Do you like tomatoes? Personally, I can’t stand them. I find the texture strange, the taste unpleasant, and the water they emanate that gets into surrounding food items disgusting. They are not all bad, though. I do love some of the tasty byproducts that come from tomatoes. I am of Italian descent after all, so, besides ketchup, I do enjoy the various tomato products that go into pasta sauces and gravy (note: you might have to be very Italian to understand the gravy reference).

So, what do my random thoughts about tomatoes have to do with water heaters? Well, there has been some debate as to whether a tomato is a fruit or a vegetable. To me, it doesn’t matter because how they are classified is not going make me dislike them any less. But to some people, it matters. Whether a tomato is a fruit or vegetable depends on how its attributes match or don’t match the attributes associated with fruits and vegetables. In the plumbing world, properly identifying whether a water heating appliance is technically a water heater or something else works the same way. By looking at what the appliance does, and often what it doesn’t do, this determination can be made and, thus, the proper code requirements can be applied to it.


What is a water heater?

In the Uniform Plumbing Code, water heaters are regulated by the provisions in Chapter 5. The types of water heaters regulated by this chapter are fuel-gas burning, oil burning and electric, all of which heat potable water. Chapter 5 also regulates the air supply and venting systems associated with these water heaters. These appliances should not be confused with boilers, which are regulated by the Uniform Mechanical Code. By understanding the correct attributes of water heaters, this confusion can be avoided.

Water heaters are constructed to comply with nationally recognized design standards for water heaters. Conversely, a water boiler is considered to be a “pressure vessel,” and it must be constructed in compliance with the American Society of Mechanical Engineers and National Board standards, which are considerably more stringent than those applicable to water heaters. There are several criteria that determine when a water heating device has reached its limits as a “water heater,” at which point, both by definition and in fact, it becomes a water boiler and steps up to a new set of design and construction standards. The industry tends to think of anything that looks like a water heater as actually being a water heater. In reality, there are a variety of somewhat arbitrary performance values that distinguish water heaters from water boilers (pressure vessels), even though there may be considerable similarity in the appearance of one when compared with the other.

A water-heating device that exceeds any one of the following should be classified as a water boiler:

  • 120 gallons (454.2 liters) nominal water storage capacity;
  • 160 psi (1103.2 kPa) operating pressure;
  • 210 degrees Fahrenheit (99 degrees Celsius) operating temperature; or
  • 200,000 Btu/hr. (58,620 kW) heat input.

Most typically, it is elevated heat input that results in larger water-heating devices becoming classified as water boilers. The distinction between water heaters and water boilers takes on special significance in many jurisdictions. A number of state or local regulations require a National Board inspector to approve all boiler installations before those devices can be fired or have their power source(s) energized. It is not uncommon for there to be additional regulatory considerations with which the installer must become familiar.

Much of the language in Chapter 5 is extracted from NFPA 54. However, one of the sections not contained in NFPA 54 is the reference to Table 501.1. The minimum capacity of residential water heaters are to be in accordance with the provisions of Table 501.1. In other words, the water heater installed in a house, apartment or condo must be sized to meet Table 501.1 “First Hour Rating.” The first hour rating represents how much hot water the unit can supply in a one-hour period if it starts with a full tank of hot water. For example, in Figure A the Energy Guide Label illustrates a water heater with a first hour rating of 57 gallons (215.77 liters). This water heater would be able to be installed in a residence with up to three bedrooms and 1.5 bathrooms or a residence of two bedrooms and two to 2.5 bathrooms. The note at the bottom of the table requires solar water heaters to be sized this way.


Non-gas fired water heaters

Section 505.0 and its subsections concerning water heaters using fuels other than gas, such as oil-fired water heaters, reiterate the protection requirements in the previous sections. An external combination T&P valve is required in addition to the integral primary temperature controls and integral over-temperature safety controls for these water heaters. Oil-fired water heaters should be installed according to NFPA 31.


Indirect water heaters

The indirect water heater is basically a heat exchanger and storage device where the heating of the water takes place in another device, such as a boiler. The indirect water heater can be either a dual- or single-walled exchanger. The single wall is the more controversial device due to the possibility that if a leak developed in a single-wall device, it would create a cross connection in the potable water system. Therefore, the medium being circulated between the heating element of the system and the indirect-fired water heater must be potable water or another medium that meets FDA standards as “food grade.” If the heating medium is food-grade liquid, then a cross-connection condition is avoided. The requirements for these types of water heaters are delegated to the applicable requirements in the ASME Boiler and Pressure Vessel Code or one of the standards indicated in UPC Table 501.0 (2).


Manufacturer’s installation instructions

Many people are familiar with the segments the popular show “Mythbusters” has done on residential water heaters and their potential hazards. In these segments, the pressure in the tanks is increased until they launch like a missile through the structure around them. When not installed and maintained correctly, this is a very real threat that has sadly resulted in loss of life in the past.

For this reason it is worth spending a moment discussing the manufacturer’s installation instructions. The requirement to conform to the manufacturer’s installation instructions abounds in this code and for good reason. The manufacturer designs a product to a specific standard for a specific installation. If those standards are not met or the installation requirements not followed, not only will the warranty for the equipment be voided but any liability from a failure or an accident will be on the shoulders of the installer, which can include the contractor and the individual plumber. Always consult and follow the manufacturer’s installation instructions. If there are conflicts between the code and the instructions, the installer should contact the AHJ and the manufacturer for clarification; he or she should never decide the issue on his or her own.


Kind advice

Since the installer is sometimes blamed for failures that are caused by misuse or lack of maintenance by homeowners, it is recommended that installers spend time, when feasible, going over the basic operation and safety precautions that could help prevent undue equipment failure and ensure the safety of those who may not understand the ramifications of some seemingly innocent actions that are sometimes taken with residential water heaters.

From the outset, by properly identifying a water heater, the installer can be sure to install the appliance according to the correct requirements. Failing to do so can result in a variety of consequences from failing an inspection to a catastrophic failure. Categorizing these appliances correctly carries a lot of importance with it, significantly more so than correctly categorizing a tomato, which is a technically a fruit by the way.  


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