Quick Answer:
If the temperature & pressure (T&P) relief valve drips after the water heats, the system is likely building pressure from thermal expansion with nowhere to go. Start by confirming the drip is from the T&P discharge line, use a max-needle gauge to capture the peak pressure during a heat cycle, and if the peak climbs shut off the incoming water, open a hot faucet to relieve pressure, then check and adjust the expansion tank precharge at the Schrader to the cold static pressure. Only replace the relief valve after the expansion issue is controlled.
Why This Happens
When water is heated it expands. In an open system that extra volume can flow back into the supply. In a closed system — one with a check valve, pressure reducer, or backflow preventer — the expanded water raises system pressure. If the pressure reaches the relief valve’s set point it will discharge to protect the tank.
If you notice the problem only while the water heats, it is a classic thermal expansion issue rather than a constant high-pressure supply problem. Check for related signs like noisy pipes, cycling pumps, or Pressure spikes only on hot water.
Step-by-Step What to Do
1. Verify the drip is from the T&P discharge line
- Follow the visible discharge pipe from the valve to its termination (floor drain, pan, outside). The T&P pipe is usually 3/4″ copper or plastic and points downward.
- Do not disconnect or block the pipe to test it. Just confirm the water is coming from the valve outlet and not a nearby fitting.
2. Capture the peak pressure with a max-needle gauge
- Attach a pressure gauge that records the maximum (a max-needle gauge) to a hose bib on the hot side or a service port near the water heater. The goal is to see the highest pressure reached during a normal heat cycle.
- Start with the house at normal cold pressure, run the water heater through one heat cycle, then read the peak on the gauge. Record that peak.
3. If the peak climbs, relieve system pressure
- If the recorded peak approaches or exceeds the relief valve set point or clearly rises during heating, shut off the cold-water feed to the water heater (the inlet shutoff valve).
- Open a hot faucet on the highest floor to relieve pressure and drain a small amount until the system pressure drops to the cold static reading.
4. Check and recharge the expansion tank precharge
- With the system at cold static pressure (inlet off, hot faucet open to relieve), check the expansion tank air charge at the Schrader valve with a tire gauge.
- Set the tank precharge to match the cold static water pressure (for example, if the house static pressure is 50 psi, precharge the tank to 50 psi). Use a hand pump or compressor and then close the Schrader.
- Close the hot faucet, restore the inlet water, let the heater complete a heat cycle, and retest with the max-needle gauge.
5. Replace the relief valve only after expansion is controlled
- If the expansion tank was waterlogged or its precharge was wrong and recharging fixes the pressure spikes, the valve should stop dripping. Do not replace the valve until the expansion problem is solved.
- Replace the T&P valve only if it continues to leak after the expansion tank is repaired or if the valve is visibly corroded or stuck.
What Not to Do
- Don’t cap or plug a relief line. That removes the system safety and can lead to dangerous pressure build-up.
- Don’t ignore a drip. A relief valve is a safety device telling you pressure or temperature is too high.
- Don’t replace the valve first without addressing expansion — a new valve will simply discharge again if the root cause remains.
- Don’t confuse a valve discharge with unrelated wet spots. Check seals and connections, and if you see moisture at the expansion tank air fitting, check for Expansion tank leaking from air valve.
When to Call a Professional
- If you can’t capture the peak pressure or the readings are inconsistent.
- If the expansion tank is badly waterlogged, damaged, or you’re not comfortable draining/pressurizing it.
- If the T&P valve continues to discharge after the expansion tank and precharge are corrected, or if the valve shows corrosion or hard leaks.
- When in doubt about working with hot pressurized systems — call a licensed plumber.
Safety Notes
- Always relieve system pressure and allow the water heater to cool before working on fittings. Hot water and steam can cause severe burns.
- Never cap, plug, or bypass a T&P valve or its discharge pipe.
- When checking the Schrader, avoid releasing water through that valve. Use a tire gauge on the Schrader only after the system is at cold static pressure.
- If you are unsure how to safely isolate the heater or handle pressurized tanks, call a professional.
Common Homeowner Questions
- Why does the valve only drip after heating? Thermal expansion raises pressure during heating, and the relief valve opens when pressure or temperature hits its limit.
- Can I just replace the T&P valve to stop the drip? No — replace the valve only after you fix the expansion issue; otherwise the new valve will also discharge.
- How do I know the expansion tank needs service? If the precharge at the Schrader doesn’t match cold static pressure or the tank feels heavy and waterlogged, it needs recharging or replacement.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Thermal Expansion Pressure Behavior.