Water Heater Overflow Valve: 11 Reasons It’s Leaking and What To Do
If you’re like most homeowners here in American Fork, UT, you’ve likely never seen a water heater leak. This is because modern water heaters are extremely reliable and rarely fail in that way. However, there is one place where a storage tank water heater may leak, and it’s by design.
It’s called a water heater overflow valve, and every water heater has one. Its purpose is to prevent a catastrophic water heater failure. So, if your water heater begins leaking water from its overflow valve, you should figure out why immediately.
If the valve is discharging continuously, shut off the cold water to the tank and turn off gas or electricity. Do not cap the discharge pipe. If you need help now, request fast service for water heater repair or, if the tank is done, schedule water heater replacement.
1) Your Tank Is Over-Pressurized
Your water heater’s overflow valve also serves as a safety device in the event of over-pressurization. Storage-tank heaters can handle only so much internal pressure before they fail. The valve at the top of the overflow pipe opens as the tank approaches its maximum pressure, typically around 150 psi, and lets water drain to relieve that pressure. Without this feature, a malfunction could lead to a dangerous rupture. So, if there’s water coming from the overflow valve, suspect a pressure issue.
In many cases, excess pressure starts with conditions elsewhere in the system. You can check your home’s pressure by attaching a gauge to an outdoor hose bib. If you get a reading over 80 psi with the spigot fully open, that’s too high.
What to do: If the discharge tube is wet after most heating cycles, follow the immediate steps in what do you do if a water heater is leaking, then plan a pressure and temperature check.
2) Your Tank Is Overheating
Another reason your water heater might leak from its overflow valve is if it is running too hot. Most residential units heat water to 120–140°F by default and can tolerate up to about 210°F. If the temperature exceeds that, the overflow valve opens and releases water so cold water can enter and lower the temperature.
If you suspect overheating, test with a kitchen thermometer. Let hot water run for a minute or two, then test. Never use your hand. If the water is too hot, check the temperature control. If it is already set appropriately, call for help.
What to do: Set the control near 120°F, retest, and if the temperature still spikes, consider age and condition, and what is the life expectancy of a water heater? If it’s set appropriately, call Gunthers Heating, Cooling, and Plumbing for help immediately.
3) You Recently Installed a Backflow Preventer
If you have recently installed a backflow preventer or PRV, it can cause an overflow-valve leak. As water heats, it expands; normally that expansion nudges back toward the incoming line. A backflow device creates a closed system, so pressure cannot move backward. The relief valve opens instead. The fix is an appropriately sized expansion tank that provides a place for the pressure to go.
What to do: If the drip started after a PRV or backflow device was added, have an expansion tank installed and pre-charged to house pressure. For context on common tank issues, see a comprehensive guide on common water heater problems.
4) A Failing Valve
Your water heater may leak from the overflow valve if the valve itself fails. That is the most common reason for a leak at that location. Do not ignore it. A failed T and P valve removes a key safety layer, leaving the tank unprotected from the problems above.
What to do: Have the T and P valve replaced and the discharge piping inspected for full size, slope, and termination height. If corrosion is visible or the unit is near end of life, consider water heater replacement.
5) High Incoming Water Pressure (separate from tank pressure)
Municipal or well pressure that consistently exceeds 80 psi can force the relief valve to weep even when the tank itself is normal. Readings often vary by time of day, so check more than once.
What to do: Install or adjust a pressure-reducing valve and recheck after a few heating cycles. For comfort tips that avoid over-pressure, review 4 ways to increase water pressure in your home.
6) No Expansion Tank or One That Is Undersized
In closed systems, thermal expansion has nowhere to go. Without a correctly sized expansion tank, pressure spikes trip the overflow valve.
What to do: Add or upsize the expansion tank and match its air charge to house pressure.
7) Failed or Water-Logged Expansion Tank
If the expansion tank’s internal bladder loses air charge, it cannot buffer pressure and the relief device drips instead. A quick tap test that sounds solid end-to-end is a red flag.
What to do: Replace the failed expansion tank and set the pre-charge to system pressure, then monitor the discharge tube.
8) Sediment Buildup in the Tank
Scale at the bottom of the tank creates hot spots that can flash to steam, spiking temperature and pressure.
What to do: Flush and service the heater. If maintenance has been light, see how often do I need HVAC maintenance and get on a routine.
9) Incorrect Temperature Setting
A bumped dial can push water above safe levels and trigger the valve.
What to do: Set to around 120°F and retest. If you frequently run out of hot water, check sizing with what size hot water heater does my home need.
10) Partially Closed or Stuck Valves
A stuck cold-inlet or outlet valve can create pressure swings that nudge the overflow valve open. If you also notice damp walls or floors, there could be a separate supply leak.
What to do: Verify all valves are fully open and walk through how homeowners can detect a plumbing leak.
11) Damaged Dip Tube or Faulty Mixing Valve
A cracked dip tube or a stuck tempering valve can cause temperature instability that occasionally trips the relief valve.
What to do: Replace the faulty component. If repairs are stacking up and you want endless hot water with faster recovery, compare options in tankless water heaters.
How to check a water heater overflow valve safely
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Look, not hands. Confirm whether dripping happens only after heating cycles or all day.
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Verify the thermostat near 120°F.
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Gauge pressure at a hose bib; anything consistently over 80 psi points to PRV and expansion-tank needs.
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Never cap the discharge tube. If water is pouring, shut off the cold inlet and power or fuel, then call for help.
Repair vs. replace for water heater overflow valve problems
Choose repair when pressure is high, the expansion tank is failed or absent, or the thermostat is mis-set. Choose replacement for heavy corrosion, repeated overheating, or end-of-life. If you prefer on-demand hot water, review the pros and cons in benefits of a tankless water heater.
Local Water Heater Experts
Since 1910, Gunthers Heating, Cooling, and Plumbing has been the company that American Fork homeowners trust for their water heater needs. We offer comprehensive plumbing services, including water heater repairs and installation. Our skilled plumbers can find and fix the cause of your water heater leak quickly. We also offer HVAC services, ductwork, indoor air quality, and sheet metal fabrication. So, if your water heater begins leaking and you need it fixed, call the experts at Gunthers Heating, Cooling, and Plumbing!
Water Heater FAQs
Is it safe to keep using hot water if the overflow valve is dripping?
A steady drip means pressure or temperature is out of range, so keep showers brief and set the thermostat to 120 while you book a licensed tech through plumbing services for a full system check.
Do I really need an expansion tank if I have a PRV or backflow preventer?
Closed systems trap thermal expansion, which is why many homes add a tank. Your instal tech can size it correctly and tie it into the cold inlet during a standard visit arranged via sewer line service when broader pressure issues are suspected.
When does a dripping overflow valve count as an emergency?
If the discharge shifts to a hot, continuous flow—or you smell gas or see scorch marks—treat it as urgent and follow shutoff steps while you review what plumbing leaks create an emergency to decide your next move.
Our family keeps running out of hot water. Will a tankless system reduce stress on the system?
Right-sized on-demand units avoid storage-tank expansion cycles and stabilize supply during peak use, so start your options review with tankless water heaters.
If I do have to replace the heater, do you offer payment options?
Most homeowners spread the cost over time; check current promotions and application steps on financing options before you schedule an install.
