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Ideal iStor boilers....

The Ideal iStor was designed and introduced as a competitor to the Potterton PowerMax.

The iStor is an appliance using the same simple concept of a boiler and hot water cylinder fitted into a single case, for easier installation and reduced risk of theft from site and appeals in particular to builders and developers so is most commonly encounters in recently-built flats and houses.

The boiler inside appears to have a lot in common with the Ideal Icos, and probably uses the same heat exchanger and pre-mix burner fan. It is a condensing boiler, and the hot water storage is a low-capacity, high recovery-rate mains pressure unvented cylinder.

Unfortunately despite the good basic design concepts this boiler is turning out to be a troublesome appliance. As with cars, an apparently good design can turn out to have multiple flaws once sold in large volumes into public use and in the iStor we have a good example. My heart sinks when someone calls me to repair their boiler and tells me that have an iStor, but a growing body of experience with them (and a good stock of iStor spars in the van) makes them easier to deal with. Repairs are nearly always time-consuming and therefore expensive though.

Common fault reports from iStor users:

1) iStor has stopped working. Fault code H9-L9 showing on display.

On arrival I find the iStor is indeed not working. Pressing the re-set button appears to start the boiler again but it fails almost immediately and locks out again. 

On examination/testing, I find the pump has seized and the central heating system pressure has fallen to zero. Un-jamming the pump and re-setting the system pressure will get the boiler working again but the fault returns an a hour, a day or a week later. Experience is beginning to show that having seized once, the pump on an iStor is very prone to seizing again so it's best to replace it even though it appears to be working again.

Further checking usually reveals the central heating expansion vessel diaphragm has lost its nitrogen/air charge,  and the neoprene diaphragm has usually failed too, indicated by water leaking from the Schrader valve when testing for nitrogen/air pressure. Replacement of the expansion vessel is then necessary at the same time as the pump. 

Failure of the CH expansion vessel is often associated with water discharging from the automatic air vent inside the boiler too. Sometimes debris enters the needle valve inside and stops it closing properly on re-pressurising the system. A new AAV is advisable when this happens. 

2) iStor has stopped working. No fault code displayed or display completely blank.

This is generally control board failure. The transformer is prone to overheating and melting the control board box. An easy but expensive repair. An new control board just needs to be plugged in. The hard part is getting hold of one. Very few boiler spares merchants stock parts for the iStor as it is such a new model. The part usually has to be ordered directly from Ideal Boilers resulting in a delay of several days.

3) Boiler is working but water is leaking onto the floor and dribbling from the safety valve discharge pipe outside.

This is usually a long term problem caused by the hot water expansion vessel having lost it's nitrogen/air charge, leading to excessive pressure in the hot water cylinder during re-heating. Initially, the pressure relief valve opens and dribbles water during re-heating only, but after a few months (or years) of this it begins to wear out and fails to close properly. This results in continuous dribbling of water from the discharge pipe. Users still don't notice (or care, usually). Eventually the tundish inside the boiler - a visual indicator to show that water is flowing to waste - begins to scale up. Eventually water scale accumulates to divert the water originally discharging to drain OUT of the tundish and onto the floor under the boiler.  

Re-pressurising the hot water expansion vessel, replacing one (or both) of the pressure relief valves and descaling/cleaning up the tundish restores the system to correct operation.

4) Hot water warm but not hot.

This can be initially perplexing as the hot water cylinder is piping hot yet water from hot taps is only warm. The cause is a jammed thermostatic blender valve on the iStor. The jamming appears to be caused by water scale as operating the temperature adjustment control and working it back and forth throughout its whole range generally gets the hot water working again, but the fault returns after a few days. A new blender valve is needed. As with many repairs on the iStor this is not simple or quick to replace even though it initially appears so. The hot water cylinder must be drained and  


Other problems I've seen with the iStor are:

1) A fiendishly awkward-to-use programmer which defeats users and heating engineers if the instruction book is missing. Whilst not exactly a breakdown, I've attended breakdown calls where the user (often a tenant) has been unable to get the boiler to operate, simply because they don't have the user instruction book.

2) Partial pump failure. This one presents as a persistent overheat error. The flow and return temperatures in and out of the heat exchanger show a large difference suggesting a blockage but the problem is sometimes a lazy pump. The pump appears to be working normally but a new pump results in a much reduced flow/return differential and the overheat error disappears.  



I've just published a new website specifically for the istor called istor Repairs. A lot of overlap with this page and I'd better merge the two soon. The new site is here




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