Water Heater TPR / PRV Diagnosis FAQs

TPR Valve Opening Problems FAQs
TPR Valve Size & Capacity FAQs
Q&A on calorifier, geyser, cylinder relief valves Set #2

Water Heater TPR Valve Opening Problem FAQs

I can't test the water heater relief valve - it won't open

I tried to open the pressure relief valve on my water heater and It won't open by my hand and I don't want to use a wrench or pliers to force it. How should I get it open. It has been 7 or 8 years since it was installed.

There is no visual corrosion. (May 2, 2014) Anonymous

Reply:

Good question anon.

Watch out: IF the test lever is stuck this is an important discovery, as it quite possibly means that the valve opening mechanism is stuck as well - that would be very dangerous - should an overpressure occur and the valve fail to open as it should the water heater becomes a rocket or a bomb.

See BLEVE EXPLOSIONS

And if you use a tool to force the valve open there is a risk that it won't close and you'll have a flood.
And you'd risk damaging the valve (as you understand).

I would replace the valve.

No hot water after someone opened the pressure relief valve

Someone opened the pressure valve by accident and water heater went into vacation mode and now we don’t have hot water. On 2018-12-29 by Anonymous

by (mod) - what to do if after opening the relief valve you have no hot water

Anon:

IF you water heater is electric

AND the water level in the heater dropped below the electric heating element

AND the heater was turned on

THEN the element may have burned up.

Try first turning the system off and back on

Try also checking for a tripped button on the temperature limit control

Then use the on page search box to find our article entitled

How to Test for a Bad Electric Water Heater Heating Element

let me know how that works for you.

About water heater vacation mode and how to get out of vacation mode:

Some water heaters have a "vacation mode", often a setting on a gas water heater pilot light, that leaves the water heater set to a low temperature. In a freezing climate that can help prevent freeze damage to the water heater itself and its nearby supply and outlet pipes and valves.

If that is your water heater type, simply set the dial back to a normal water heater temperature setting, or if you're unsure what that was, start by setting the temperature at the middle of the range allowed.

If your water heater uses a digital control panel, you need to open its thermostat control menu and either change the temperature setting to a higher number or if your heater has a "vacation mode" message shown in the display, select "exit vacation mode".

Water Heater Relief Valve Type & Size FAQs

Tip: See details at WATER HEATER TPR VALVE SIZE

What size relief valve is normally on a 50 gal hot water heater?

What size relief valve is normally on a 50 gal hot water heater? On 2018-04-08 by Anonymous -\

Shown above is a model whose input BTUh is given on its data tag.

by (mod) - relief valve capacity must be equal or greater than the input BTUH of the water heater

Anon

There is not a single "right" answer to your question.

The relief valve must be capable of discharging hot water fast enough to not b e overtaken by the input BTUH of the water heater.

You'll see that the input BTUH varies significantly by water heater fuel source: oil, gas, electric, solar, and not by the water heater size in gallons.

Take a look at the data tag on your water heater where the input BTUH will be given;

Then look at the data tag on the pressure/temperature relief valve where you'll see a BTUh Range that needs to include your water heater.

For example, the Watts Series LF1L, LF1XL, LF10L and LF100XL temperature and pressure relief valves like the one I show above, are ASME rated and C SA listed as approved for water heaters up to 105,000 BTU/Hr.

Details are in this

• Watts Series LF1L, LF1XL, LF10L and LF100XL TPR VALVES SPECIFICATION SHEET [PDF] (at Watts Website) - local copy saved as Watts-L1FL-Specs.pdf
  
  Excerpt: ASME Rated**, CSA Listed. Self-closing T&P Relief Valves for Water Heaters up to 105,000 BTU/Hr.

The combined 2 in 1 T&P relief valve provides the least expensive and proven means for protection against both excessive temperature and pressure emergency conditions.

Provides fully automatic temperature and pressure relief protection for hot water storage tanks and heaters up to 105,000 BTU/HR.

See also this helpful

• Hellemans, Marc. THE SAFETY RELIEF VALVE HANDBOOK: DESIGN AND USE OF PROCESS SAFETY VALVES TO ASME AND INTERNATIONAL CODES AND STANDARDS [PDF] Elsevier, 2009. [copy provided by https://www.daboosanat.com ]

What's the difference between a pressure relief valve for a water heater and a heating boiler?

what is the difference? and what is commonly used on a 40 gallon water heater? On 2020-11-11 by levi

by (mod) -

Levi:

The TPR valve selected depends on the type of heating appliance and its fuel and thus the operating temperature range and pressure range.

Across water heaters, the relief valve will normally

- open at 150 psi pressure
and/or
- open at 210 degF temperature

and must be capable of discharging energy (as BTUs) at a rate greater than the input rate of the water heater's energy source

To simplify life you'll typically find TPR valves sold in BTU capacity ranges

1 group for heaters rated at Under 200,000 BTU Per Hour Input Rating

a second group for heaters rated at Over 200,000 BTU Per Hour Input Rating

You can easily verify the TPR valve on your 40 gallon water heater by looking at the information on the relief valve data tag and comparing that with the INPUT BTUH rating of your water heater.

See details at

WATER HEATER TPR VALVE SIZE

Which Type of pressure / temperature relief valve is right for my water heater?

Lennox Boiler model CMWB-5 as XEB-5 series with Crown's Mega store 50 for hot water storage is my heating system.

BTU/HR by CMWB-5 is 150000 for INPUT and 123000 CGA OUTPUT.

Due to the problem I have replaced Pressure Relief value with WATTS 354A model M with set to 30 PSI as recommended by pluming parts store. (unfortunately I did not have the tag for old parts.)

Can you tell if WATTS 354A model M with set to 30 PSI is right parts for my heating system? On 2018-10-02 by Peter Kim

by (mod) - Two types of temperature/pressure relief valves - water heater vs heating boiler

Watch out: don't mix up types of temperature/pressure relief valves between heating boilers (building heating sytems) and water heaters (domestic hot water for washing and bathing) completely different pressure and temperature ranges are involved.

Putting the wrong type of relief valve on a heating device is unsafe.

The relief valve on the boiler needs to be rated at or above the input BTUH of the boiler it protects.

A boiler relief valve is usually set to open at or above 30 psi.

The WATTS 374A model M [shown here) is designed for hydronic heat (hot water boilers) and has a spill or protection capacity of 550,000 BTU/hr - well above (thus in the "safe" direction) the input BTUH rating of your boiler (which you give as 150,000)

The relief valve on a separate calorifier or hot water tank or hot water storage tank such as an indirect water heater will have different specifications. And it is a different design as well.

For example a Rheem ProTec TPR valve (shown above) designed for a water heater has these specs:

Temperature Relief: 210°F (99°C),

Pressure Relief Range: 75 to 150 psi (5.2 to 10.3 bar).

Notice that it is opening at a higher pressure than the TPR valve on your boiler itself. That's because the normal pressure inside of a water heater or hot water storage tank will be greater.

A heating boiler starts at about 12 psi cold and gets up to just under 30 psi hot (at under 200F).

While a water heater (calorifier or geyser) starts at building water pressure that could be anywhere from 30 to 70 psi and goes to a higher pressure as it's heated.

I'm not sure which TPR application you are asking about. So I address both above.

Below the image shows what your Watts TPR valve looks like -

What's the opening temperature for a TPR Valve?

At what temperature a T&P value would open? On 2017-05-04 by Bob Trueblood

by (mod) - opening pressure for a T&P valve depends on type of heating equipment being protected: boiler, steam boiler, water heater or geyser

Bob

It depends on what type of heating equipment is being protected;

For example a home heating boiler TPR valve like the Watts 335 ASME Boiler Pressure Relief valve, used on smaller boilers up to 510,000 input BUTH at 30 psi (2.11 bar) opens at 30 psi. No temperature rating is even given in the basic specifications.

• The valve opens at 30 psi

In comparison, a Rheem ProTec TPR valve used on water heaters provides

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

What's the difference in a long stem and a short stem on a hot water heater pop off value?

What is the difference in a long stem and a short stem on a hot water heater pop off value? On 2020-01-31 by Eddie

by (mod) - clearance distance needed from other components inside the water heater tank

Eddie

On some water heaters the shorter stem is needed because given the TPR valve mounting position, such as a side-mount TPR valve, the long stem might touch an internal part of the heater such as a center flue or a dip tube.

NEXT TPR Topic

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

What is an "open main" water system and does it have safety implications for water heaters?

Does the water pressure in my house plumbing system ever increase under any circumstance (hot water heater or something else) on an open main water supply system? - Watts

Reply:

In an "open main" system nothing is closed between the house [plumbing] system and the street main.

Therefore, water pressure [in the house] cannot increase above the street main pressure from the reservoir regardless of any expansion [in the hot water system] from temperature. Consequently, a straight pressure relief valve cannot operate unless a system is closed.[10]

Note: By "straight pressure relief valve" we think Watts means a valve that responds only to water pressure and that does not include a response to hot water temperature. Such a system could be unsafe.

 

Can I use a combination of check valve and pressure-only relief valve to protect against overheating my hot water tank?

Will a "straight" pressure-only relief valve prevent overheating and thus keep my hot water system safe if a check valve is also used?

Reply: No. Such a system is unsafe

No. Overheating in the hot water system depends entirely on the BTU input rate to the water heater.

A relief valve that operates on pressure only (ignoring temperature), regardless of its size or rated discharge capacity, can't prevent overheating nor reduce temperature. Such a sysetm is unsafe.

 

Will the relief valve affect building water pressure delivered to faucets?

Can the relief valve affect the pressure of water coming out of faucets? - Pam Gregg

Reply:

No, Pam, not under normal circumstances. That's because hot water leaving the water heater may pass BY a relief valve en route to your faucets (depending on where it is installed) but the water is not passing THROUGH it.

But if a water heater pressure-relief valve were stuck wide open flushing hot water continuously down a drain, building water pressure would probably fall noticeably - that would be an unusual circumstance and surely you would know it, from the flooding water out of the valve and quickly by the loss of hot water.

 

Since a TP valve releases excessive pressure in a water heater or boiler, isn't pressure-release enough protection against a water heater tank explosion?

If the condition of too much hot water pressure is corrected by the automatic opening of the pressure/temperature relief safety valve or "blowoff valve", why was I told that a water tank can still blow up?

Reply: relieving pressure alone won't prevent a hot water tank from exploding - that's why a TP valve also responds to temperature and the TP valve has to release heat at a rate equal to or greater than the BTU input rate for the water heater

One would think that the TP valve alone would always be enough protectin on a water heater tank. But as Watts Regulator Company explained back in the 1970's and as was demonstrated in the film "Explosion Danger Lurks", even a wide-open pressure relief valve can not prevent water in a water heater tank from becoming overheated.

At WATER TANK PRESSURE CALCULATIONS we discuss the calculations behind the increase in temperature and pressure in an enclosed water tank or water heater tank. In the case of a hot water storage tank, Watts points out that

The reason that the volume of pressure discharge [through the relief valve] is not enough to overcome the BTU heat input [into the hot water heater tank] is because thermal expansion pressure is approximately 2 1/2% of volume for every 100 degree rise. [The] fluid heat discharge ... necessary to relieve the extra BTU heat input for every 1000 heat units is about 20 times greater in volume.

In a [hot water heating] system with a check valve and a pressure relief valve, the relief valve opening by pressure increase due to expanding water will discharge approximiately 1/2 lbs. of water for each 1,000 heat units (BTUs) put into the water - because that is the rate at which the water expands when heated.

To release 1,000 heat units when the temperature is at 210-212 degF, there must be approximately 6 2/3 lbs. of water released from the system. In other words, to prevent overheating [water in the hot water storage tank] a means must be provided to relase about 20 times as much water from the system as a pressure relief valve can discharge from thermal expansion [alone], when there is a check valve in the supply line or [when there is] an accidental stoppage [of the water piping] to make the system a closed one.[10]

In other words, in a closed hot water piping system (closed by the presence of a check valve or a blockage in the water piping to prevent expanding hot water from "pushing" water volume back out of the watertank, to prevent a hot water tank from overheating, the TP valve has to discharge heat [not pressure] at or greater than the rate that the heater's burner or electrodes are putting heat into the water in the tank.

But on a typical water heater, the heat input rate in BTUs is about 20 times more than the heat output rate through a TP valve if it operated on pressure alone.

Watch out: the conclusion of this technical discussion is that it is absolutely essential that the BTU (heat or energy) dumping rate for a TP valve must be properly matched to the BTU input rate of the heater. And a pressure-only relief valve on a water heater, that is excluding a valve that also responds to temperature, would be an unsafe installation.

 

Is a TP valve required on a wall mounted water heater?

(Oct 2, 2012) Bill Miller

Is there a rule or code on how many 90 degree 3/4" fittings can be used on the drain line of a T&P at a 75 gallon State water heater?

(Oct 4, 2012) James

IS a TP valve required for storage wtaer heaters that are wall mounted, 50 liter capacity. I ahve noted that most household storage water heaters in the middle east have on TP valve except a releif valve on the water INLET of the heater.

Yet all these are listed and registered to be complying with the IEC 60335-2:2002 Household and similar Electrical appliances: Storage Water heaters? would appreciate guidance. cheere

Reply:

James a temperature and pressure relief valve is required on any hot water tank or cylinder regardless of how the tank is mounted.

James, yes. I worry that a relef valve on the cold inlet is not what the manufacturer specified, and that even if it responded to overpressure it won't respond safely and reliably to an over temperature condition.send us brand and model numbers for research.

 

I tested the TP valve, now it keeps leaking

After lifting the lever of temperature and pressure relief valve for annually maintenance,hot water leaking for hours, I wonder when its stop? (Dec 2, 2012) Sun

Reply:

Sun,

Tap gently on the pin that pulls the valve open. If that doesn't reseat the valve, and if flushing the valve doesn't remove debris from the valve seat it's time to replace the valve.

Watch out: a temperature/pressure relief valve that keeps dripping is unsafe and could risk becoming clogged, failing to open on overpressure and leading to a dangerous or even fatal boiler explosion.

See BLEVE EXPLOSIONS

When in doubt replace the valve. You should be using a heating service professional.

Also check for abnormally-high system water pressure and water hammer problems in the building.

 

Thermal expansion valve on cold water line leaks

I have a thermal expansion valve that is installed on my inlet line (cold water line going into my hot water tank) that is leaking, now the question that I have is, is it required to be on the inlet line . Talking to a few friends that do plumbing and they are saying that it is not required, but they recommend that I install a expansion tank.

So, another question is if the system was installed without a expansion tank is it a good idea to install one are just replace the thermal expansion valve that was installed when the hot water tank was installed, are just remove the old system and install a new hot water tank system per new system directions ? So many questions, but can't find all the answers! NEED HELP ! (Dec 22, 2012) charles

Reply:

Sounds like you need an expansion tank but first I'd check for any other sources of abnormal system pressure such as water hammer.

 

Rheem PVW50 TP valve leaks

(Jan 11, 2013) Wayne

I have a 9 year old Rheem PVW50. Last year I replaced the drain valve, T&P valve and the anode rod. The old anode rod was worn out.

A couple of months ago I noticed the T&P valve was leaking every once in a while. Then it started every day.

I read an article that said if the T&P is leaking to open the cold water faucet, if the T&P stops dripping it is not the water heater.

But to check it out get a watts gauge with high pressure marker. So I purchased one and found out the cold water was going up to 110 psi and hot water heater up to 150 psi. Found out I need to replace the Watts Pressure Reducting Valve AU25B.

After replacing valve cold side is now 50 to 60 psi. However, the hot water heater registers 50 to 60 and when the water is being heated may go up to 100 psi. No water is leaking out of the T&P valve or anywhere else. Putting my ear to the heater I can hear a little grumbling out of the tank but not overly so.

So my question is 2 fold. First, is the water pressure of 50 to 100 in the tank ok? Secondly, when is the best time to replace the tank but still get maximum usage out of the water heater? I've been told by a friend at Rheem that they have seen 15 to 20 years out of the tanks.

Reply:

Opening a cold water faucet is not a proper fix for a leaky TP valve. Replace the valve; if the new one leaks check the system temperature and pressure; Your system may need a hot water expansion tank.

 

Nipple between water heater and TP valve?

(Feb 12, 2013) Ethan

Hello! I have a question here, not addressed by any web info I can find:

Is it against any code to have a nipple (about 1" long) installed in between water heater and the T&P valve? (instead of having the T&P valve threaded directly into the tank). I'd rather not use an extra-long T&P valve. Much thanks!

Reply:

In my OPINION the installation you describe is improper and unsafe and if it violates the manufacturer's installation instructions, indirectly by that error it's "against code" as you put it.

The short nipple you describe raises the valve, reduces the depth of extension of its temperature sensor into the water heater, so is improper.

 

O ring blew in mixing valve - is there a sticking relief valve problem?

(Mar 4, 2013) ron goodrick

hi there just replaced an expensive basin mixer cartridge leaked installed new one .. 3 days o ring blew in hose //sticking temp relief valve ? dodgy taps ??

Mar 6, 2013) SOMIDI MAHESHWRA

IF A SAFETY VALVE IS GETTING RELEAF AT 250 Deg centigrade and presuure 20 kgs/sq cm if my temperature is 30 deg centigrade what is the pressure of release in kg/sq cm.

Reply:

These questions are a bit unclear leaving us reluctant to guess at an answer.

Watch out: if your TP valve is "sticking" - that is not opening when it should, the system is unsafe. See BLEVE EXPLOSIONS

 

Washing machine blows the TP valve

(Mar 16, 2014) mike

I have a front loader washing machine and it blows the T&P Valve on my electric hot water tank. Any solution on how to fix this problem. I have put in a pressure reducer coming into the house and water hammer behind the washing machine and nothing.

Should I replace the T&P valve again since it has blown 50 plus times. Only with the washing machine nothing else it blows

Reply:

I'm unclear how the washer is causing the TP valve to blow or leak - and because there could be a very serious safety hazard involved, some accurate diagnosis is in order.

For example, if the problem is due to water hammer caused by the washer operation, there are water hammer control devices you can install at strategic locations on the system to solve the issue - search InspectApedia.com for

WATER HAMMER NOISE DIAGNOSE & CURE to read details.

But if the washer cycle is for some reason causing the hot water source - a water heater - to overheat or malfunction, that could be a serious safety hazard.

 

Do I need a new TP valve when installing a new water heater?

(Mar 29, 2014) henry parker

when I buy a new water heater do I also need to buy a temp relief valve?

Reply:

Absolutely, if a TP was not included with the heater. Be sure to buy the proper valve matched to the water cylinder (heater) capacity and design and mount location.

I would never risk re-using an old TP valve on a new heater.

Why is just one TP valve leaking?

We have a 30 gallon water heater less than a year old with pop-off value leak.

I have replaced the valve, city water pressure is 40 -45 lbs., heat controls on 120 degrees top and bottom, this is the only water heater out of 12 apartments in the same building that leaks. What happened?

Reply:

C. David,

Several conditions besides a too-high water heater temperature can cause the relief valve to leak, such as water hammer in the building water pipes.

You want to diagnose the reason the valve is leaking.

So step through our diagnostics at RELIEF VALVE LEAKS