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Guide to Hot Water Heater Temperature Pressure Relief Valves InspectAPedia®  -     How to use, test, inspect water heater temperature and pressure safety relief valves Water heaters: how to inspect, test, adjust, repair, and choose among types of hot water heaters

This article discusses the function, inspection, and maintenance of temperature pressure relief valves on water heaters used to produce hot water for washing and bathing.

This website contains links to in-depth articles on inspecting, testing, and repairing problems residential hot water heaters of all types, including their parts, controls, and alternative sources for hot water as well as tips for improving hot water temperature, hot water pressure, and hot water quantity. Reproduction of this web page electronically at other websites is prohibited. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. The sketch at page top is courtesy of Carson Dunlop.

Water Heater Temperature & Pressure Relief Valves

Here at Relief Valves - Water Heaters we discuss temperature and pressure relief valves used on residential water heaters. For a general discussion of temperature and pressure relief valves used as safety devices on heating boilers and other pressurized please see Relief Valves - TP Valves. Pressure relief valves (that sense pressure only, not temperature) are also required on pressureized tanks such as water tanks in buildings. Water tank pressure relief valves are discussed at WATER PUMP & TANK SAFETY.

Suggestions for Testing the Water Heater Temperature Pressure Relief Valve (TP Valve or "Safety Valve") on a Water Heater

Some manufacturers (Rheem) recommend that the temperature and pressure safety relief valve be tested once efery six months. To test the valve the lever on the valve is lifted, which should result in (hot) water flushing through the valve and its discharge tube into a bucket placed below the discharge tube for that purpose or into a floor drain. In Australia and New Zealand the test levfer is called an easing lever but performs the same purpose.

After lifting the easing lever or relief valve opening lever, lower it gently back to its closed position. Rheem points out that rough handling of the relief valve test lever, such as allowing it to "snap" closed, can damage the temperature and pressure relief valve - a dangerous condition that could lead to a water heater explosion or BLEVE explosion.

If water does not flow freely when the temperature/pressure relief valve is opened, the valve should be replaced by a licensed plumber, making sure that an approved, properly-rated TP valve is installed.

If your temperature/pressure relief valve has not been tested or replaced sooner than five years, Rheem recommends that the TP valve should simply be replaced. More frequent replacement of the safety valve may be needed in areas where hard water is found.

Other Versions of Pressure or Temperature & Pressure Relief Valves on Water Heaters

Constricted, Under-sized, or Mis-Routed Relief Valve Discharge Tubes

Evidence of dangerous leaks and corrosion at water heater temperature/pressure safety valves

Other safety valves and devices found on some water heaters include:

A Temperature LImiting Valve that limits the outgoing water temperature to 50 degC in order to prevent scalding. This valve should be checked annually by measuring water temperature at a nearby water tapk, making sure that the water delivered closest to the water heater is not hotter than 50 degC.

An Expansion Control Valve is used on water heaters in south and Western Australia where hard water is found, and in some other countries or other jurisdictions. The Expansion Control Valve discharges water into a drain to relieve excess pressure in the hot water tank. The purpose of this valve is to release pressure through a separate control so that the safety provided by the Temperature and Pressure Relief valve is not compromised by clogging from minerals should that valve frequently open.

The expansion control valve should be tested every six months, following the same procedure as for temperature and pressure relief valves as we described above.

Readers of this article should see RELIEF VALVES - TP Valves where we include additional relief valve information including for hydronic heating and steam heating boilers used for central heating.

How Mineral Deposits and Heating May Weaken a Water Heater Tank Bottom, Contributing to BLEVE Explosion

How to Let Air Into the Hot Water Tank to Speed the Tank Draining Process It is possible to speed the heater draining and also to ease the flushing procedure if you open a hot water tap nearby so that you can let air into the heater as water leaves it Some plumbers open the water heater pressure/temperature relief valve for this air-in purpose - but there is risk that you'll later be unable to get the valve to shut fully again - debris can clog the valve seat - sometimes we can stop a relief valve from dripping by tapping on the valve lift rod that protrudes through the valve lift lever Other plumbers simply remove the relief valve entirely - this is the process recommended by some water heater manufactures such as A.O. Smith. Removing the relief valve makes it easy to inspect this critical safety component itself, and it's easy to clean or replace the safety valve at that time. When replacing the relief valve use an approved teflon tape or pipe sealant and work neatly and with care so that there will be no leaks at this location. Safety Warning - a damaged, improperly-selected model, or modified pressure/temperature relief valve is dangerous and could lead to a serious BLEVE explosion should the water heater later overheat.

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• Daniel Friedman - principal author/editor of the InspectAPedia^® Website
• Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
• Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• Pounds of lime deposited vs. temperature and hot water usage: see Purdue Bulletin #74 (also provided by A.O. Smith in the article below). Purdue's chart shows the number of pounds of lime deposited per year as a function of the water usage in gallons per day, with an assumed 10 grains of water hardness.
• "When, Why, and How to Remove Water Scale from Tank Type Glass-Lined Water Heaters (for non glass-lined tanks, consult water heater manufacturer)" PDF provided by A.O. Smith Water Products Company - hotwater.com/lit/training/4800r9.pdf 800-433-2545 - 01/07/2009.
• A.O. Smith's Form No. 4800 Rev. 8 Why? When? & How? /UN•LIME Specific Deliming Instructions for use with Up-N-Down Transfer Kit for Tank Type Heaters. (Normally supplied in UN-LIME shipping cartons), Supersedes Form Nos. 4800 Rev. 7 and 4813-100.
• A.O. Smith's Form No. 4778* All about Deliming Coil-Type/Tube-Type Commercial Water Heaters and Hydronicc Boilers *Normally supplied when ordering Part No. 4930 Motorized Deliming Pump Kit
• Rheem Electric Water Heater "Owners Guide and Installation Instructions", (Australian models) rheem.com.au/images/pdf/owners_dom-elec_121996C_0707.pdf

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