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Episode 89: Exploding Water Heater, Jeans Myths

  • Exploding Water Heater: a water heater can rocket through the roof of your house: confirmed
  • Jeans Myths
    • Pants on Fire: Your jeans can catch on fire while being dragged by a horse: busted
    • Jean Death Shrinkage: Wearing jeans in a bathtub can cause death: busted

Exploding Water Heater

Myth: A water heater can explode like a rocket through the roof of your house and land 500 ft away

Water heaters have two protection mechanisms to prevent them from rocketing off:

  • Thermostat in an emergency cutoff switch
  • Temperature and pressure valve (T&P valve)

The T&P valve is a pipe sticking out near the bottom of the water heater. Adam hypothesized that a homeowner might see the T&P valve and think that it was an old pipe that was no longer being used. Seeing that it occasionally leaks water, this hypothetical homeowner might cap it off, thus disabling it.

6 gallon tank test 1

The MythBusters first tested how the tank responded to the failure of its safety mechanisms. They setup a six gallon water tank in a container outside their headquarters. They removed the thermostat in the emergency cutoff switch, rendering it useless. At 165 degrees, the T&P valve started to release, preventing explosion. They next disabled the T&P valve and rigged up their own emergency release valve. At 248 degrees, the tank started to rapidly lose pressure and they had to end the test.

So much steam was released from their own emergency valve that they decided to take their setup to Alameda for additional safety. The tank failed after 20 minutes at 370F/191 psi. Rather than launch like a rocket, the top split enough to allow steam vapor to spread across the concrete.

30 gallon tank

Satisfied that they could get a tank to rupture with the safety mechanisms disabled, they moved up to a larger tank. They filled a 30 gallon tank 80% full with 24 gallons of water to see if they could get a good mixture of steam and water. After 50 minutes (350 psi), the tank launched like a rocket -- it took several seconds for the tank to land and there was tank lining scattered everywhere.

52 gallon 'Home' test

Their final test was to confirm the conditions of the myth. They built a tiny home with roof to California code. The house included a door, a window, and simulaid dummy in a bathtub. At 350psi, the water heater shot rocketed through the roof as the rest of the house exploded into little bits. The tank went through two layers of 2x4 trusses in the roof while the steam and water ruptured the rest of the structure.

The MythBusters' researchers found two confirmed cases of this happening in real life. In 2001, a burrito shop's water heater launched 439 ft into the air. In Minnesota, a water heater went through all the floors and roof of the house and another 150 ft into the air. The house was knocked off its foundation, two were injured, and a dog was killed.

Adam's lesson to the viewers is:

When they see that pipe sticking out of their water heater that doesn't go anywhere that's dripping water, that they don't go, "oh, maybe I should cap that."


Jeans Myths

See also: Exploding Trousers

Pants on Fire

Myth: A cowboy falls off a horse and gets dragged. The friction causes the jeans to catch on fire

John Growney, Legendary Stockman, explained to the build team that a common way for a cowboy to get dragged was for a foot to get caught in a stirrup, or in old days get caught on the rope. Growney has never had his jeans catch on fire, at least not literally.

Denim's ignition temperature is about 215F, so the MythBusters decided to do some real-life dragging tests and measure the temperature of the jeans afterwards. Tory was the human guinea pig for this experiment:

"In the shop they're like, 'oh you're going to be dragged behind a horse,' and I'm like, 'Yeah, that sounds fun.' But then you get out here and go, 'God, this seems like a really stupid idea,' and I'm starting that feeling more than, 'that sounds like it's going to be fun.'" -- Tory

An infrared surface thermometer measured the temperature of Tory's jeans before and after being dragged behind a horse on a 25' lead through dirt and dung:

  • Before: 103.5F
  • After 20 mph: 116.5F
  • After 30 mph run: 145.5F and much rippage

While warm, there was no smoke nor fire.

Belt Sander test

Unable to get fire with a real-life test, they stuck a slab of pork in a pair of jeans and lowered it on to a belt sander. They actually did worse than the real-life test. The denim reached 138.5F and shredded quite readily.

Frustrated at the lack of flames, Tory put a torch to the pants and finally got results.


Jean Death Shrinkage

Myth: A tight pair of jeans killed a woman after she sat in a warm bath for 6 hours to try and get a perfect fit. The combination of swelling legs and constricting jeans caused the fatality.

Dr. Edward Kersh, Chief of Cardiology at St Luke's Hospital, confirmed that the mythical scenario could cause a fatality. The pressure could result in a leg clot (deep vein thrombosis [DVT]), leading to a fatal pulmonary embolism. The MythBusters' researchers found a 2006 case in New Zealand that was similar to their myth: after soaking in a bathtub, she dried off in the sun. She had to be rushed to the hospital and was in danger of losing her legs from compartment syndrome.

Mock leg test

The build team decided to test first by measuring how much blood flow was restricted in a fake pair of legs. Grant's legs were used as a model to cast a set of permagel legs with mock blood vessels 2" below the surface. Pig's blood circulated through the vessels at a rate calibrated to a human's: 1L/minute.

Kari: "It's kinda a neat little blood fountain"

After 6 hours, there was no decrease in flow in their test legs.

Grant test

They decided to use Grant for human guinea pig testing: Kari helped Grant pick out shrinkable jeans at Wasteland in SF. They setup a small pool in a hospital room where Grant's health could be monitored during the test. Medical technician Keith Atkinson monitored the status of Grant's legs using a doppler machine. After 6 hours, there was no sign of constriction in Grant's blood flow.

While they know that compartment syndrome and deep vein thrombosis (DVT) are real, they couldn't show any sign of constriction with either their fake leg's or Grant's. Perhaps with many more test subjects, they could show different, but for now they ruled the test: