Water Heater Temperature & Pressure Relief Valve FAQs

Index to All Q&A on Water Heater Temperature/Pressure RElief Valves

Water Heater Relief Valve Accessibility & Clearance Distance FAQs

How close Can a T and P discharge Pipe be to dry wall

How close Can a T and P discharge Pipe be to drywall ? - On 2020-11-23 by R J -

by (mod) - Two Answers: Legal and Practical clearance distances for TPR valve access

Great question, RJ.

I have a practical answer and a legal answer.'

Practically, you have to have enough space that when replacement of the TPR valve is needed there is working space to unscrew and remove the old one and get the new one in place - that might be just an inch or two past the test lever; and separately, you don't want any dripping or discharge to wet the drywall lest you invite a mold contamination issue.

Practically - adding another messy detail, if your water heater sits in a drip or spill tray then the bottom of the discharge pipe will have to jog out and over and down to get past the outer edge of the drip tray, so the drip tray itself is going to space the heater and most of the discharge pipe a bit more-distant from the wall.

Legally: you have to respect the working clearance space or distances specified by the manufacturer or your specific water heater brand and model. That may be considerably more.

And depending on the kind of water heater and its fuel you will need to meet clearances from combustibles (that includes your wall).

For example this gas fueled American Standard water heater - just an EXAMPLE - your heater may differ, lets the heater itself be as close as 2 inches from a wall, but that does NOT consider space to get the TPR valve on and off.

Take a look at where the TPR valve is connected to your water heater; if it's on a side (near the top of course) then you need working space for the valve.

If the TPR valve is connected off of a tapping on the top of the water heater (as is on some models) then there's probably plenty of working space there and the discharge tube could be quite close to the wall - but considering the practical warnings I gave above.

Above is an illustration of an [American Standard] electric water heater installed with a drain pan - notice the relief valve discharge pipe routing.

What is the overhead clearance space required for a TP valve installed on a water heater top?

I have a hot water heater with the temperature relief valve on the top. It is a short 30-gal water heater that is under a counter top.

What is the minimum clearance distance that is required from the bottom of the countertop to allow the temperature relief valve adequate clearance? Is there a code spec for this? - Julie

Reply:

Julie a look at installation details for TP valves on water heaters didn't come up with an over-head clearance, though some common sense would indicate you'd need enough space for the test lever to be operated AND enough space to remove and replace the valve when needed.

Since a top-mounted TP valve has a temperature sensing stem that protrudes downwards into the water heater the removal space will be more than you think

On a small water heater (cylinder) the extension of the sensor is about 3" while on larger water heater cylinders that added length could be as mmuch as 9" below the inlet. Adding that 9" of sensor to the TP valve body and lever height (anywhere from 5 5/8" to 9 1/4") means the minimum (for the smallest valve size) valve total length is about 9" and the larger valve could be much larger.

Now we can slightly tip a valve to get it out of the heater, buyt - I'd like to see 12" or more to give both working space and space to remove the valve.

Water Heater Relief Valve Opening Temperature & Pressure FAQs

What's the relationship beween building water pressure and pressure in the water heater?

If my residential pressure is 70 psi which of the pressure inside my hot water heater tank be?

It’s fluctuating from 140 psi to as low as 45. I think the expansion tank is my next replacement of course. But what should the pressure in the tank normally be in relation to the system pressure? On 2020-01-11 by Tom

by (mod) - Rate of expansion of water or increase of water pressure in a water heater per degree of temperature rise

Tom,

Thanks for asking an important and interesting question on the expansion of water or increase of water pressure in a water heater.

The pressure in a closed tank of water will indeed increase as the water temperature is increased.

Your water heater pressure of 140 psi may be quite normal. The actual pressure increase will depend on

- the temperature of the incoming water

- the temperature to which the water is heated

- and an effect of the thermal expansion of water as it is heated

When the water heater (or in other countries cylinder or calorifer or geyser) is cold the pressure inside it should be the same as incoming building water pressure - in your case 70 psi.

At the article we cite below we note, as do water heater manufacturers, that

"For each 10°F increase in temperature, water expands 0.2% "

So if your incoming water were cold, say 40 degF, and you heat it to 140 degF, the volume of water will increase by 2%.

If the expanding water were in an "open" system that expansion would just push back into the cold water pipes and pressure wouldn't increase. But where there are check valves or other obstructions in the piping system, pressure can build rapidly as water is heated.

A domestic water heater is typically designed for up to 150 psi working pressure. At higher pressures we expect the temperature/pressure relief valve to open to relieve pressure.

But in some homes the water heater pressure just from thermal expansion causes dripping at the TPR valve (an unsafe condition); in that case, if all else is normal, yes it makes sense to add a thermal expansion tank to the water heating system.

SeeHOT WATER EXPANSION TANKS

Also see THERMAL EXPANSION of HOT WATER for further explanation of the actual amount of expansion that occurs in heated water.

What are the Pressure and temperature on a 30 gallon Rheem temp press safety valve?

What is the limits of pressure and temperature on a 30 gallon Rheem temp press safety valve? (Nov 27, 2014) Anonymous

Reply: typical water heater pressure relief valve opening temp is over 210F or at / over 150 psi

Anon I can't guess from just your question - we don't know fuel, btu input rate of your water heater, type of heater - nada.

In general the pressure/temperature relief valve on any water heater will open at a pressure low enough to protect the tank from a BLEVE explosion (discussed above) AND/OR at an abnormally high temperature (e.g. over 210F) but the TP valve also must be sized to be capable of dumping hot water and/or steam fast enough to exceed the BTU input rate of the heating appliance.

A TYPICAL water heater TPR valve will open at

• Temperature Relief: 210°F (99°C),
• Pressure Relief Range: 75 to 150 psi (5.2 to 10.3 bar)

Temperature/Relief Valve Missing on my Heater - FAQs

How do I add a water heater TP valve if there is none on my calorifier?

How do I add a water heater TP valve - there's none on my heater

Reply:

Watch out: a water heater that has NO temperature/pressure relief valve is unsafe and illegal, risking a dangerous BLEVE explosion. I would leave such a water heater turned OFF until the relief valve has been found or installed.

There should be a tapping on the upper side of the heater intended to accept a pressure/temperature relief valve. Be sure to install a valve with the proper capacity matched to the unit's input BTUH.

Anonymous said

Well, there is a plastic cap on the edge of the top, but when it is removed, there is only yellow styrofoam insulation under it.

Moderator said

I'd ask for help from a plumber to find the tapping and install a TP valve. You may need to remove insulation to expose the tapping and plug in that threaded opening.

I have two Water Heaters with no pressure relief valve on the heaters themselves

Hello, I just bought a house that was built in the early 80's.

It has two water heaters located in small outside closets on opposite ends of the house and neither tank has a T&P valve installed on them, instead there's a plug which looks original where the relief valve should be.

There's a hose bib outside the house with what looks to be a PRV tee'd into it and maybe a common line between the two tanks.

Was this an approved installation back in the 80s in lieu of having the t&p on the side of the heater? On 2017-02-08 by Oscar -

Reply by (mod) - sounds unsafe

A remote temperature/pressure relief valve might respond to water heater pressure but not to water temperature, so it's likely that you are describing a jury-rigged plumbing installation that is improper and unsafe.

I am guessing that there were relief valve leaks (which mean something was wrong and maybe unsafe) and that someone rigged up what you describe so that the drips occur outdoors.

It's time to get help from a licensed plumber

Water Heater Relief Valve Noise FAQs

TP valve is noisy and keeps spilling - should I turn off the heater?

I heard a noise coming from my AO SMith compact water heater under the sink and the T&P valve is turning constantly.

I do not want to touch it and the repair person can not coming for a couple days. Should I just turn it off? (Jan 25, 2015) Juanita

Reply:

Juanita

The system you describe is unsafe and should be turned off. We can't know if the problem is a leaky T&P valve or a problem with the water heater and overheating.

But since an overheated water heater can explode causing a dangerous BLEVE explosion you should TURN OFF THE HEATER immediately.

See RELIEF VALVE LEAKS

Whenever the hot water gets turned on, there is a intermittent bang sound

My water heater was got replaced 6 months ago. Since then whenever the hot water gets turned on, there is a intermittent band sound occurs. It only happens when hot water turns on.
Just wondering if this is related to the pressure valve. Any idea? - Will

Reply:

Will, please search InspectApedia.com

for WATER HAMMER NOISE DIAGNOSE & CURE to read the probable cause as well as solution to this problem.

Watch out: banging pipes due to water hammer can also cause the water heater's temperature/pressure relief valve to discharge or leak. Over time that could become unsafe.

NEXT TOPIC

[The water heater TPR valve Q&A below are in process of being sorted & edited]

What is the cost to change a TPR valve in a water heater?

What is the price range for changing a prv in a Racold 20 litre storage water heater? (Apr 6, 2015) Jayant Bakkshi

Reply:

Jayant

TPR valves are typically priced in the $10 TO $20. U.S.D. Range for the part.

Plumbers charge by the hour, some plus travel and have fees that vary quite widely depending on where you live. The rate is often between $75. and $150. USD. Then, your question presumes that the changeout is straightforward and no unusual difficulties are encountered.

What should happen if I test the relief valve by lifting the test lever?

Hi there just wondering why my TPR doesn't discharge at all and I tested it by lifting the lever and it was flowing freely outside and stopped when i released it back?

Is that normal? Thanks - On 2015-10-01 by Eddie

by (mod) -

Provided that the heater is full of water and under normal water pressure, lifting the relief valve test lever should cause water to come out of the relief valve discharge tube.

When you close the lever again that flow should stop.

Watch out: if the heater is hot you could be scalded by very hot water.

See details and our warnings at RELIEF VALVE, TP VALVE TEST

Does pressure relief valve piping affect water heater operation? Insurance co. says piping pointing down is wrong.

I have a question about the pressure relief valve piping. I completely understand the function and operation of the relif valve.

I am in the middle of disputing a denied claim with a Home Warranty company who is trying to tell me that the water heater does not operate properly with the piping being pointed toward the floor.

So my question is: does the piping on the pressure relief valve effect the operation of the water heater.

Is there somewhere that documents that although this may be a safety issue, the water heater still will function. (Apr 16, 2015) Dave K

Reply: relief valve piping must be routed DOWN to the floor - piping "up" is dangerous, risking a clog

Dave,

Watch out: The TPR valve (and its discharge tube piping) MUST be pointed DOWN for proper, safe operation.

You'll see this in the valve installation instructions dpfrom the manufacturer as well as in researched articles found here.

Pointing a tp valve up is improper, risks clogging, failure, and a catastrophic BLEVE explosion.

If your insurance company says the valve should not point down you are welcome to quote me as observing that they are dead wrong and at are risking killing someone.

But the relief valve itself does not affect the water heater's normal operation. It's there only to relieve excess (unsafe) pressure or temperature.

See TPR DISCHARGE DRAIN PIPED UP - usually unsafe

Blow off valve on hot water heater is required to be six inches from the floor?

Village inspector for pending home purchase said "blow off valve on hot water heater is required to be six inxhes from the floor." But arent these valves pre-installed into the heater? How can I raise or lower it? - (July 20, 2016) MFA

Reply: no, not the valve, the opening of the valve's discharge pipe

No, the relief valve itself, properly called a temperature/pressure relief valve, is installed in a tapping on the water heater tank, typically near or on the tank top where water is hottest.

But yes, the TPR valve must have a discharge tube that terminates close to the floor or at an approved location to reduce risk of scalding someone if the valve opens.

I think that either you misunderstood or the inspector mis-spoke.

Can I increase water pressure at the boiler to get more heat in my home?

I want to increase water pressure from my boiler to get heat to more baseboard heaters. The boiler recommends 30 lbs relief valve. Should I change it to 45 lb (Oct 26, 2016) Michael

Reply: no, that's dangerous

Nope. You'll make your boiler unsafe. See BLEVE EXPLOSIONS

Don't try it. Instead check with your heating service tech to review the boiler's temperature settings on its aquastat. Search InspectApedia for AQUASTAT SETTINGS to read details.

 

Our water heater relief valve discharges through a wall

My existing gas hot water heater has the t&p valve on the top of the tank and it ties into a pipe that discharges through a wall to the outside.

My question is my new water heater has a side mounted t&p line that is located on the side of the tank and on the opposite side of where the piping stubs out of the wall for discharge.

What do I do in this situation? (Nov 14, 2016) Bill

Reply:

Watch out: never pipe a relief valve discharge tube "up" from the relief valve - doing so risks BLEVE EXPLOSIONS from a clogged valve.

You'll need to make a new side wall discharge to a location that is always visible.