The expansion vessel on an old Ideal Isar heating boiler is on its last legs.
Replac "If preferred, and for convenience, a new expansion vessel may be installed elsewhere in the heating system, providing it ensures equivalent system protection"
Does this mean the expansion vessel can be attached elsewhere on the pipe runs than inside the boiler?
If so then are there special expansion vessels designed for this? The original has a strange shape and mounting it externally would need some sort of highly customised mounting.
Yup, it does not matter where in the system it is.
Yes. E.g:
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An external one does not need to fit in the back of a boiler - so they tend to be more conventional in shape. Typically they come with a metal band that goes round their middle, and that can fix it to a wall bracket.
Connect it up with a flexible hose (like a filling loop), then it is easy to hook on to the bracket rather than have to get that accurately located. Also, connect it with a "service valve" and a drain point on the pressure vessel side. Isolate and drain the wet side before you check the gas pressure, and top up. You shouldn't need to do that for five years. Don't forget to open the service valve afterwards.
Someone stayed with me and from some genuine OCD washing habit they turned the hot tap on and off repeatedly about 100 times ever few seconds. Maybe more.
The boiler stopped working and the next day I noticed water dripping out of it which I now realise must have been the expansion vessel rupturing or the value leaking.
Now the pressure reading when cold is zero. It rises to about
1.2 bar with the boiler temp set low. Any more and the boiler makes grumbling noises and the pressure reading darts around. I have no idea how it manages to work at all.
the expansion vessels consist of an enclosure with a wet side and a dry side - usually separated by a diaphragm, or perhaps containing a flexible bladder. The idea being that one side is connected to the wet bit of the primary heating system, and the other side is full of gas (typically air) at pressure.
It's designed to allow the wet side to increase its volume by compressing the gas side. So to work properly, their needs to be some gas pressure in it to balance the water pressure - otherwise the thing would just fill with water and you would have no expansion capacity.
So common failure modes are the gas side losing pressure, or a leak in the diaphragm allowing the gas to diffuse into the water.
However the use of the hot tap will not have any practical influence on the expansion vessel which is connected to the primary water circuit (i.e. the water that flows through the rads)
Sounds like it has low water volume - the grumbling noise is probably the (inadequate) amount of water in the primary heat exchanger at best "kettling" or at worst actually boiling. Keep using it like that and you will damage something expensive!
As a temporary workaround for a failed expansion vessel, you can bleed some water from a rad, so that it has an air pocket at the top. That will work as an expansion space.
That particular comment was about whether it is possible to use a boiler designed for open vented use (i.e. header tank, and no expansion vessel etc) on a system piped for unvented or closed system operation.
Combi boilers are more typically unvented / sealed system boilers, although those designed for vented operation do exist as well.
What else than the exapansion vessel could her "routine" have affected? Afterwards perhaps 100 ml of water was splashed around the internals of the boiler and dripped out the bottom.
In the past, the combi would fire up at night (presumably to heat its internal hot water reservoir) and make an almighty banging sound which I thought was coming from the neighbour. Turns out to have been from the boiler -- without any heating on. Are you saying this can't possibly be from the expansion vessel as it's on a separate CH water circuit?
How would that work? Why would CH water expand into the air inside a radiator in preference to the expansion vessel, which presumably has little gas pressure now?
The air space in the radiator contains a compressible medium, into whose space the water will expand. If the expansion vessel has no pressure, the internal diaphragm will be pushed tightly against the inside of the housing, and there will be little or no free (compressible) space.
You've now lost me. Was the OCD so severe she felt obliged to spray
100ml from the tap into the boiler?
Which leads to the obvious question, why do you think the EV has failed?
If designed for a close system, it implies a loss of water, and pressure from the primary. Most boilers have a pressure switch to turn off the boiler if this happens.
Yes, except you would need to empty the rad of water and made sure it fills with air.
I would suggest removing the boiler cover and find where this water is coming from. As I said earlier I have known boilers overheat from lack of water and then leak from damaged O-rings.
Plumber turns up. Sees problem is failed expansion vessel. Quietly lets some air into a radiator and charges £400 for the "repair". By the time it's done the damage, matey boy has long since changed name.
It was one reason why we were careful not to show customers the dirty quick fixes when we attended breakdowns ...
Well the hot water pathway is basically mains in, secondary heat exchanger, hot water out.
A combi like the ISAR has two heat exchangers (HX), the main or primary one is directly heated by the combustion of gas, and the wet side of it is connected to the primary heating circuit. That is usually the radiators.
There is also a secondary heat exchanger (usually what is called a "Plate Heat Exchanger" (PHE) that is used to heat the hot water. This is a water to water HX - so clean mains water is fed through one side, and the dirty primary heating water is fed through the other. So there is never any contact between the wholesome hot water in the house, and the primary heating circuit water that is potentially full of rust, and inhibitor, and sludge etc.
There is a flow switch that detects when the hot water tap is turned on, and that will do two things: cause the boiler to fire, and activate a diversion valve in the boiler. The valve will redirect the flow of water so that instead being pumped around the radiator circuit, it is instead pumped through the primary side of PHE, and its this that indirectly heats the incoming mains cold water passes through the secondary side of the PHE.
So the expansion vessel is not in any way connected to the DHW side of the boiler. Even if you cycle the hot tap, the boiler will not fire on an off at the same rate - it will typically need a period of demand to get running, and then will run for a few seconds after the demand goes.
The banging is usually what they call kettling - small cavitations or expanding bubbles in the water in the primary HX. This can occur if the main HX is not completely full of water, or if the flow rate through it is too slow, or sometimes if the primary side of the main HX is badly scaled up meaning the surface temperature has to get much higher to effect the same amount of heat transfer.
Now kettling will cause lots of shock to pass through the primary circuit, and the expansion vessel will see that. If its functioning normally, then you would expect it to absorb that shock without any problem. However if deflated, then the system will "feel" that percussion more acutely.
If you think it through from the basics, it hopefully makes sense...
(apologies if I am teaching granny to suck eggs here!)
Water will expand in volume as its heated. Now on old vented systems, this just means that some of it would push back into the header tank in the loft. However with a sealed system like yours, there is no header tank. So there needs to be another way of allowing that expansion to happen without blowing the pipe joints apart! So sealed systems basically include a bag of pressurised air inside an expansion chamber.
When you fill the system to the typical 1 - 1.5 bar, that bag of air gets squashed a bit, but there still remain a good few litres of space. As the system fires and warms up, the water expands, and the pressure will rise. This will compress the air in the bag more. So a hot system pressure will perhaps rise to 2 to 2.5 bar depending on the actual temperature and the total volume of water in the system, and that will cause a matching reduction in volume of the bag of air.
If however that bag of air is not there, then the water will still expand, but now its "pushing" against rigid pipework and hence the pressure rise will be much higher. To prevent something being damaged the boiler also includes an emergency pressure release valve (PRV), that will allow some water to be expelled from the primary heating circuit to a pipe that discharges outside. The PRV will normally be set to "let go" at 3 to 4 bar.
If the PRV does activate, some of the system water is let out, dropping the pressure back to a safe level. However once the system cools, that volume will shrink again, and the pressure will now be lower than the normal resting pressure that was set by the installer. In some cases this may be so low that the boiler detects an low pressure error and locks out. (there is also the risk that once the PRV has operated, it can get dirt into it, that prevents it sealing again properly in the future).
So, to your question about expansion into rads etc:
The rise in pressure due to heating will be "felt" everywhere in the primary system, and hence all the things connected to it. That includes the expansion vessel, but also the rads, the pipework, and the boiler itself. So the extra water volume *will* find a place to go. Normally it would be the expansion vessel, but if that can't take it, then it will go elsewhere - say causing the PRV to operate and release some. If however there is a pocket of air in a rad, then that will be compressed, instead, and that will prevent the pressure rise being large enough to operate the PRV.
If the expansion vessel is working normally, *and* you have a pocket of air in a rad, then they will both share the expansion.
The bubble in the rad solution is not however a good long term fix, since there is no separation between the air and the primary water - so over time it will diffuse into the water, and that extra oxygen will cause more corrosion.
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