House is wrecked by blast

[electronicsuk]

Just saw this on the local news and it caught my interest. The details seem to be a little vague:

Our investigation leads us to believe the explosion was caused by pressure in the central heating tank which had built up behind frozen pipes.

That doesn't make a lot of sense to me. From what I saw on TV, two or three walls of the house had been well and truly blown out. Something tells me that there must have been a pre-existing fault here that allowed this to happen. Entertain me for a moment, if you will...

It seems fair to assume that the tank being referred to was a hot water cylinder. If it was vented, then it's possible the feed and vent pipes could have frozen, and similarly for an unvented system the connections to the expansion vessel and PRV discharge pipe could have become blocked with ice.

In either case the water in the cylinder would be prevented from expanding, however, it should never have been able to rise above the temperature set by the thermostat and, if fitted depending on type of cylinder and heat source, the overheat cutout. The extent of damage that appears to have been caused to the house looks more akin to an explosion of superheated water, rapidly expanding to hundreds of times its original volume in a very short space of time. Again, I fail to see how this could have happened with a limited water temperature - I would suspect that the tank would simply rupture and leak in a far less explosive manner.

This is purely for my own interest and education, so all comments and theories welcome.

 

[Hugh Jaleak]

The City and Guilds Plumbing Craft syllabus covers boiler explosions. It can (and as seen does!) happen, but in order to do so several things have to happen.

The cold feed and vent to the system freeze, thus effectively making a vented system a sealed system.

Then, should the boiler stat fail, the boiler continues to heat the water. With no means of expansion, (vent and cold feed are blocked with plugs of ice), then this water boils and instantaneously turns to steam, expanding by IIRC about 1600x its volume.

At this point something has to give to release this pressure, often the boiler, exploding like a bomb......

 

[Garfeild]

Cue video of exploding cylinder.

 

[electronicsuk]

Then, should the boiler stat fail, the boiler continues to heat the water. With no means of expansion, (vent and cold feed are blocked with plugs of ice), then this water boils and instantaneously turns to steam, expanding by IIRC about 1600x its volume.

Thanks - so that more or less confirms it. There has to be another point of failure besides the buildup of ice, be it a faulty boiler stat or a faulty immersion stat with no safety cutout fitted. Thanks.

 

[Onetap]

In either case the water in the cylinder would be prevented from expanding, however, it should never have been able to rise above the temperature set by the thermostat and, if fitted depending on type of cylinder and heat source, the overheat cutout. The extent of damage that appears to have been caused to the house looks more akin to an explosion of superheated water, rapidly expanding to hundreds of times its original volume in a very short space of time. Again, I fail to see how this could have happened with a limited water temperature - I would suspect that the tank would simply rupture and leak in a far less explosive manner.

It was most likely one of two things, both involving an uncontrolled heat source.

1) a run-away immersion heater with an old-type thermostat. Thermostats often fail on with the contacts arc-welding themselves together. There were 2 deaths by scalding in this country recently involving this and new thermostats now must have an additional high-temperature cut-out.

2) A solid fuel or wood burner. A former work colleague told me he'd seen a house virtually demolished by such an explosion, I think in the winter of 1947, when the cold feed and open vent pipes had frozen.

 

[Hugh Jaleak]

It would be interesting to know a few more details, possibly an unvented cylinder. (That'll give Drivel something to drivel about.... ) With a property of that size and age it may have been fitted with an unvented cylinder, but they have various safety devices fitted and the likelihood of all failing should be remote.

The house, it stated in the report was unoccupied, good job it was or there could easily have been fatalities. I imagine some heat source has been left on to prevent freezing, but the extreme temperatures had still caused something to freeze with catastrophic results.

I seem to recall an incident near me some years ago when a back boiler exploded due to frozen pipes. It blew the chimney to bits, and destroyed the living room wall. The older cast iron exchangers in boilers would shatter with immense force I think, although i'm not too sure if the new lightweight exchangers would behave in a similar manner. Something I will leave for the lab guys I think!