Homeowner says the T&P valve on the water heater drips every evening and they've already replaced it twice.

Diagnosis: Thermal expansion in a closed system.

The 65 psi static pressure is fine, but the evening drip pattern is classic thermal expansion. When the heater fires after heavy evening use (showers, dishes), cold water enters and heats. Water expands ~2% from 50°F to 140°F. With no room to expand (check valve, PRV, or backflow preventer on the supply), pressure spikes to 150 psi (T&P set point) and the valve weeps.

Why replacements didn't fix it: You're treating the symptom, not the cause. The new valves work correctly — they're relieving dangerous pressure.

Verify:

1. Confirm a check valve/PRV/backflow preventer exists on the cold supply (IRC P2903.4).
2. Install a pressure gauge with a lazy hand (tattle-tale) on the drain valve. Check it next morning. If it reads >100 psi, expansion is confirmed.

Fix (IRC P2903.4, UPC 608.3): Install a thermal expansion tank on the cold inlet.

• Size: 2.1 gal (e.g., Amtrol ST-5) for typical 40-50 gal heaters.
• Pre-charge: Must equal static supply pressure (65 psi). Check/adjust with tire gauge before installation.
• Location: Cold inlet, between heater and any shutoff valve. Support per manufacturer.
• No expansion tank access? A 1/2" hose-bibb style relief valve (Watts 530C) set at 100-125 psi on a tee at the heater drain valve is a code-accepted alternative (UPC 608.5), but an expansion tank is superior (prevents pressure cycling, saves water).

Do not: Plug the T&P, cap the drain line, or increase T&P setting. Explosion hazard.

If no closed system device exists: Check for a failed PRV letting pressure creep, or a cross-connection. But 99% probability: missing expansion tank.

The recurring T&P valve discharge indicates either excessive pressure or temperature in the water heater — not a faulty valve (since it’s been replaced twice). At 65 psi incoming pressure, the issue is likely thermal expansion in a closed system.

What’s happening: When water heats, it expands. If the system has a check valve, pressure-reducing valve (PRV), or backflow preventer on the cold inlet (common in modern homes), it creates a closed system. Without an expansion tank, heated water has nowhere to go, causing pressure to spike — triggering the T&P valve to relieve excess pressure (and sometimes temperature).

Diagnosis:

• Confirm if a PRV, check valve, or backflow preventer exists on the cold water line feeding the heater.
• Test pressure at the water heater during peak heating (evening). If it exceeds 80 psi (T&P valve setting), thermal expansion is the culprit.
• Check if the T&P valve is discharging hot water (indicates temp/pressure) or just drips (could be minor seepage, but recurring suggests pressure spikes).

Solution: Install a thermal expansion tank on the cold water line near the water heater. Size it per manufacturer specs (typically 2-gal for 40–50 gal heaters). Pre-charge the tank to match incoming pressure (65 psi) using a tire gauge. Also, verify the T&P valve is correctly installed (discharge pipe routed to drain, no valves or traps, air gap). If pressure exceeds 80 psi even after expansion tank, inspect or replace the PRV.

Note: If the valve discharges scalding hot water or steam, shut off power/gas and call a licensed plumber immediately — this risks explosion. But based on timing (evening) and history, thermal expansion is almost certain. Always follow IPC Section 607.3 and manufacturer guidelines. If unsure, consult a licensed plumber.

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