What happened here?

[Bschooly]

See video (link below). This is a Bosch gas hot water heater in our home in Mexico. One evening my wife heard an explosion. Upon investigation she found a busted water line and gas flames emitting from a tankless hot water heater! The local plumber says the unit failed first then heated the water line to the point that it exploded. My initial thought was the water pressure was too high (there is a pressure pump) and the line blew thus causing the damage to the hot water heater resulting in the flames.

The fire was put out easily and strangely there was no gas smell thereafter. The unit as well as three others in the house were replaced. I don't trust them. They are unknown age. But my guess is 10 or so years in age. My guess is they never saw service either.

Ideas on what happened?

WhatsApp Video 2026-02-20 at 9.01.32 AM.mp4

drive.google.com

 

[JohnCT]

See video (link below). This is a Bosch gas hot water heater in our home in Mexico. One evening my wife heard an explosion. Upon investigation she found a busted water line and gas flames emitting from a tankless hot water heater! The local plumber says the unit failed first then heated the water line to the point that it exploded. My initial thought was the water pressure was too high (there is a pressure pump) and the line blew thus causing the damage to the hot water heater resulting in the flames.

The fire was put out easily and strangely there was no gas smell thereafter. The unit as well as three others in the house were replaced. I don't trust them. They are unknown age. But my guess is 10 or so years in age. My guess is they never saw service either.

Ideas on what happened?

WhatsApp Video 2026-02-20 at 9.01.32 AM.mp4

drive.google.com

The link doesn't work but was there a PRV installed on the failed heater?

John

 

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[Fitter30]

Not seeing the video. A single passage in heater was full of minerals. Water from both directions filled with water soaking the minerals. Burner flashes water to steam. Steam expands 1600 times blows a hole in the heat exchanger. Waters spraying everywhere and burner stays on and not having a correct path burner flames are going everywhere. Bursting pressure tube pressure has to be 300 lbs.

 

[Bschooly]

The link doesn't work but was there a PRV installed on the failed heater?

John

OK try the link now. What's a PRV?

 

[Bschooly]

pressure relief valve? I don't know but I doubt it

 

[Reach4]

pressure relief valve? I don't know but I doubt it

I am pretty sure that a pressure+temperature relief valve is required, even on a tankless WH. I have not worked with a tankless.

T&P relief valves can fail, and should be tested periodically. Tested that the lever opens the valve and drains water. Tested that the valve closes when you release the lever.

 

[JohnCT]

I am pretty sure that a pressure+temperature relief valve is required, even on a tankless WH.

I spent a bit of time looking that up, and the information is conflicting to say the least - some say a pressure relief (but not temp) is required on a tankless and some say no relief is required. My guess it comes down to local code.

John

 

[Bschooly]

Not seeing the video. A single passage in heater was full of minerals. Water from both directions filled with water soaking the minerals. Burner flashes water to steam. Steam expands 1600 times blows a hole in the heat exchanger. Waters spraying everywhere and burner stays on and not having a correct path burner flames are going everywhere. Bursting pressure tube pressure has to be 300 lbs.

video should work now

 

[Fitter30]

Lucky your wife was home. Have you contacted Bosch? Still have the heater? Explosions sound more like gas first. Still have the heater Bosch might want to see it.

 

[Bschooly]

Well, I've dealt with Bosch before. Sort of a nightmare. Their support is very regimented and not very helpful. There's that and then there is the fact that this occurred in Mexico.

I've asked the contractor to open up the unit and see if he can see anything that might tell us what happened.