Cracking the Codes: T&P Valves
Keeping homeowners out of hot water
Have you ever known someone in life who gets hung up on something and just can’t let it go — kind of like Wile E. Coyote and his endless pursuit of the Road Runner?
Sure, it can be fun to watch, but after a while, we pretty much get the gist of it. Perhaps, instead of ordering odd devices from ACME and concocting ridiculous schemes to capture and eat the world’s fastest bird, why don’t you just order a pizza and move on with your life? We all know people who ride a particular issue until you are tired of hearing about it and you want to ask, “Isn’t there anything else you can talk about?” Well, to be embarrassingly honest with you, I am about to seem like that guy.
I know my last several articles have touched on T&P and water heaters, but I do believe the dangers are worth discussing again, if at least from a different angle. Here is my motivation to touch on this again: I recently attended a state plumbing board meeting on the East Coast in which a homeowner was appearing before the board concerning, what he thought, was an excessive and unfair requirement in the code.
This gentleman was in the process of building a home and was upset when the plumber he hired refused to install his water heater without a temperature-limiting device. It was his desire to have 140° F water at the point of use in his shower and bathtub. When a member of the board plainly and appropriately described the need for scald prevention devices, the gentleman responded with a dangerous line of thinking. He stated that most scalding victims tend to be children. This is true. He then made the argument that this fact indicates that the scalding issue is really one of poor parenting rather than a plumbing issue that should be forced upon all homeowners in all situations. Eventually, the board rightly denied his appeal and sent him on his way.
The question that came to my mind was: What if this homeowner had happened to hire a plumber who did not show as much integrity as the plumber in this case did? What if some other plumber thought it was no big deal?
A brief history
In the Uniform Plumbing Code, sections 504.4, 504.5 and 504.6 provide the primary safety features of the water heater installation. Pressure-relief valves first appeared during the mid-1800s in response to a number of major mishaps involving steam-powered equipment. These devices were simply designed to protect against explosive overpressure conditions caused by “runaway” boilers. At that time, there were few domestic water heaters; consequently, overheated storage water had not become a major concern. As time passed, domestic water heaters became fashionable. Water heated to intended water temperatures represented no threat; however, unintentional overheating of stored water soon became a major cause of domestic tank explosions. In rare instances they still do.
In the early 1930s temperature-relief valves were developed to protect against contained water reaching temperatures in excess of 212°. Temperatures below 212° may cause scalding and other personal injury to individuals, but at temperatures above 212° contained water becomes explosive when it flashes into uncontained steam. The transformation of water into steam can be catastrophically explosive when it flashes as it is suddenly released from a containment vessel, such as a water heater storage tank to the free atmosphere.
Later, the pressure and temperature relief functions were combined into a single valve that would allow one valve to protect against both overpressure and over-temperature. Data plates attached to these valves became more comprehensive and complex as a reflection of this dual purpose. The data plate information reflects each of these independent safety functions.
Levels of protection
There are three levels of protection required for a water heater. The first level is a temperature-limiting device that must be incorporated with the water heater. This will limit the water heater from becoming a steam boiler. This control is integral to the water heater and is included with the heater’s primary controls. They are installed directly on the water heater by the manufacturer. At a temperature not to exceed 210°, this factory-installed device will activate shutting down the energy source to the water heater or boiler. Activation of the high-limit switch [210°] requires manual resetting of the circuit breakers or relighting of the standing pilot, when applicable. The presumption here is that an equipment failure has occurred that requires repair or replacement of the water heater or boiler.
The second level is protection against a pressure increase caused by the heating of the water. This is not accomplished by expansion tanks. A pressure-relief valve is required to ensure that the pressure is dissipated quickly. Section 608.3 addresses protection from thermal expansion and requires either a listed expansion tank or other device designed for thermal expansion control as an acceptable method of preventing excessive pressure within the system. This expansion device is in addition to the pressure-relief valve.
The third level of protection requires the water heater to be protected from vacuum situations that may occur in the piping system due to its installation. This is discussed in Section 608.7, Vacuum Relief Valves, of Chapter 6, which explains the installation of the vacuum-relief valve.
Section 504.6 allows the use of a combination valve to accomplish these safety measures, usually the temperature and pressure protection. Section 608.3 of Chapter 6 also requires that a temperature- and pressure-relief valve be installed on all water heaters and hot water boilers in addition to the factory-installed over-temperature safety devices. Thus, no matter what protection is used that is integral to the water heater, it must be protected by a T&P valve.
The CSA Z21.22 standard requires markings for relief valves and is found on the relief valve tag plate. Three of the most important markings required for the combination temperature-and-pressure relief valve are the set pressure, temperature limit and the CSA rating. The set opening pressure is the minimum pressure at which the valve will open. The temperature limit is the temperature at which the valve will discharge. The CSA rating is the temperature steam rate and thermal expansion water rate in terms of Btuh.
The temperature steam rate and thermal expansion water rate are used to size the relief valve for its discharge capacity. This CSA discharge rating in terms of btuh must exceed the btu input indicated on the water heater label. The valve must be capable of discharging more Btuh than the heater is capable of.
Another marking shown on T&P data plates may include the ASME Pressure Steam Rating. Frequently, this rating is simply ignored. Why? Because it normally reflects a larger btu input rating (based upon ASME standards) than the CSA rating. In all cases, when selecting a T&P relief valve, one must select the lowest rating shown on the data plate (the CSA rating) regardless of the water-heating method (whether gas or electric). The btu input from the heat source must never be allowed to exceed the CSA Temperature Steam Rating as indicated on the data plate. The ASME rating reflects the maximum amount of heat input that a given pressure-relief valve is able to vent as it strives to maintain the maximum pressure (psi) for which it is rated. This btu input limitation is worthwhile information to know; however, it is never the determining factor when selecting a T&P valve.
Another matter for consideration by the design professional is temperature and pressure protection of stand-alone hot water storage tanks. Protection must address both temperature and pressure within these tanks. The pressure limitations of these tanks may be well below the common 150 psi design standard applied to most water heaters. Protection is frequently not possible with use of a single valve. Storage pressure tanks are designed and listed as ASME pressure vessels and, as such, they are limited operationally to the pressures designated on their data plates. Frequently, this pressure limitation will be considerably less than the pressure rating of a standard T&P valve. T&P valves can be special ordered with activation pressures set as low as 75 psi. However, the more common T&P pressure ranges of 125 to 150 psi may be all that is available to the installer on short notice. In that case, it may be necessary to install two independent valves in the shell of the tank: one specifically to relieve pressure before it reaches the design limit of the tank (possibly as low as 75 psi) and the other to maintain water temperature at 210° or less.
The installation of the T&P valve is covered in Section 608.3. It also is discussed in the UA Water Supply Manual. In summary, it is important to remember the following:
- Both gas and electric water heaters require T&P valves when they are connected to storage tanks with a dimension greater than 3 in. in diameter.
- When two or more Btu ratings are referenced on the nameplate of a combination T&P valve, always use the lowest ratings given for temperature-control purposes.
- Electric water heaters generally have a relatively smaller Btu input than that of a comparable gas-fired water heater. Nonetheless, electric heaters still require installation of T&P valves of appropriate capacity. To convert electrical energy input into equivalent Btuh, multiply kilowatt hours by a factor of 3.413.
At this point, pretty much everyone understands the dangers of pressure with regard to water heaters and the need to mitigate that danger. After all, water heater explosions get news coverage and several dedicated segments on the show “Mythbusters.” We don’t hear as often about the scalding incidents that occur. Organizations like ASSE International create and maintain dedicated standards regarding the valves intended to safeguard the public from scalding and pressure threats because they are important and because they save lives and prevent injury. But I will stop beating this dead horse for a while and move on to another topic, I promise.