief-valve discharge, corrosion, or faulty water heater controls, don’t delay — these are signs of persistent overpressure.
Safety Notes
- Turn off power to electric heaters or set gas heaters to pilot/turn off gas before doing work on the heater or valves near it. If you’re unsure how, contact a professional.
- Relieve pressure slowly and carefully; opening the T&P valve can discharge very hot water. Stand clear and use a drain hose if you must drain the tank.
- When in doubt, stop and call a plumber. Do not attempt gas or electrical repairs yourself.
Common Homeowner Questions
- How much pressure rise is normal during heating? — A small rise (a few psi) can be normal; spikes above 10–20 psi from the cold reading suggest a problem.
- Can I just add another expansion tank myself? — Only if you are comfortable with plumbing and the tank location; otherwise have a plumber size and install it so it works correctly on the cold inlet.
- Will adjusting the water heater thermostat fix pressure spikes? — No. Thermostat changes affect temperature but don’t solve thermal expansion in a closed system; an expansion tank or modification to the closed-system components is needed.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Thermal Expansion Pressure Behavior.