Copyright 2007 to 2016 Michael Bryant

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Ideal website
Replacing an istor boiler
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First of all, thanks for visiting istor Repairs.

WARNING: Long page, this boiler suffers from a lot of faults!

Just by way of introduction, I'm Mike Bryant, also known as Mike the Boilerman. I specialise in repairing central heating boilers. In particular, I like repairing the old, the awkward and the unusual ones that most technicians announce cannot be repaired and tell you must be therefore be replaced. Always treat such advice with skepticism ;-)

Anyway back to the subject. The Ideal istor (a name which grates with me for being spelled in all lower-case characters by the manufacturer, Ideal) is a combined gas boiler and mains pressure unvented hot water cylinder all jammed into in one large box. It suffers from a number of expensive faults. In addition, the instruction book is spectacularly unhelpful in tracing and diagnosing many of the problems an istor suffers from. Ideal have abandoned the fault-tracing flow-chart that most boiler manufacturers publish in favour of a chart of 'fault codes'. The fault codes are incomplete and no help at all if you have a failed istor with, say, the common problem of a completely blank display screen. (The fault code chart does not mention what to do if the fault code screen is blank!) Ok, I'll stop ranting about it now and write some stuff covering the more common, undocumented, faults :-)

I've written this site mainly as an aide-memoire for myself but I see no reason not to publish it. Anyone is welcome to use it. Make sure you are competent within the meaning of the Gas Safety (Installation and Use) Regulations 1998 if you attempt to sort out your own boiler though. Gas accidents kill a dozen or two people every year and the cause is usually over-confident and under-competent technicians working without a full understanding of the appliance, or complete lack of service or safety-checking of the appliance, ever.

But back on topic again, here are the problems I usually encounter when I attend an istor:

Istor common faults:

1) The most common istor fault these days is failure of the internal heating coil in the hot water cylinder. A serious and terminal failure unfortunately as the hot water cylinder is no longer available as a spare part. This has various symptoms but the user (often a tenant) usually sees little need to call for assistance until the problem has been present for quite a long time.

Initially the only symptom is the overflow pipe (safety discharge pipe) running continuously to drain, which to users seems trivial, but it means the radiator water is being constantly refreshed with new clean mains water (via the internal leak) from the hot water cylinder. This leads to accelerated corrosion of the insides of the radiators and the products of corrosion contaminate the pump and the gas boiler heat exchanger. The boiler begins to hiss, pop, bang, overheat, lock out and generally run noisily and ever more unreliably. 

At this point people call me in asking for a service or thinking a minor repair is needed, but when I arrive it becomes clear the problems are deeper. The heating and radiator system are usually found to be over-pressured, and the boiler running noisily from the heavy contamination. Diagnosis is suggested by a 3.0 Bar reading on the boiler system pressure gauge instead of the correct pressure of 1.0 Bar. On a healthy system this can be corrected by disconnecting the filling loop and opening a radiator drain cock or the boiler pressure relief valve to discharge excess water. This should reduce the  reading on the pressure gauge to 1.0 Bar. If the system pressure immediately returns to 3.0 Bar diagnosis is confirmed as a leaking internal hot water cylinder coil, and the only course of action available is to fit a whole new boiler. 

2) Fault code alternating 'L' & '9' displaying:

This is the "dry fire thermistor", according to the fault section in the book. What they don't actually explain is the dry fire thermistor is a heat sensor attached to the top of the heat exchanger which shuts off the boiler if the temperature of the heat exchanger rises to about 90 degrees C. So if you have error code L9 you have to figure out WHY the heat exchanger is getting so hot. Usually a jammed or under-performing pump. They are horribly prone to sticking. Un-jamming it will work for a while but the problem will return. Fit a new pump. Sometimes the pump appears to be running correctly when actually it is not. The istor pump is noted for partially failing but continuing to run - unsuitably slowly - this can fox even the most experienced repair technician! A new pump to replace the apparently working existing pump will often fix the L9 fault code.

A completely separate reason has also emerged for the alternating L & 9 error message. Most modern heating systems are installed with thermostatic radiator valves (TRVs). These progressively reduce circulation as the room temperature rises until they are fully closed when room temperature reaches the selected temperature. The istor installation instructions specify that an external bypass circuit must be installed with TRVs so circulation through the boiler heat exchanger can be maintained. This bypass is often omitted by installers who didn't read the installation manual leading to absence of heat exchanger circulation and the L9 error. If TRVs are fitted to all radiators and a bypass has not been installed, keeping one or more radiator TRVs set permanently to MAX will prevent the L9 errors.

3) Fault code alternating 'L' and 'F' displaying:

This is listed in the manual as 'Flame Detection'. The boiler electronically detects the presence of a flame what the boiler is running snd should it detect the flame has gone out (or failed to light in the first place) turns the boiler OFF and puts it into a safe condition. The error message displays and can be re-set by pressing the tiny little reset button next to the display. If the boiler still fails to light, or to stay alight, there can be a wide variety of reasons starting with no gas (pre-payment gas meter run out of money sometimes!), gas valve failure, PCB failure, or it can also be caused by the condensate drain or trap being blocked. The trap is supposed to be cleaned during annual servicing but access to it is virtually impossible on the istor so probably never gets done.

4) Blank screen, no codes or anything displaying:

This is usually control board failure. Fit a new one. Be warned, they are EXPENSIVE. Prove the diagnosis by checking there is mains power passing through the On/Off switch. If the On/Off switch is supplying power, this is main control board failure. If not, then it is failure of the user display board. But the On/Off switch is always supplying power, in my experience....

5) Water dribbling continuously from the copper overflow pipe (or "safety discharge pipe" to give it it's correct title):

This is another fault not listed in the fault code chart. One or more of the three safety valves is operating. These are all complex faults. The investigating technician first needs to investigate which of the three relief valves is letting by. The three valves are the central heating pressure relief valve, the hot water cylinder pressure relief valve, and the hot water cylinder temperature and expansion relief valve. So now you know ;-)

The CH PRV letting by is usually associated with fault 1) above, but if not it will be failure of the CH expansion vessel and, perhaps, the central heating filling loop being left turned permanently on as a stop-gap measure to maintain heating system pressure. This is seriously bad news as by the time the problem has penetrated the consciousness of the user, serious corrosion of the radiator system has usually occurred and the system will be contaminated with black muddy stuff (known in the trade as 'sludge'). The short-term fix is just to replace the expansion vessel and the CH PRV, and ignore the sludge problem. If the customer wants things 'just so', then an expensive powerflush will usually be necessary too.

The discharge from the copper overflow pipe may also be coming from the pressure relief valve on the hot water cylinder. This is usually caused by loss of air pressure in the hot water cylinder expansion vessel. This should be checked during the annual service but few users actually get istors serviced properly every year, if at all! Even if they get them serviced, the servicing technician rarely reads the service schedule and checks the air pressure in the hot water cylinder expansion vessel. The fix is to re-pressurise the expansion vessel and fit a new HW PRV. 

The third valve, the temperature and expansion relief valve, only discharges if the EV has lost it's air charge and the HWC PRV has stuck shut, in which case the T&ERV does the job instead. i.e. opens to relieve the excess pressure. (Please excuse all the abbreviations but my fingers are tired and I expect you can figure out what they all mean!)

6) The boiler runs for an hour or two, or three, or six, then shuts down for no apparent reason with an alternating 'L' and 'F' showing on the display panel. Pressing the re-set button gets rid of the L and F and the boiler lights again and runs happily for a further random period of a few hours then the lockout repeats. You'll be disappointed to hear I have no answer for this one yet beyond checking the condensate trap for blockage. This is not a five minute job by the way, as to check it, one has first to remove it. The location of the condensate trap is between the boiler section and the hot water tank, right at the back and behind several pipes which are normally VERY HOT. So you need arms of asbestos and the size of knitting needles with three elbows in each to get to it off, unless all the hot water in the tank is first drained off, in which case one's arms only need to be like multi-elbowed knitting needles. (Can you tell I don't like this boiler?)

That's it for now. I'll be back and add all the faults I've forgotten about today, next time I encounter them :-)

Now to cap it all, the last problem is a generic installation fault common to many boilers in this format. A concealed extended flue. when we work on a gas appliance we have to carry out four statutory safety checks. Two of these are checking the effectiveness of the air supply and the flue. In practice this means visually inspecting the air supply and flue exhaust tubes connecting the boiler to outside air. In most installations these tubes are fitted above the ceilings out of sight. Until the 31st December 2012 we were allowed to carry out a risk analysis on these concealed flue systems but not any more, Gas Safe Register now insist we visually inspect or condemn the boiler. Great eh? To you the user, this means having access panels installed in your ceilings following the route of the flue to outside prior to you next having the boiler serviced or repaired. This is so the gas bod has visual access to the whole of the flue and can assess it for safety. There are specialist firms out there now installing these access hatches. Google 'flues in voids' to find one run by a friend of mine (No financial connection.)  


UPDATE 5th October 2016:

Although I used to end this page with an invitation to call me if your istor needs repairing, I'm afraid I now no longer work on the istor. A fault they commonly suffer from (failure of the heating coil inside the hot water cylinder) can no longer be repaired, as Ideal have stopped making the hot water cylinder as a spare part. Other problems can be very time consuming and expensive to fix and it makes little sense to spend potentially large sums of money on a boiler prone to an unrepairable failure in the near future. I don't usually say this but in the case of an istor, should it go wrong my advice is to replace it rather than repair it.

I'm happy to leave this site up as reference for owners and other repair technicians, and to give email advice to anyone wanting it, but not telephone advice. I had to stop that years ago when the weight of calls grew too great!

For my main site, check out

Once again, thanks for visiting.

Mike Bryant, AKA Mike the Boilerman. 



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First published 6th May 2011
Last updated 5th October 2016

Copyright 2007-2016 Michael Bryant