Perils of pressurised systems?

[David WE Roberts]

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.

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.

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. 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.

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.

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. This would explain the warning sticker from the person who serviced it late last year. If there was an explosion it would probably be due to heating a depressurised system and generating steam in a sealed system. 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. 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? Should a green label go over the red label? 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. 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. 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.

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

Cheers

Dave R

[The Natural Philosopher]

not necessarily as you have seen later

all sealed systems SHOULD have pressure relief safety valves. Not necessarily part of the boiler at all.

shrug.

frankly its really overkill: ALL you have to do is make sure the pressure relief system works

You have to have a stuck safety valve AND a stuck thermostat before the tank will go up.

*shrug* caveat emptor. First thing you do when gettigh a n old house is have it looked over.

[John Rumm]

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.

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

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!

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)

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.

Yup.

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

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

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.

Fixing the reason it was labelled as such!

Taking the old label off would seem more elegant.

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.

They are usually labelled quite conspicuously.

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.

These things being relative, you probably were ;-)

[David WE Roberts]

Thanks as usual, John.

To clarify just this little bit, I was talking about the general explosion risk if the boiler had not been modified (which as you say should be quite low) as opposed to the minor gas explosion that they had which should be another issue entirely (although worrying).

[John Rumm]

Careful not to conflate things though... it can get a bit confusing when talking about sealed / unvented / pressurised primary systems and sealed / unvented / pressurised hot water systems. The two are unrelated.

Using a boiler on a sealed primary that has not been appropriately modified or augmented, is a risk, and it may only take something simple like a failed pump to cause that risk to manifest and break something in a not good way.

This is a separate issue from the inherent risks of an unvented hot water system. Those risks are very low when the heating of it is indirectly via a CH boiler.

A bit depends on exactly what and how bad... for example, years ago we had a Main multipoint water heater that would after a while suffer percussive ignition - so rather than lighting the main burner quietly it would start with a noticeable thump. This was usually an indication that the pilot light nozzle had accumulated a little soot, and was not igniting as fast as it should (it was a two stage ignition, spark to light pilot, thence main burner). Jiggling the burner jests aside and removing the pilot burner, poking some fine fuse wire through it would clear any obstruction and it would be fine again for about 18 months).