explosion waiting to happen

[ccha]

who knows what their domestic hot water (dhw) pressure is as they turn off a tap?

i have found out to my cost that it will exceed 10 bar
the guage needle was bent!

who has a gauge on dhw?
turn off and pressure rises dramaticallly.

only a blowout twice on a flexi alerted to this
all the rest was in copper but it must be a real problem due to expansion in dhw with no where to go

 

[Corgigrouch]

Do you have a water meter fitted?

 

[doitall]

Don't believe a word of it, unless its a home made job.

More information, about where the DHW is stored to get to that pressure, a couple of pics would be good as well.

 

[Corgigrouch]

Only time that I can think that it might happen (Discounting a headder tank fitted at the top of a very tall building) may be snapping the tap shut on a combi where ther is a water meter fitted and no expansion on the cold supply... Having said that, if a flexi fails at 10 bar then you need to buy better quality flexis and use spanners to tighten them up

 

[namsag]

Not by anychance a gas water heater or combi heating your HW and staying on after tap has been closed

 

[fatplumber]

Is he talking about peak transience on shut off? The kinetic effect may just give this result.

 

[ccha]

no water meter
no expansion
direct from combi to taps
Exceeding 10 bar(gauge max) to blow flexi
prv on intake 3 to 4 bar
diverter valve fault? as pressure showing at prv over 10 bar when tap first turned off

am I corrrect in thinking the diverter valve should lose this pressure by passing water to CH system

anyone tried a gauge on theirs ?

 

[namsag]

Have combi set for just hot water.
Get someone to turn tap off as you obseve burner on combi it should go off within a second of tap going off if its staying on that is more than likely your problem water is still being heated with no where to go so pressure builds up

 

[bengasman]

Got to be an installation fault; never heard of a mains of more than about 6.

 

[ccha]

boiler shuts down immediately the tap is closed
mains is about 7 bar (cold)but as i said only 3 to 4 bar infeed to boiler through prv (adjustable)
transient pressure sounds plausable
how do other systems lose their latent heat and therefore pressure to prevent this?

any input welcome !

anyone got a pressure on dhw when first turned off ?

 

[gigz]

Put a mini exp vessel on hot pipework, never had to do it but it should work. Or even a PRV ?

 

[Master of None]

I would imagine any redundant heat would heat the dhw in the heat exchanger and be forced back down the mcw incomer (unless there is a dcv on it). I only read this forum I am no expert......

 

[ccha]

I also thought that
ie it would all balance at mains pressure if i fully opened the prv.
no check valve. except on filler loop to CH which is on the boiler side of the prv

tonight replaced the diverter valve diaphragm but although the pressure built up slower it still went over 10 bar.
also observed that there is no direct connnection allowing excess pressure from dhw to be 'dumped' to CH.

latest is that i have now set the prv down to about 2 bar.
the CH filler loop is open and the CH pressure is stable at 1.6 bar
I can now run dhw and when turned off it loses pressure into CH by back pressure flow through fill loop.
with little or no effect on CH pressure
so all balanced at 1.6 bar and the CH safety at 3bar as a backup

my next move will be expansion vessel and then a safety on dhw

any more thoughts?

 

[swbjackson]

Got to be an installation fault; never heard of a mains of more than about 6.

We've got areas around here with up to 10. My incoming is between 6 and 8 dynamic depending on time of day.

 

[ccha]

now have 8litre expansion connected
turn off tap now and burner stays on until pressure at 2 bar then shuts down when the flow into the exp vessel slows
expansion continues and pressure goes to about 2.7

sorted methinks

but will still fit a safety soon at 3 bar