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TP valve Discharge pipe Code
TP = Temperature and Pressure

Water heater TP valve is a safety device that prevents water heater from exploding
'TP relief valve provides 2 in 1 protection against excessive pressure and excessive temperature in emergency conditions. Mechanical spring-loaded TP valve is reliable and fully automatic protection for hot water storage tanks and heaters.' TP valve will automatically close when emergency conditions are over.
''Water heaters are pressure tested to 300 pounds per square inch (PSI) and have a working pressure of 150 PSI.
T&P valve will open when the pressure inside the tank exceeds 150 PSI, allowing pressure to vent safely.
T&P valve will open if water temperature at top of tank reaches 170°F-210°F depending on water heater and valve rating.


Pressure issues from heating water
Heating water causes water to expand in volume. The hotter the water, the more expansion, and the higher the pressure.
When thermostat is set to 120°F, then there is less pressure than a tank that is heated to 140°F. \
However, even 120°F can easily cause pressure that exceeds safe working limit of 150 psi.
Normally excess pressure is not a problem because it expands back into the cold water line and into the city's water main -or- into the pressure tank of water well ... where the pressure is absorbed and mitigated by the pressure controls that are required for the water supply.
Pressure problems occur when heated water is blocked from expanding back into cold water line. Blockage can be caused by a check valve, pressure reducing valve, shut off valve or other obstruction in the cold pipe.
Result: TP will drizzle out water whenever tank is heating.
Typically the problem is solved by installing an expansion tank. Resources:  Read about expansion tank   TP leaking lots of hot water

Temperature issues and water heater ECO
Residential gas water heater has an ECO (energy cut off or high limit) located on back of gas control valve inside the thermostat probe. The ECO trips when water inside tank reaches 180-188°F, sometimes requiring gas valve replacement. By design, the TP should release water before high temperature inside tank trips the ECO and damages gas valve. More recent versions of electronic gas valves generally come with reset protocol when ECO trips. Resource Troubleshoot gas valve

Residential electric water heaters have a red reset button or ECO that is located on upper thermostat. The ECO trips at 170-180°F +/-5°, shutting down electric power to elements. Because high limit of electric thermostat is close to same temperature as TP release, the TP may or may not release water before the ECO trips. With electric water heaters, no damage occurs because of ECO trip.

If the ECO malfunctions on either gas or electric heaters, or something else happens to cause runaway heating, the TP will release the high temperature water, saving integrity of heater and preventing steam explosion.

''If the temperature probe on the T&P of a residential water heater senses water temperatures of 170°F (or 210° F on some commercial heaters, boilers etc), it will open the TP valve. The valve will stay open until the temperature probe cools."
Since the probe is at top of heater while cold water enters the bottom area of the tank, it may take quite a while for the probe to cool. In mean time, you will have a large volume of water discharged before the temperature probe cools.
Because super heated hot water is dangerous and can cause considerable property damage, it is best to have adequate floor drain, or route TP overflow tube to outside, or into a drip pan that has a drain line leading to outdoor or to an open drain etc. In this instance, do not expect a metal can to catch the full volume of overflow.

Footnotes, misc information.
-Commercial heaters are often rated higher temperatures, with commercial thermostat range as high as 180°+. For these commercial heaters, the TP valve and ECO trip at higher range. Maximum temperature for any water heater is 210°F. Why? The risk of catastrophic steam explosion if tank ruptures.
-Heating water inside a tank causes pressure that is contained by the steel tank. "Water will not boil under pressure until the temperature reaches approximately 297° F. The energy potential in the superheated water is called latent heat energy and will flash to steam when exposed to normal atmospheric pressure. This flash to steam (steam explosion) has the explosive potential of over two million foot-pounds of energy."
-The highest allowable temperature for any type water heater is 210°. This includes residential heater, commercial heaters, booster heaters, boilers of all type etc.
-"A water heater is an appliance for supplying hot water for residential or commercial use other than space heating. The maximum outlet water temperature for any water heater is 210°F ( 98.5°C)." Resource: Designing energy efficient commercial systems .pdf
Code:

-TP valve cannot be installed so it points in upward position. Must be installed horizontal on top of tank, or aimed downward on side of tank.
-TP valve must be installed so probe inserts directly into water heater tank.
- Install TP in port located on top of water heater or side of water heater near top of tank.

-TP cannot be installed in lower part of tank (hot water rises). The probe must be located within top 6" of tank.
-If heater does not have enough ports, an extra-length TP valve can be installed straight down into a Tee that is inserted into Hot outlet, as long as TP probe reaches into tank, but does not exceed maximum 6" deep into tank.

-Each valve has a 'steam rating' or BTU per hour:
'T&P Valve must have a rating label that shows it is capable of discharging more BTUs than the heater is capable of putting into the water.'
-So if heater is rated 40,000 BTU per hour (check label on side of heater), then the TP valve must have a rating that shows it can expel more than 40,000 BTU.
-''TP valve must be m
arked with a maximum set pressure not to exceed the marked hydrostatic working pressure of the water heater (150 psi = 1,035 kPa) and a discharge capacity not less than the water heater Btu/hr or kW input rate as shown on the water heater’s model rating plate.'' ANSI Z21.22- CSA 4.4 and ASME. ...'' T&P valves tested under ANSI Z21.22 with ratings as certified and listed by CSA. Resource .pdf

-TP valve rating cannot exceed 150 psi F for residential water heater.
-Household water pressure cannot exceed 80 psi (50 psi is best), or TP can begin leaking water.
Resources: Read about thermal expansion    
TP leaking lots of hot water

-TP valve overflow tube must connect directly to TP valve and run down to at least 6 inches off floor.
-Cannot terminate near or on electric wires or junction box. Released water cannot contact person or electrical parts.

-Pipe must be approved for hot water distribution: copper, pex, galvanized, CPVC (cannot use garden hose or PVC etc).
-Overflow tube must be same diameter as valve discharge connection. Do not downsize pipe into smaller pipe.
-Overflow tube cannot have shut off valve installed, or any obstructive valve.
-No restriction, reducer coupling or valve can be installed on overflow tube.
-If water heater has condensate drain (found on tankless, power vent, heat pump, condensing etc), the condensate drain cannot be inserted into overflow tube ... the overflow tube cannot have any holes or tees or other intersections.

-Do not cap off TP valve or obstruct overflow tube.
-End of overflow tube cannot be threaded. Must be protected from freezing, and cannot be concealed from view.

-Overflow tube must pitch downward at all points. If tube has horizontal run, then it must pitch downward.
-Shall be installed so as to allow complete drainage of both the Temperature-Pressure Relief Valve and the discharge pipe.
-Can have maximum 4 elbows and max 30 feet length. Over 30 feet (9.14 m), or more than four elbows can cause restriction and reduce discharge capacity of the valve.
--- Some manuals specify maximum 15' length and 2 elbows

-If water heater is sitting inside a drip pan, overflow tube cannot be lower than top edge of drip pan.

-Tube must terminate 6" above open drain, not inserted into drain. TP valve can draw water back into water heater if overflow tube is under water.
-Some water heaters, for example under counter water heater or hybrid water heater may be installed in location that does not have available floor drain. TP cannot be inserted into a drain line for sink or washing machine etc. The TP tube can terminate 6" above a metal can. Discharge should not cause potential water damage. Homes can suffer damage if thermostat fails and TP begins releasing hot water.
-If overflow tube is terminated inside a pipe, the end of tube must be visible to observe if water is being released from tank, and there must be air gap to prevent drain water from being sucked into water heater.
-It is always best to have a floor drain, or a catch pan with minimum 3/4" line to outdoors, or terminate the TP overflow outdoors as long as freezing temperatures will not cause ice formation and blockage.
-End of overflow tube cannot be restricted, blocked or obstructed by belongings, structure, snow, ice, rising water, damaged end of pipe etc.
-Using a Temperature and Pressures Relief Valve that was subject to flood water is not advisable.

-If water heater is used to heat a secondary tank or storage tank, then that second tank must also have it's own properly sized and rated TP valve.
-If installation has more than one water heater, each heater must have it's own TP valve and independent overflow tube. Overflow tubes from multiple tanks cannot be combined into one.

-T&P valves should be inspected and tested on every service call. Twice a year recommended. ''Ensure that (1) no one is near the outlet tube, and (2) the discharge will not cause injury or damage.''
-If TP fails to reset and continues to release water, raise again slowly and see if it will reset. If not, then close cold water inlet, turn off gas or electric, let tank cool, drain down a few gallons of water, then replace TP with properly rated/sized new one.

-"T&P valve is not intended for the constant relief of thermal expansion (drizzle). A properly-sized thermal-expansion tank must be installed on all closed systems to control thermal expansion."

-TP valve is last line of safety for functioning water heater.
First line is the thermostat ECO reset button for electric heaters, and for gas, the thermostat, ECO, TCO etc depending on heater.
Resource:
How to replace TP valve
Closed system
Buy TP valves:
Replacement TP valves at Amazon
Teflon tape at Amazon
• Shall not be smaller in size than the outlet pipe size of the valve, or have any reducing couplings or other restrictions.
• Shall not be plugged or blocked.
• Shall not be exposed to freezing temperatures.
• Shall be of material listed for hot water distribution.
• Shall be installed so as to allow complete drainage of both the Temperature-Pressure Relief Valve and the discharge pipe.
• Must terminate a maximum of six inches above a floor drain or external to the building. In cold climates, it is recommended that the discharge pipe be terminated at an adequate drain inside the building.
• Shall not have any valve or other obstruction between the relief valve and the drain.\
• Shall be approved material shown below. PVC and other piping material is not approved.
 Copper
 CPVC
 Galvanized
 Pex
TP valve codePrevent explosion

If thermostat fails, it can cause run-away heating inside tank.

When water inside tank reaches boiling point, a steam explosion occurs. Water boils at 212°F – at sea level – unless contained under pressure. At 52 psi gauge pressure, water would not boil until it
exceeded 300° F
A steam explosion will occur if pressure on a rusted tank, or a weld ruptures. Water heater explosion causes massive damage.

To prevent this, a TP valve or temperature-pressure-release valve is installed on each water heater
Instead of explosion, the TP valve releases hot water and cold water automatically refills tank, causing temporary cooling
The cycle of overheated water and TP release continues until water heater is repaired.

If the tank can deliver more BTUs into the water than the TP rating, or if the TP overflow tube is not installed to code, then the pressure can build up and cause explosion.

Installing properly sized and rated TP valve, and following installation code is critical for water heater safety
Applies to all water heaters/ gas, electric, tankless, condensing, oil-fired, solar, hybrid etc....

''Why is hot water over 212° F dangerous? The containment vessel or storage tank used to store the hot water causes the danger. The tank is under pressure - the normal working pressure caused by the cold water supply. As water is heated under pressure, the boiling point rises. For example, with a nominal incoming supply pressure of 50 PSI, water will not boil under pressure until the
temperature reaches approximately 297° F. The energy potential in the super heated water is called latent heat energy and will flash to steam when exposed to normal atmospheric pressure. This flash to steam has the explosive potential of over two million foot-pounds of energy.''
Resources:
Thermal expansion
How to install electric water heater
How to install gas water heater

Leaking water heater
Thermal expansion
Overflow Tube is required
Released water cannot contact person.
TP valve code Add 6 wraps teflon tape when installing or replacing TP valve
Temperature probe must be inside water heater
Buy TP valves:
Replacement TP valves at Amazon
Teflon tape at Amazon
Symptom          Action
What to look for:
TP valve leaking water Water heater should be dry at all times.

Is TP leaking, or water heater leaking?
Use masonite to check if TP valve is releasing water.
Some TP valves drain directly into drain pipe. Turn off all taps and put ear against pipe to hear sound of escaping water.
If TP valve is continuously running out hot water, tank will have less hot water.
Resource:
Leaking water heater
Quick fix for leaking TP: Raise TP lever
Raise TP lever for 5 seconds.
See if obstruction is flushed out
Keep extra TP valve on hand when doing this in event the TP continues leaking
Caution: Keep Feet away from hot water coming from discharge tube!
Resource:
Leaking water heater
Do NOT cap off TP valve Caution:
Do not cap off TP valve or discharge tube.
Do not plug or obstruct TP valve or discharge tube.
Obstructing TP valve can lead to explosion of water heater tank.
Steady flow of water from TP Raise TP lever to clear blockage.
Then if problem continues:
Replace TP shown in steps below
If water is unusually hot, then thermostat is bad:
Bad thermostat: Replace Gas thermostat
Bad thermostat: Replace Electric thermostat
Bad electric element: Test elements
Weeping TP valve Dribbling, intermittent weeping:
Foreign material in seat valve: Open valve to clear problem. Replace TP valve/ steps show below.
High pressure:
Check pressure with gauge on drain valve as shown below.
Install pressure reducing valve and expansion tank.
Reduce tank temperature:  How to adjust temperature
Replace TP shown in steps below

FYI:
If pressure reducing valve is installed, it will produce closed system .... expansion tank required .
Closed system
: Water confined to a storage tank or piping system, when subjected to a temperature rise of 10°F (increasing from 75° to 85°), increases pressure from 50 psi to 250 psi.
Normal system (not closed) without pressure reducing valves, check valves etc: Water expands approximately 2% in volume for a 100°F temperature rise (from 40°F to 140°F) ....40 gallon water heater, water being heated to its thermostat setting will end up expanding to about 40.53 gallons.
Large volume hot water released sporadically Bad thermostat
Malfunctioning thermostat is only cause
High temperature limit is around 210 degrees F
Resource:
Electric: How to replace electric water heater thermostats
Gas: How to replace water heater gas control valve thermostat
Identify/ troubleshoot by type of gas control valve
Leaking threads on TP valve Reseal threads with 6 wraps teflon tape
Probably mean tank is rusting: inspect tank each 6 months
Resource:
Replace tank: How to install gas water heater
Reduce pressure using pressure reducing valve. Read more below
Turn down thermostat temperature: How to adjust temperature
Replace anode rod to prevent rusting: Replace anode
Leaking weld around TP valve Tank rusted beyond repair: Replace tank
Resource:
Leaking water heater
How to install gas water heater
Closed system/ high water pressure pressure Install pressure reducing valve and expansion tank (read more lower on page)
Resources

Read about expansion tanks
Read about expansion tanks and closed system
Expansion Tank Installation pdf
Discharge pipe Code

TP overflow tube runs down to at least 6 inches of floor.
Overflow tube must be same diameter as valve discharge connection. Do not downsize pipe into smaller pipe. Overflow tube can have maximum 4 elbows and 30 feet length.
End of overflow tube cannot be threaded.
Tube cannot be lower than top edge of drip pan. Tube must terminate 6" above open drain. TP valve can draw water back into water heater if overflow tube is under water.
Do not cap off TP valve or obstruct overflow tube
Overflow tube cannot have shut off valve installed, or any obstructive valve
TP valve is first line of safety for water heater.
TP valve releases water when pressure or temperature is too high, and prevents possible water heater explosion
Reasons TP valve releases water:
1) Tank is overheating. Action: Open hot tap, and check for overheated water. Adjust water heater temperature. How to adjust temperature  Bad gas control valve thermostat
2) Bad TP valve. Action: replace TP valve. Open TP valve once per year to test and dislodge obstruction. Clean sediment out of tank. How to clean sediment
3) Factory Heat trap nipples or inserts clogged. Action: remove heat traps Read more
4) Closed system caused by check valve, resulting in frequent TP replacement or constant drizzle. Action: remove check valve or install pressure reducing valve. Install expansion tank. Read more about closed system and expansion tank
5) High pressure. Action: put pressure gauge on water heater drain valve. Anything over 80 psi can damage tank. Install pressure reducing valve. Read more
TP valve with teflon tape
Standard 3/4" pipe thread



Wind 6 wraps teflon tape on new valve.

Most new valves already have Teflon tape on threads


Buy TP valves:
Replacement TP valves at Amazon
1/2" TP valve at Amazon
Teflon tape at Amazon
Step-by-step: How to replace TP valve:
1) Turn off water.
2) Open tub spout to release pressure
3) Turn off power to heater
        Electric heater: turn OFF circuit breaker
        Typical Gas heater: rotate knob to vacation setting, or move knob to pilot position
4)  Open bathtub tub spout on Hot side to release pressure
5) Let water cool
Caution: Make sure tank has cooled before removing TP valve
5) Open TP valve to lower water level.
        Or use drain valve on bottom of tank
        Resource:
        If water heater won't drain


6) Unscrew TP valve counterclockwise using 12-14" pipe wrench.
7) Install new valve and make sure threads have white teflon tape.
Teflon tape will seal threads against leak.
8) Connect overflow tube that was attached to old TP valve.
9) Refill tank with water.
Caution: Do not turn water heater ON unless tank is full of water or it can damage tank 
Wait for water to come out of bathtub spout in full stream before turning on water heater.
When bathtub spout is running, then tank is full. Turn off tub spout.
10) Open new TP valve for a moment to let last bit of air out of tank.
11) Turn on water heater.
       Electric: turn on circuit breaker
       Gas: Rotate knob to previous temperature setting
                Resource:
              How to adjust temperature on water heater

 
Use 6 wraps teflon tape
☑ White for water lines
☑ Yellow for gas lines

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Icon gas valve   
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If tank is overheating or not heating correctly, replace thermostat
Symptoms: Cold water, uneven temperatures, TP releasing water
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