Right now I'm at my wife's condo near Jomtien. The condo has two bathrooms with a 'mixer' shower in each.  Each shower mixer takes a cold water feed and a hot water feed. The water is heated by a MEX 'Cube 6000 multipoint water heater' like this.... https://www.homepro.co.th/m/p/1072870

One heater feeds one shower. So we have two of these heaters.  These are installed in all the condo rooms (its about 8 years old).

So the hot water heater outlet is not 'vented' - it goes to the shower mixer valve. There is no control valve on the inlet to the heater.

This morning I finished my shower and shut off the mixer.  It seems that the heating element was still being supplied with power because I could hear 'rumbling' in the water heater and then a bang as the inlet water hose blew off its fitting and some hot water and steam escaped.  The ELCB and circuit breaker then tripped.

Examining the unit, it seems the element is now short circuit.

There was actually a brand new spare unit which was (inadvertantly I think) left in the condo at construction. I've simply fitted that and all working again.

I'm puzzled how this failure arose since there is both a flow sensor and a thermostatic cut out switch in the heater. Yet power seems to have continued to be supplied to the element after I closed the shower valve and water was boiling in the heating vessel.  Possibly the flow sensor, triac or PCB failed and kept power applied to the element.

There seems to be no pressure release valve on the heating vessel. Any excess pressure can only go back into the cold water feed.

Bit puzzled by all this...  Doesn't seem safe.

I can read Thai and there seem to be two kinds of water heater - 'warm water heater' - which needs to be vented (no valve on outlet) and 'hot water heater' - which can feed into a mixer valve. But how do the designs differ?

 

That is the difference between normal wall point of use water heater as shown on left and the multi point heater you use (which with the normal high temp of source water in Thailand have never found to be a good option - almost always hard to get and keep good shower temp using mixer valves but guess looks better for those from cold countries used to having hot/cold water).

 

Suspect you are right that flow control failed.  

 

To properly diagnose, some photos of your wife in the shower would help. 

52 minutes ago, lopburi3 said:

That is the difference between normal wall point of use water heater as shown on left and the multi point heater you use (which with the normal high temp of source water in Thailand have never found to be a good option - almost always hard to get and keep good shower temp using mixer valves but guess looks better for those from cold countries used to having hot/cold water).

 

Suspect you are right that flow control failed.  

 

It actually works pretty well with the shower mixer. Its a 6kW unit and always plenty of hot water at a high flow rate and stable temp.  However I am concerned at lack of safety, e.g. no PRV . The braided inlet hose let go with a bang, the ferrule shot across the room and  steam and about a cupful of very hot water came out.....

7 minutes ago, JeffersLos said:

To properly diagnose, some photos of your wife in the shower would help. 

Wife away right now. I'm in the condo alone (most of the time..). But I can send you some photos of me in the shower (it'll cost you though...).

36 minutes ago, HauptmannUK said:

The braided inlet hose let go with a bang, the ferrule shot across the room and  steam and about a cupful of very hot water came out.....

It sounds like a simple hose failure.

There are hoses that are rated for hot water use... but they do get hard and inflexible over time and need replacing.

2 hours ago, HauptmannUK said:

Wife away right now. I'm in the condo alone (most of the time..). But I can send you some photos of me in the shower (it'll cost you though...).

I,m free to pop round and soap your back while the missus is away..........

1 hour ago, Encid said:

It sounds like a simple hose failure.

There are hoses that are rated for hot water use... but they do get hard and inflexible over time and need replacing.

No not a hose failure. It was the cold inlet hose that blew off. The outlet was closed off (shower valve closed), so flow stopped, but I could hear water boiling in the heater. Then a loud bang as the hose blew off its fitting and steam shooting out. Element continued to receive power and fused, knocking out the ELCB and also main circuit breaker.