Expansion tank burst on outside tankless water heater system

[hughten]

Hi. Temps dropped to single digits last night in Austin, TX. Loss power @ 2AM. This morning around 11:30am, heard a loud explosion coming from outside area where the tankless water heater system is installed. Noticed that the expansion tank exploded. You can see ice inside the tank. I have been dripping hot and cold water from all of my faucets since Friday.

I have a few questions.
What caused it? Did losing power help cause it? Tankless system is heated by natural gas but system is electric. Did losing power and not turning off the gas cause the pressure to build? Just wondering if there's anything I did wrong so that I can NOT do that again next time.

What do I do now? I know that when the temps finally allow for a thaw (probably Friday or Saturday), water is going to rush out of the gaping crack in the expansion tank. As such, I shut off the water from the hot and cold pipes to the tankless system. My concern is will those pipes now freeze because it's not being circulated? What else should I be doing or not doing?

 

[Twowaxhack]

Since you lost power the tankless lost its freeze protection. Yes, it all will freeze.

The tankless is likely ruined at this point. I’m sorry.
Better get some heat out there or if it isn’t ruined it will be.....

You need to locate a cut off valve ahead of the expansion tank ( supply side ) to turn the water off.

Personally I think an expansion tank in a system with only a tankless is silly anyway.
No significant expansion can take place with water flowing and the tankless doesn’t heat unless waters flowing.
The heater has sensors and if that fails it has a pressure relief valve.

 

[Geofd]

I'm guessing the single digits caused it if the temps get that low why is it outside???????

 

[Twowaxhack]

I'm guessing the single digits caused it if the temps get that low why is it outside???????

I’m assuming it usually doesn’t get this cold there.

In my opinion it shouldn’t have been installed there.

Losing power would doom the water heater in extended freezing temps. A malfunction in the freeze protection circuit would as well.

it was probably installed in that location for installation convenience.

 

[hughten]

Temps do not normally get his low in Austin Texas. This is a once in a 100 year storm. We will not get above 32 degress until Friday, at the earliest. It's been since 1890s that we've experienced something like this.

I did shut off the water supply to the tankless system. In the before pic, you can see the hot and cold water lines connecting to the system. They are covered in form. I turned off the water at the valves at those connections.

My question is should I shut off those water lines? Won't the water in those lines now freeze since it's not running through the system? Should I open up those lines to allow water to flow through the heater system. I have my inside faucets dripping so that would prevent freezing of those lines. But I guess the gaping crack in the expansion tank will also encourage the water in the system to freeze if I don't shut off those lines?

 

[Twowaxhack]

Do all of your water faucets have water flow ? Hot and cold ?

 

[hughten]

Do all of your water faucets have water flow ? Hot and cold ?

Cold. Not hot because the tankless system is without power. But water is coming from hot and cold taps from all inside faucets. However, the "hot" water has mush less pressure/flow.

 

[Jeff Handy]

This “once per century” cold snap might happen multiple times now, every year, it is called climate change for a reason.
Move all the piping and new heater inside the house, to conditioned space.

 

[Twowaxhack]

Cold. Not hot because the tankless system is without power. But water is coming from hot and cold taps from all inside faucets. However, the "hot" water has mush less pressure/flow.

You better keep it flowing......

If the tank is the only thing broke just remove and plug the female adapter. 3/4 mip pipe plug.

 

[CT18]

That expansion tank should have had a service valve to isolate it for removal. I think if you close the cold supply valve at the tankless and open both service ports and remove the caps this may allow the water inside to drain out. Someone else that is more familiar with tankless should chime in to verify.

 

[Hamberg]

Relatives down South and all of their systems are in unconditioned space - most just out in the open (HAHAHA). Guessing @CT18, @Jeff Handy, @Geofd don't get to see much of that where they are located

 

[Jeff Handy]

You’re right, too cold around Chicago to leave water pipes above ground, and water heaters outside.
But I always thought it was dumb, risky, and strange, when I saw plumbing and water heaters and wells tanks etc left exposed to the elements, in locations that supposedly never would freeze.
Weather doesn’t always follow the rules.

 

[Twowaxhack]

There are ways to protect exposed plumbing but it’s more difficult when the powers out. Backup generator is a good idea.

 

[breplum]

OP, you seemingly should not need the exp. tank, so abandon and plug.
Real sorry for y'all going through this.
The climate shift and uncertainties bring up a whole new set of crossroad issues.
I doubt there is a frost proof hose bibb in all of Texas. So everyone around should have opened those external faucets to let them drip. But there are so many w/o power nor knowledge, that more bursting will happen.
Plenty of plumbing in exterior walls facing disaster.
Plumbers will be busy for weeks.

 

[arctic bill]

If i was you i would order a replacement now , In about one week everyone will be looking for the same.