Why is my water heater relief valve dripping?

When to stop and call right away

Shut off the gas (control to "off") or the breaker, close the cold supply valve, and call a licensed plumber now if:

• The valve is releasing a steady stream rather than an occasional drip.
• The water coming out is very hot, or you see steam.
• The tank is making loud rumbling or the hot taps run scalding hot.

Those signs point to the tank overheating, which is what the valve exists to catch. Stay clear of the discharge pipe while it is releasing.

What the relief valve does

The temperature and pressure relief valve (T&P valve) sits on the top or upper side of a tank. It opens when the tank's pressure or temperature goes past its rating, then closes again. Its discharge pipe runs down the side of the tank so released water lands near the floor. The model code is strict about that pipe: it must be full size, drain by gravity, end where people can see it, and have no valve, no tee fitting and no threaded end^[2]. Every one of those rules exists so the valve can never be blocked.

The three causes of a dripping valve

Cause	Typical pattern	Usual fix (by a pro)
Thermal expansion in a closed system	Drips during or right after a heating cycle, then stops	Install or recharge an expansion tank
High water pressure from the street	Drips at random times, other fixtures also show wear	Install or adjust a pressure-reducing valve, plus expansion control
Overheating (failed thermostat or gas control)	Steady flow, very hot water or steam	Replace the faulty control; replace the T&P valve
Worn or debris-fouled valve	Constant slow seep regardless of heating	Replace the T&P valve

Thermal expansion: the most common cause

Water expands as it heats. In older homes the extra volume could push back into the city main. Many homes now have a check valve, a backflow preventer, or a pressure-reducing valve on the incoming line. Those devices stop water from flowing backward, so the expansion has nowhere to go and the pressure climbs until the relief valve opens.

The model residential code addresses this directly. Where a backflow preventer, check valve or similar device lets thermal expansion raise the pressure, a device for controlling that pressure must be installed. The same goes for systems with a pressure-reducing valve when expansion pushes pressure past the valve's setting^[1]. In practice that device is usually an expansion tank, a small tank on the cold line with an air cushion that absorbs the extra volume.

A clue: if the drip started after the water utility replaced your meter, or after a plumber added a pressure-reducing valve, expansion is the likely cause.

Expansion tanks fail too

An expansion tank loses its air charge over time. If you already have one and the valve still drips, the tank may be waterlogged. A plumber checks the air charge and replaces the tank if the internal bladder has failed.

What a homeowner can safely check

• Timing. Note whether the drip happens after hot water use (heating cycle) or around the clock. That single fact narrows the cause.
• Is there an expansion tank? Look for a small tank, often blue or white, on the cold water line near the heater.
• Recent changes. A new water meter, a new pressure-reducing valve, or a new heater.
• Water temperature. If tap water seems far hotter than usual, shut off the heat source and call.

Manufacturers list a yearly lever test as owner maintenance (see our relief valve guide). The catch: a valve that is old or has never been tested often fails to reseat after it opens, and then it leaks constantly. If yours has not been tested in years, do not lift the lever to "clean" it. Test it only when you are ready to have it replaced, or let a licensed plumber do it.

Costs

HomeGuide's national figures, published August 2025: relief valve replacement $50 to $300, expansion tank replacement $300 to $800^[3]. Its January 2026 installation guide lists a new expansion tank at $100 to $400 when added during a replacement^[4]. Adding one while a plumber is already on site costs less than a separate visit.

When to call a pro

• Any steady flow, hot water, or steam from the valve.
• A drip that follows heating cycles: likely expansion control needed.
• A valve that seeps constantly or has been leaking for weeks (mineral buildup).
• Any replacement of the relief valve or the discharge pipe.