Water expands when it heats. A 50-gallon tank heated from cold to 120 degrees gains roughly a gallon of volume, and that extra water has to go somewhere. In many homes it has nowhere to go, so the pressure in the plumbing climbs every time the burner or element runs.
Two parts deal with that. The expansion tank absorbs the extra volume every day so pressure stays steady. The temperature and pressure relief valve is the emergency backstop that opens if pressure or temperature ever gets dangerous. When one of them fails, the other usually shows it.
We install and replace both on gas and electric water heaters, check the house water pressure, and make sure the relief valve discharge line follows the plumbing code.
- Duke Energy Trade Ally
- Rinnai certified
- Navien certified
Signs of a pressure or relief valve problem
- A relief valve that drips, or a wet discharge pipe beside the heater
- Water pressure that surges after the heater runs
- Toilet fill valves and faucets that start to leak or wear out early
- An expansion tank that is full of water when you tap it, with no hollow sound
- Water from the air valve on top of the expansion tank
- A new backflow preventer or pressure reducing valve on the main water line
Why a closed system needs an expansion tank
In an open system, heated water that expands can push back out through the service line into the water main. Many homes no longer have that open path. A check valve or backflow preventer at the meter, or a pressure reducing valve on the main line, stops water from flowing backward, and that makes the house a closed system.
In a closed system, every heating cycle raises the pressure. It can spike well above normal house pressure, wearing on faucet cartridges, toilet fill valves, washing machine hoses and the water heater tank itself, and pushing the relief valve open. The plumbing code requires a thermal expansion device where a closed system exists, and an expansion tank is the usual answer.
A pressure reducing valve is also required where incoming water pressure is above 80 psi. If we find high street pressure, we will recommend one, and that in turn makes the expansion tank necessary.
Sizing and precharge
An expansion tank is a small steel tank with a rubber bladder or diaphragm inside. Water from the plumbing fills one side, and compressed air fills the other. As the heater warms the water, the extra volume pushes into the tank against the air cushion.
Size depends on the water heater capacity, the house water pressure and the stored water temperature. A typical 40- or 50-gallon residential tank at ordinary house pressure takes a small expansion tank, but high pressure or a larger heater calls for a bigger one. We size from the manufacturer's chart rather than guessing.
The air charge matters as much as the size. Tanks ship precharged, and that charge has to be set to match your static house water pressure before the tank goes in service. We measure your pressure with a gauge and adjust the air side with a pump. A tank charged too high or too low leaves part of its capacity unused.
How the T&P valve works and the discharge rules
The temperature and pressure relief valve is set to open at 150 psi or 210 degrees Fahrenheit, whichever comes first. Water above its boiling point at normal pressure can flash to steam when a tank ruptures, which is why this valve is not optional.
The plumbing code, in section 504, sets strict rules for the pipe that carries water away from the valve.
- It must be no smaller than the valve outlet and must run by gravity, with no traps or upward sections.
- It cannot have a valve, a cap, a plug or a threaded end anywhere on it.
- It must end within six inches of the floor, at an indirect waste receptor, or outside, where discharge can be seen and will not hurt anyone.
- It cannot connect directly to a drain, and the material has to be rated for hot water.
- Each relief valve gets its own discharge line rather than sharing one.
Why a dripping relief valve is a warning
A relief valve that drips is doing its job. Something is pushing pressure or temperature high enough to open it, and the cause needs to be found. The usual causes, in order, are thermal expansion with no expansion tank, an expansion tank that has lost its air charge or ruptured its bladder, high incoming water pressure, and a valve worn out from years of opening.
The rarest cause, and the most serious, is a heater that is overheating because a thermostat or gas valve is not shutting off. That is why we check the water temperature on every relief valve call.
Never cap, plug or remove the discharge line to stop the drip. That silences the warning and leaves the tank with no protection.
How a relief valve or expansion tank call runs
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Measure pressure
A gauge on a hose bib or the drain valve shows static pressure, and whether it spikes as the heater runs.
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Check the system
We look for a check valve, backflow preventer or pressure reducing valve, and inspect any existing expansion tank.
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Test the relief valve
Lift, reseat and confirm the discharge line meets code.
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Install or replace
A correctly sized expansion tank, precharged to your house pressure, supported so its weight does not hang on the pipe.
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Verify
We run the heater through a cycle and confirm pressure stays steady and the valve stays dry.
Not sure this is the problem? Describe what is happening and we will tell you.
(864) 415-7482Permits
When an expansion tank or relief valve is part of a water heater replacement, it is covered under that job's permit and inspected with it. In the City of Spartanburg, residential work requires a permit; in unincorporated Spartanburg County, plumbing work is permitted through the county Building Codes office. We confirm with the office that covers your address whether stand-alone work needs its own permit.
What affects the cost
Every job is priced before work starts. These are the things that move the number.
- Tank size
- Larger heaters and higher pressure need a larger expansion tank.
- Pressure reducing valve
- If street pressure is above 80 psi, a PRV may be needed as well.
- Discharge line corrections
- An undersized, capped or badly routed relief line has to be rerun.
- Location and support
- Tight closets or attics take more time, and the tank needs proper support.
- Relief valve replacement
- A worn valve is a small part, but the tank must be partially drained to change it.
Why homeowners call Gas Guys for pressure problems
We find the cause
A new relief valve on a system with no expansion control will drip again. We fix the reason.
Charged to your pressure
We set every expansion tank to your measured house pressure, not the factory default.
Discharge lines to code
We correct capped, undersized or uphill relief lines on every heater we touch.
There when it drips at midnight
Call any hour and a person picks up.