Free Technical Information for Building Inspectors, Contractors and Consumers. Code and Manufacturer's Information Resource
Water Heaters
What kind of damage has water heater explosions caused?
The Seattle Post-Intelligencer on July 28, 2001, reported on an electric water heater tank explosion where the pressures built up until the tank exploded. According to the article the force of the explosion blew the tank across six lanes of traffic and ended up in a parking lot more than 439 feet away.
In 1993, an older water heater exploded in a home in South St. Paul, Minn. The 200-pound tank shot through a floor, ceiling and roof like a missile to a height of 150 feet in the air. The explosion destroyed the home and injured two people.
In 1982 six children and one teacher were killed when a hot water heater exploded in the Star Elementary School cafeteria in Oklahoma City. Regulations were later enacted there requiring annual inspection of domestic hot water heaters in all public facilities.
BURN FACT's:
The leading cause of deaths and injuries to children at home is accidents. Scalding from hot water is one of the most dangerous of these accidents. Small children are busy and can get to sinks or bathtubs quickly. They can burn themselves severely before they can get out of the water. Infants are unable to move away from hot water if it is accidentally left on too hot or if the cold water is unintentionally turned off.
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The Temperature/Pressure Relief ValveOne of the most common code violations our inspectors observe in the field involves the Temperature/Pressure Relief (TPR) Valve on the water heater unit. The purpose of this article is to assist property owners and inspection professionals in identifying these common code violations so they may be prevented and corrected. What is a Temperature/Pressure Relief Valve? Most model building codes require the installation of a combination Temperature/Pressure Relief Valve (commonly referred to as a TPR valve) or a separate temperature relief valve and a pressure relief valve on every water heater. The purpose of the TPR valve is to provide an important safety mechanism that allows for the immediate discharge of excess temperature and pressure that builds up inside the tank as the water is heated.
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| Codes also indictate that a TPR valve must discharge once the pressure inside the tank reaches 150 psi (pounds per square inch). Refer to ypur loacl code which should specify that the TPR valve must be installed within the top 6 inches of the water heater tank in order to enable the valve to discharge at a temperature of no greater than 210°F. As a result, any discharge piping must be rated to withstand at least 210°F to prevent melting and blockage of the TPR valve when discharged. | ||||||||
Common Violations in the FieldAs we have seen, the presence of a functioning Temperature/Pressure Relief (TPR) Valve on a water heater is extremely important. However, there are other factors to consider in ensuring the protection of property and people who may be standing near the water heater when the valve actually discharges. Namely, where does the scalding water or steam go once the valve is allowed to discharge? The following are the more common discharge piping violations we observe in the field: | ||||||||
1) Improperly-Sized Discharge Pipe Code requires that the TPR valve is fitted with a discharge pipe that provides the same size opening as the valve outlet itself. | ||||||||
| Figure 2 (right) shows a TPR Valve that has been fitted with a discharge pipe that is smaller than the size of the valve outlet. This is a common defect that is strictly prohibited because smaller discharge pipes restrict the flow of the pressurized water released by the valve. This creates a dangerous bottleneck, building-up even more pressure at the point of discharge that can cause the tank to explode. Personal injury (or death) and property damage may result. |
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2) Missing Discharge Pipe In many instances, the discharge pipe required per code was never installed at all. This normally occurs during replacement or initial installation of the water heater. | ||||||||
Figure 3 (right), shows a missing TPR discharge pipe. This creates a dangerous condition whereby the TPR valve will spray scalding steam or scalding hot water onto anyone (or anything) that happens to be near the water heater when discharge occurs. When you consider that the TPR valve discharges water at 210° F., serious injury or property damage will result. |
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| 3) Improper Termination of the Discharge Pipe | ||||||||
| Ideally, the TPR valve discharge pipe will be vented to the outside of the building where the scalding water or steam can be discharged without posing a threat to people or property. However, in many installations direct piping to the outside is not possible or is deemed to be too costly to provide. Accordingly, the code does allow for the discharge pipe to terminate inside a receptacle (or spill pan) as long as an air gap is provided between the end of the discharge pipe and the top rim of the spill pan itself without creating a hazard or potential cause of damage. | ||||||||
| This is clearly not the case in Figure 4 (right). Here we see the copper discharge pipe runs down the right hand side of the water heater tank to terminate into a metal spill pan below. However, the problem lies in the fact that the end of this discharge pipe terminates below the upper rim of a shallow spill pan. This installation of the discharge pipe poses several potential dangers. First, the discharge pipe may be able to siphon any liquid that may accumulate inside the pan. This is due to the fact that the end of the discharge pipe terminates below the top rim of the receptacle itself. Once siphoning occurs, the liquid (be it dirty water, household chemicals, oil, etc.) will be drawn back into the water heater tank, thereby contaminating the household drinking water supply. |
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While this occurrence is admittedly unlikely due to the fact that the TPR valve would have to fail (stay open) while a simultaneous drop in line pressure would create a siphon, the mechanical potential does exist for this contamination to occur. Thus the air gap provision in the building code. Secondly, and even more likely, is the risk of personal injury when this TPR valve discharges. This is because the discharge pipe terminates into a spill pan that cannot safely catch the flow of water or steam during discharge. When you consider that the TPR valve discharges at 150 psi, which is five times greater than the force of your garden hose, the scalding water can strike the bottom of the shallow pan and spray out in all directions. Therefore, a deeper spill pan receptacle with adequate circumference would be advisable over the shallower, tightly-fitting spill pans commonly observed in the field. Refwerring again to figure #4. If this unit is located in a garage additional code requirements for protection of the unit must be met. These concerns are primarily for impact issues. Ballards are typically required. | ||||||||
Figure 5 (right), shows a spill pan that tightly fits the base of a water heater. Unfortunately, such an installation accomplishes little in the way of catching water or steam from the TPR safety discharge valve, rendering the spill pan useless. Spill pan installations depicted in this photograph are common in condominiums and some residential homes where space is a premium. |
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ABOUT YOU WATER HEATER:
Sediment
Heating water causes calcium carbonate to precipitate out and settle to the bottom of the water heater. Water that gets under this layer of sediment can turn to steam when the burners come on and cause popping and other noises. The build up of sediment can reduce the efficiency of your water heater and reduce the holding capacity.
Regular flushing of the water heater through the drain valve can help prevent sediment build up. Sediment can reduce the efficiency of the water heater and get into re-circulating lines and cause problems with circulating pumps and check valves.
Smelly hot water
Anaerobic bacteria reacting with the magnesium anode forms hydrogen sulfide gas. This gas smells like rotten eggs. Hydrogen Peroxide can be used to get rid of the bacteria but it usually returns. Replacing the magnesium or aluminum anode rod with a zinc/aluminum alloy rod will often get rid of the problem.
Insufficient Hot Water or Hot Water Not Hot Enough
Check for a broken dip tube, wrong setting on a thermostat, a defective thermostat, burned out heating elements (electric), or a heavy build up of sediment. Check out our trouble shooting guides.
Dip Tube
The dip tube is a long slender tube that fits down into the water heater inlet. The dip tube directs the incoming cold water down to the bottom of the tank. If the dip tube is broken, the incoming cold water can mix with the out going hot water and cause it to seem as though the water heater is running out of hot water.
Noisy Water Heater
Noise coming from gas water heaters can often be caused by the sediment build up in the bottom of the water heater. As the burners heat the bottom of the tank gas bubbles form under the sediment. The thumping and popping noises are created by the gas bubbles escaping from under the sediment. Sizzling noises can be caused by condensation dripping onto the hot burner.
Thermal Expansion
When water is heated it expands. If the inlet to the water heater is not blocked by a check valve, pressure reducing valve, or other device, the increase in volume simply travels back into the water source. If the inlet is blocked, this increase in volume will cause an increase in pressure, sometimes to dangerous levels. The T&P valve should relieve this pressure by discharging some water. A thermal expansion tank can be installed in the water line that will absorb the increase in volume preventing the relief valve from discharging water unnecessarily.
Plumbing codes require that thermal expansion control be addressed in plumbing systems. A temperature and pressure relief valve is not considered a thermal expansion device. This is because when water is allowed to continuously drip from the T&P relief valve, minerals from the water can build up on the valve, eventually blocking it. This blockage can render the T&P valve useless and potentially lead to hot water heater explosions. The International Plumbing Code (IPC), Uniform Plumbing Code (UPC) and Standard Plumbing Code all require thermal expansion control to be addressed.
Water Hammer
The hammering and pounding noise and associated vibration that occurs when a column of moving water within a piping system is suddenly stopped by a valve is known as "water hammer". When water is traveling in the pipes it has kinetic energy (energy of motion). When a valve shuts off suddenly a shock wave results. Not only is an annoying noise created, but damage to the plumbing system can occur. Water hammer most often occurs when a valve shuts off suddenly as with solenoid valves. Commercial water hammer arrestors are available to combat this problem. They consist of a small air bladder within a cylinder plumbed to the piping system near the valve causing the problem. They cushion the moving column of water. Some hardware stores carry them. Sometimes if the water piping is sagging then supporting the pipe solves the problem.
Condensation
Condensation often occurs on water heaters when a large hot water draw occurs and thus a large amount of cold water enters the water heater. This condensation is sometimes mistaken for a leak in the water heater storage tank.
Scalding Dangers
Homes that have small children, elderly and disabled persons may wish to lower the water heater setting to 120 degrees to prevent potential scalding. Below is a Table giving the approximate time it takes have a scalding accident for different temperatures of hot water.
Length of time for hot water to cause scalding
- 125 F 1 1/2 to 2 minutes
- 130 F About 30 seconds
- 135 F About 10 seconds
- 140 F Less than 5 seconds
- 145 F Less than 3 seconds
- 150 F About 1 1/2 seconds
- 155 F About 1 second
Table courtesy of Shriners Burn Institute
Earthquake Safety
A 50 gallon water heater holds about 400 pounds of water. In an earthquake it is quite possible for the water heater to fall over. Having a 400+ pound water heater fall on you, a pet, or even your property is not desirable to say the least.
If a gas type water heater falls over it could very easily damage the gas connection and create a dangerous gas leak.
Preventing the water heater from falling over will also eliminate the need for cleaning up a 50 gallon hot water spill and the expense of replacing the water heater should it be damaged in a fall.
Should the water heater fall over it could also damage the water piping and create a very large mess as well as wasting valuable water and contributing to a reduction in the water pressure available for fighting fires that can accompany earthquakes.
Water heater strap kits are available and sometimes even required by local building codes.
Milky Hot Water
Water contains dissolved oxygen and other gases. When water is heated it has less ability to hold these gases and when the pressure is lowered as the water comes out of the tap these gasses can form tiny bubbles giving the water a milky appearance. Letting the water stand for a few minutes will allow these bubbles to rise out of the water and the water resumes its clear appearance.
Water Heater Sizing
To determine the right size water heater for your home you must figure out the peak demand that will be required from your water heater. The table below lists typical amounts of water for various uses. Decide what activities will occur simultaneously and choose a water heater that can handle the required load.
Hot Water Consumption Table
Activity Gallons per use
- Shower or Bath 10 to 20 gallons
- Shave 1 to 2 gallons
- Hand/Face Wash 1 to 4 gallons
- Dish washing (hand) 2 to 4 gallons
- Dish washing (automatic) 12 to 14 gallons
- Food Preparation 3 to 5 gallons
- Clothes Washing 10 to 32 gallons
The hot water supplied by a storage type water heater will begin dropping in temperature before the total water in the heater is consumed due to the mixing of incoming cold water.
Example:
For a family of four the following activities may occur:
- 8:00 a.m. Adult takes shower and shaves 18 gallons used
- 8:15 a.m. Adult takes shower 15 gallons used
- 8:30 Two children wash face/hands 3 gallons used
- 8:40 Breakfast food preparation 3 gallons used
Total hot water required for one hour: 39 gallons
To insure full temperature hot water choose a water heater with a first hour rating of 40 gallons or more.
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Did you know?
- Hot water heaters should be properly sized for your house.
- Extra thick insulation helps prevent radiant heat loss and saves energy and money.
- Water temperatures above 120° F can cause scalding especially on young children.
- Sediment build-up can cause premature tank failure, electric element failure, and excess costs on fuel bills.
- Hot water heaters require yearly maintenance to ensure proper operation.
DO
- Set hot water heater temperature below 120° F.
- Flush hot water heater through drain valve at least once a year to remove sediment build-up.
- Test safety relief valve once a year to ensure proper operation (this device protects the heater from over-pressurizing and from exceeding 210° F.)
- Keep area around hot water heater, clean and free of combustible and flammable material.
- When leaving for vacation, set hot water heater temperature at its lowest setting. This will save money and reduce the risk of any problems while you are away.
- Follow the manufacturer’s guidelines for recommended maintenance schedule.
DON’T
Set hot water heater temperature above 120° F.
Keep combustible material or flammable material or flammable material within 18” of the hot water heater.
Never leave children unattended around a hot water heater
