Blocked relief valve Relief Valve Discharge Tube Hazards
Installation, inspection, & specifications for discharge tubes on TPR valves used on boilers, calorifiers, geysers, & water heaters

  • RELIEF VALVE DISCHARGE TUBE - CONTENTS: how should the drain line or discharge tube be installed on T&P valves: TPR valves or temperature & pressure relief valves used on heating boilers, hot water cylinders, geysers, or on other water heating devices?
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T&P valve discharge tubes:

This article describes the requirements for a discharge tube or drain line on temperature & pressure relief valves used on any appliance that heats water. These include hydronic heating boilers (hot water boilers), steam boilers, and all types of water heaters, both those that use a water storage tank or cylinder and those that heat water on demand such as tankless water heaters.

Here we describe the installation specifications for TPR valve drain line piping and we include an extensive list of discharge tube installation or condition defects, most of which are unsafe. All of them are improper.

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Temperature/Pressure Relief Valve Discharge Tubes & Drain Lines & Their Piping

TPR valve drain piping installation (C) Daniel Friedman

The Temperature & Pressure Relief Valve or TPR Valve on any heated appliance that contains water, such as a heating boiler, hot water tank, water heater, water cylinder, must have a drain line or discharge tube properly installed, routed, and made of proper materials. The purpose of this drain line is to discharge potentially hot scalding water to a safe location so that a bystander is not scalded.

At left we see a typical TPR valve installation (by the author) including the vertical 3/4" copper drain piping that will discharge any T&P valve spillage to the floor.

[Click to enlarge any image]

An unsafe TPR drain line installation is shown at the top of this page. Only a complete fool would do what we found on this boiler. To "stop" an annoying boiler drip at the pressure temperature relief valve, the mechanic installed a short length of pipe capped by a drain valve which he could simply shut. This might have been installed on a system for other reasons, such as connecting a hose to permit easy draining of pressure off of the boiler through the TP valve.

But it is in all events dangerous, illegal, and plain stupid to ever install a shutoff valve or any other sort of "cap" on a pressure/temperature relief valve.

But how dangerous is it to omit a discharge drain tube on a TPR valve? The possibility of a scalding burn is obvious but do these accidents actually happen?

Noticing that a TPR discharge tube was missing on a heating boiler during a home inspection I [DF] pointed out this safety hazard to my client while the real estate agent nearby frowned at my "old maid" trouble-making personality. My client burst into tears. Sobbing she told me that she was grateful that inspectors would routinely point-out this hazard. Her son, playing with friends in the basement, lost an eye when he and a pal opened the discharge lever on a heating boiler, scalding his face and ruining his left eye forever.

Less dramatic but scary, at a different inspection I found that a string tied through a small hole in the end of the TPR valve's test lever. The string was routed up towards the ceiling, over a horizontal plumbing line and back down to a termination in a nice knot a few feet above the floor. This interesting TPR test lever addition was explained by the building owner. His son and friends liked to play steam boat. It was fun to pull the string, pretending it was a steam boat whistle, and to see the burst of steamy hot water emerge from the end of the discharge line.


TPR Valve Discharge Tube Inspection: Signs of Trouble, Unsafe Installation, Safety Hazards

Missing relief valve discharge tube (C) Daniel FriedmanWatch out: While it is possible to "open" a boiler TP relief valve by lifting its "test" lever, unless you are a trained heating service technician or plumber, and unless you have a spare TP valve of the proper size in your hand, we advise against "testing" a TP relief valve by opening this lever.

Insulated over relief valve (C) Daniel Friedman

Constricted relief valve (C) Daniel Friedman Relief valve discharge tube reduced or constricted (C) Daniel Friedman 2005

The photos above illustrates this unsafe installation practice: a 1/2" copper tube has been installed through a reducing fitting into the mouth or piping of a 3/4" diameter TPR valve. At above left the reducer from 3/4" to 1/2" was installed at the TPR valve opening. At above right a reducing elbow was used to shrink the 3/4" horizontal T&P drain line (from the TPR valve mouth) to 1/2" for the vertical run to the floor. Both of these installations are improper and unsafe.

Leaking at the drain pipe on a T&P valve (C) Daniel Friedman Evidence of relief valve leaking

Pressure relief valve leak test at discharge tube (C) Daniel Friedman Pressure relief valve leak test at discharge tube (C) Daniel Friedman

Why We Never Pipe the T&P Valve Drain Line Upwards from the Valve Mouth

Reader Question: explain the concern for a water heater discharge line above the TPR valve - why can't we pipe a TP valve discharge tube "up"

TP valve discharge line routed too high (C) InspectApediaI am ... in the process of selling a condo I own. I got this request for repairs for the hot water heater with a picture of the heater. On the picture it shows the that the discharge line is above the TPR valve, and that this is a problem (see description on attachment). This doesn't make any sense to me. Can you help me decide what the best action would be? thanks. - R.N. 7/11/2013

Quoting from the inspection report:

The overflow line is higher than the Temperature and Pressure Relief (T & P) valve. The over flow line should always allow to water to drain to a point lower than the valve without backing up to the valve.

Water trapped in the discharge line may cause the T & P valve to corrode and malfunction. (The concern is if the water overheats and turns to steam, and the temperature control valve fails to function, the over pressure may cause the heater to burst or explode.)

Lowering of the discharge line to a point below the valve and replacement of the valve should solve the problem.


The photo is a bit difficult to read but if you look closely where the two flexible copper lines enter the wall behind the water heater, you'll see that the smaller leftmost flexible tube, connected back to the water heater TP valve, enters the wall at a height above the valve outlet opening. What the home inspector said was perfectly correct and represents a safety hazard.

Perhaps if I explain the concern in my own words it will be more clear:

The temperature/pressure relief valve on a water heater is connected to a drain line so that if the valve  opens someone nearby is not shot in the face with hot water. The discharge drain extension is typically taken to just a few inches above the floor or in some jurisdictions it may be directed outdoors - a solution that I think is risky because IF the valve should be leaking, dripping, etc., one wants to notice that and fix it to keep the system safe.

The inspector's report makes a valid point: we should never route the discharge tube "up" from the actual outlet opening of the TP valve. That's because if the valve should develop a small leak or be discharged on occasion, the up-routed discharge tube will keep water and debris remaining in the tube at the valve outlet where debris or mineral accumulation clog the valve or interfere with its operating spring.

The result over time could be that the valve becomes clogged and would then fail to open in a true emergency - risking, ultimately a dangerous BLEVE or water heater explosion.

Watch out: ALSO, I suspect from the photo that your water heater has a discharge tube that directs the valve outlet into a wall and going to who knows where. If the other end of that line is not already readily visible and in a location where it would be noticed, that too would be unsafe and improper.

The FIX for this unsafe condition is usually trivial: the discharge tube must be routed only "downwards" from the TP valve outlet opening, and the end of the discharge tube must be in a readily accessible, visible, and safe location. You'd probably find these same instructions in the installation manual for the water heater.

The COST for this repair should be no more than a simple plumbing service call and perhaps a few piping connections. What would make sense to me and what would be most economical would be to combine this repair with any other plumbing repairs that are needed at the home.

The TIME to fix this problem is as soon as possible, without handling it as an extra-cost "immediate-emergency" plumbing repair. That means, call a plumber and schedule the work for as soon as possible and practical, but not sounding so scared that the plumber gouges the customer with extra fees.


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