You don’t hear about it happening very much (thankfully!) but tank-based water heaters and boilers can (and do) explode, often with catastrophic results. Water heaters and boilers are designed to be safe, and are installed in literally millions of homes in the United States. Water heaters and boilers include failsafe devices to prevent the build–up of dangerous pressure levels inside the devices. Occasionally however, the failsafe devices fail.
This slow-motion video, compliments of the Mythbusters television show on the Discovery Channel, demonstrates (safely) what happens when a water heater pressure and temperature valve fails. Although the demonstration involves a hot water tank, the same thing can happen with a boiler. And in this case, size matters! The larger the device is, the more damage it can do!
The failsafe for a water heater or boiler is a device called a pressure-and-temperature valve (PT valve). Some people call them T & P valves – they’re the same thing. The valve usually sits on or near the top of the water tank or boiler and is designed to open when a set pressure or temperature inside the tank is achieved. Some tanks are outfitted with a pressure valve, which opens only when a higher pressure is detected, regardless of the temperature in the tank.
Basic physics says that temperature and pressure in a closed system have a direct relationship. That is, when the temperature goes up, so does the pressure, and vice versa. Turning up the heat on your hot water tank or your boiler control will increase the pressure in the system.
These systems are designed to handle “modest” changes in temperature and pressure. Manually adjusting the temperature upward on your water heater or boiler should never produce catastrophic results. So what can go wrong? Usually a combination of things!
Regulators and thermostats are designed to keep the operating temperature of a device within a specified range. When the temperature drops below the specified range, the regulator turns on the heater to heat the water in the system. When the temperature rises to the top of the range, the regulator turns off the heat. If the regulator or thermostat in the system goes bad, one of two things can happen: no heat at all, or heat all the time!
If the regulator or thermostat gets stuck “on” and heats the system constantly, the T & P valve or pressure valve is supposed to open and relieve pressure build up in the tank. It’s supposed to prevent the tank from ever reaching the point where an explosion is imminent.
Here’s the bad news. T & P valves and pressure valves can fail. Unlike the regulator or thermostat– whose erratic behavior will indicate a failure – there’s no way to know that a PT valve is going bad or has failed without testing it. Fortunately, testing is simple and homeowners should test the PT valves on their water tank(s) every month or two.
The valve (shown in the picture accompanying this post) can be lifted or flipped into the “open” position by hand. If the valve opens and closes smoothly, it’s still doing its job. If it doesn’t open, or opens and closes only with difficulty, it should be replaced. As a rule, PT valves on boilers and water heaters should be replaced every three years, whether they’re working or not. A PT valve is an inexpensive item, and the peace-of-mind is worth every penny.
Don’t underestimate the amount of damage a defective water heater can do to your home. Depending upon its size, a water heater or boiler can build up more than 100,000 pounds of pressure before it explodes. An exploding tank may lift off its base at a velocity of 350 MPH, and will easily shatter the foundation, floors and roof of a home. Unimpeded, a fully pressurized 50-gallon water tank can achieve an altitude of more than 500 feet!
Pressure aside, a hot water tank is also carrying 40 or 50 gallons of potentially scalding hot water, which will be distributed over the tank’s exit path. And then there’s gravity – what goes up must come down! An empty water tank can weigh between 75 and 200 pounds, and when it returns, it may come back down in one piece or it may split into multiple pieces.
Boston Standard Plumbing & Heating can help you perform regular maintenance on your hot water tank and/or boiler, show you how to test your PT valve and provide valve replacement services as needed. Water heaters and boilers can be operated safely in a home, but these devices do require regular professional inspections and maintenance.
Give us a call at (617) 288-2911 anytime to schedule an appointment. Call us if you experience any operating problems with your water heater or boiler – including over- and under-temperature conditions, and pilot light or electronic ignition problems.
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