On 06/04/2012 19:06, David WE Roberts wrote:
Just been round to our (non-technical) friends.

They have recently ( 6 months or so) bought a house which has an
interesting heating system.
The house had a loft conversion just before they bought it, which may
have resulted in a traditional gravity system being removed to clear the
loft space.

Quite possibly...

It would also make a conventional gravity fed heating system difficult
since there may be nowhere to put the header tank at an height
adequately above the highest rads.

As far as I can tell they now have a spanky pressurised system with new
hot water store, pump, expansion vessels etc.
However the thermostat, programmer and boiler have not been replaced.

No reason why they would need to be necessarily...

Now, they have been having problems, including a minor explosion which
blew the cover off the boiler so it dropped and partially blocked the
flue outlet.

Can't really visualise how that would happen... I suspect there is more
to it. Gas appliances "exploding" and partly falling apart sound like
they are due for urgent replacement!

They have had a plumber in who has fixed the boiler casing and said they
were very lucky not to be poisoned by the flue gasses.
The plumber also re-pressurised the central heating as it was not
heating the top floor.
Oh, and he told them that the heating wiring was all wrong and needed
sorting.
They are now also having another firm in to give a second opinion.

More interesting stuff - the boiler has a Warning label saying roughly
that the boiler is not suitable for use with a pressurised system
because there is no interlock.
Do not use, danger of explosion!
However a plumber has told them that it is now safe to use.

Depending on the boiler, it may be possible to convert to sealed system
operation. However to do so safely requires an over-temperature cutout
in the boiler to lock it out should it end up heating inadequate water
(or flow)

(you would also need an expansion vessel, filling loop, and pressure
gauge etc)

So they are very confused people (not surprisingly).
This leaves me with a number of questions/comments.

Firstly, the gas mini-explosion should have nothing to do with
pressurised or non-pressurised usage but is a strong indication that the
boiler should be viewed with deep suspicion and probably replaced.

Indeed. Older boilers would typically be permanent pilot light types.
Its hard to think of a mechanism for these having any form of detonation
- although an undersize pilot light may cause it. Some of the more
recent ones with spark ignition, may suffer from percussive ignition if
the spark does not light the main burner as quickly as expected.

Secondly, I am assuming that the boiler (an old Ideal IIRC) was
originally used with a gravity system but possibly not designed for use
with a pressurised system because it didn't have an interlock to prevent
it running if the system lost pressure.

Yup.

This would explain the warning sticker from the person who serviced it
late last year.

Depending on the model, there are official ways of converting to sealed
operation for some.

If there was an explosion it would probably be due to heating a
depressurised system and generating steam in a sealed system.

Well that is a different situation from a gas explosion... or late
ignition.

I assume such a boiler could have a pressure sensitive switch wired into
the control circuit to provide an interlock, assuming the boiler was
rated to work at mains pressure.

They don't usually run at mains pressure, but typically between 1 and 3
to 3.5 bar. They may have a low pressure sensor, they will need an
over-temperature one, and will need an overpressure relief valve to
discharge water safely outside if the system pressure gets excessive.

This might cure the fault described on the label.

So if a boiler is labelled up as dangerous - do not use, then what is
the procedure for bringing it back into service after a repair?

Fixing the reason it was labelled as such!

Should a green label go over the red label?

Taking the old label off would seem more elegant.

A half hearted attempt has been made to remove the warning label but
only two corners have been removed.

This brings me to another issue.
The system is a pressurised system.
Now you can choose how often you have your gas boiler serviced but AIUI
you should have a pressurised hot water system safety checked every year.

Don't confuse an unvented water system, with a sealed primary heating
system - two totally different things.

Most new boilers use sealed primary systems (although you can still get
the vented sort). These use the above mentioned components to ensure
safe operation and don't require a header tank. A boiler with a sealed
system could be used to heat a conventional gravity fed hot water system
with a cold cistern in the loft.

An unvented hot water system, maintains the entire hot water cylinder
at mains pressure (or a substantial fraction of it), and does not
require the cold cistern (its supplied form the mains cold feed). It can
be heated by a sealed system boiler or for that matter a vented system one.

Because I knew this, I knew to look for a label on the hot water tank
which said this.
However, unless you are a compulsive label reader, when you move into a
house with a pressurised hot water system if you haven't had one before
you are quite likely not to realise that there is a requirement for
annual inspection and maintenance.

They are usually labelled quite conspicuously.

Our friends had no idea that this was a requirement because they've
never had a PHW system before, just gravity then combi.
So new owners are very dependant on the previous owners to warn them
about the servicing requirements.
Seems like a safety loophole to me.

Yes and no... in reality seriously bad stuff like the house blowing up
due to a steam explosion is highly unlikely to happen even with a
unvented hot water system that never gets serviced.

The main risk would be if you had a simultaneous failure of its over
temperature, and over pressure valves that caused them to seize closed.
And it was electrically heated, and both the stat and the
overtemperature protection on the immersion heater also failed, and it
was left switched on indefinitely. Then it could go bang in the way that
dribble used to prattle on about.

With an indirectly heated unvented cylinder, it can't ever get hotter
than the maximum boiler flow temperature. So the possibility of steam
explosions are eliminated.

Having just been trawling through the Wiki on a related query, I could
pretend to be an expert ;-)

These things being relative, you probably were ;-)

--
Cheers,

John.

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