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Page last edited Thursday, 18-Oct-2007 17:33:21 BST

Diverter Valve / Diaphragm Replacement on the

Worcester Bosch 24CDi (and variants) Combi Boiler

(Note: If the step-by-step images do not appear down the left hand edge click here to load the page properly)

[EDIT: Many thanks to all the e-mails and guestbook signings from those who have attempted this repair having read these pages. There would appear to a lot of people out there with this problem given that this page gets over a thousand views a month (3,000 in Jan '10 - probably due to the cold weather!). Whilst this walkthrough was never intended to be a 'How To' guide it is good to know that it has proven to have been an inspiration for others to give it a go and, with that in mind, I cannot stress enough how important it is for you to acknowledge that this still isn't meant as a guide per se but rather an overview of what's involved when changing the diverter valve or diaphragm. That said, I don't want to put you off - I've heard from people of all skill levels and experience who have successfully repaired their boiler... and the vast majority go much smoother than the story shared by one guy which I've posted at the bottom of the page...!]

As every DIYer knows all too well there's many a job that gets itself, uninvited, onto the To Do list. These jobs are invariably unforeseen, involve fixing something, and indeed often don't do much more than return you to the state you were in before! Nevertheless, they cannot be ignored and can often cost a small fortune if you get the pro's in. This happened to me, and seemingly thousands of others (dare I say all combi owners at some stage in their ownership?), when their diverter valve, or more specifically the diaphragm in the valve, gave up the ghost.

My problems manifested themselves in the form of the shower exhibiting temperature fluctuations. The shower, a Mira Excel, was only 18 months old and contained a fast-action wax-based thermostatic cartridge and so something was clearly amiss given the performance issues. With the shower on, the boiler could be seen to be 'dropping out' whereby it would fail to keep recognising the demand for hot water. Furthermore, if a (hot) tap was opened simultaneously with the shower the boiler would then usually run continuously without an issue. All this pointed to some fault with either the shower not demanding enough water to keep the boiler on, or the boiler failing to recognise the demand....

I originally suspected the shower and set to work dismantling it. Having installed this myself I recalled a pair of inlet strainers which I suspected may have been blocked thus perhaps reducing the flow of water. This turned out to be true - the hot water inlet was partially blocked with what I could only describe as black stuff. It looked like either dirt or plastic... Either way I was surprised, and confused, given that I'd installed all the pipework and taken great care with flushing the system prior to final commissioning. Still, it was in there and shouldn't have been so I cleaned the strainers and reassembled. The problem, however, persisted.

I then started to wonder what this black stuff was, and where it might have come from, and having sussed out how the boiler works realised that the diverter valve contains a rubber diaphragm which sits inline with the hot water (DHW) supply. When a hot tap is opened, the diaphragm shifts sideways due to the pressure differential and in doing so diverts the primary water circuit through the secondary heat exchanger (rather than the radiators) and activates a microswitch to fire up the boiler. This information alone was sufficient to make me suspect the diaphragm was not doing its job - perhaps it had perished hence also why the shower inlets had all that bluck stuff in it. The reason things may have been fine with an additional tap opened was suspected to be down to the increased water flow being sufficient to still cause enough sideways movement of the diaphragm.

I contacted Worcester Bosch only to be told that repair was by replacement of the entire diverter valve and, at >£75 (excl. labour so it would've been >£100 all-in) [Note: Prices as of 2004], I was a little annoyed. Whilst I could not be 100% sure at this stage, I had no reason to believe that anything but the diaphragm was at fault. Furthermore, whilst I'm no 'greeny' I can't help feel it's a bit of a waste to chuck out something that could be repaired for very little cost.

Some further trawling of the web revealed that the diverter valve was made by Giannoni - a supplier of valves for numerous other makes of boiler. Fortunately some of these other makes do sell the diaphragm without having to buy a whole valve so I bought one for £10 and set to work.

The rest is detailed through the captions on the photos (click here if they haven't appeared down the left). If you're going to tackle this job you ought to have a copy of the service manual to hand as I've kept the detail briefs on the assumption that anyone working on their boiler ought to also have this manual and have at least read through some of it! [Edit: It used to be easily available from the Worcester Bosch website but now appears not to be - you can therefore download it from me here (1.4MB PDF)]. The manual covers the 24CDi, 28CDi and 35CDi hence the innards as far as the parts that concern us are the same.

It is fair to say that you might want to consider replacing the whole diverter valve rather than just the diaphragm as given you'll be saving so much on labour by DIYing it you'll still be quids in. Furthermore, it's one less thing to worry about if you can reset the clock on when the valve itself might fail in the future. Note also that you really must get yourself an o-ring and washer pack as any dismantling of the boiler will reveal that these compress over time as they seat/bed themselves in - reassembly will otherwise invariably end in tears as you find leaks springing up everywhere!

Finally, at the risk of scaremongering, I must warn you again that this task is arguably not for the faint hearted - despite the diverter/diapgrahm being somewhat of an Achiles Heel for this boiler they really did not factor in its repair/replacement when design the layout of the boiler. As you will see a whole variety of other bits need removing first before you even get the chance to get the thing out... Also, please do not lose sight of the fact that the boiler is a gas powered device, and a potentially lethal one at that, so don't touch anything you don't need to and, if in doubt, get the pro's in as whilst DIY gas work is not illegal common sense still dictates that you ought to be competent to take the task on.

Click the photos down the left to see what you're up against and, if you're going to head off down this well-trodden path, good luck! :-)

If you'd like to share your success (or, dare I say, failure!) then please feel free to sign the guestbook! You can also contact me directly here.

As mentioned, I get many e-mails from readers who have managed to succesfully replace their boiler diaphragms/diverters... but some go more smoothly than others and with this guy's agreement (name withheld to protect the guilty!) I had to share this one...

18 Oct 2007
Hi Mathew,

Are you sitting comfortably........................... Then I'll begin.

Having read about your experiences I decided to give my repair a go. The task was a bit of a marathon though. Stripping down went well enough, though I almost fell at the first hurdle. When I opened the drain taps, nothing happened. I ended up taking the white plastic centre parts of the tap out completely, and still no water. In the end, I stuck a small screwdriver in to the centre of the tap body, and cleared out much black crud, and hence the system drained. The rest of the parts came out without issue.

Putting back together was a different story all together. The diverter valve itself went in OK, but the "bent pin" clip holding in the bypass pipe just would not go in. After a number of removals of the diverter valve to check the hole was OK, and the O-ring seated properly, I eventually decided the only way to find out what was happening was to take the bypass pipe out all together. Out of the boiler housing, the pipe fitted so easily it was scary.

Now all I had to do was get the old O-ring out of the connection at the right hand side, which I thought would take ages, but in fact came out quite easily with tweezers. So I put the diverter valve back in, put the bypass pipe back in, and still the clip would not fit. In the end, I had to lift the right hand end of the bypass pipe from it's housing to allow the left hand end to be fitted first. Now it located and the pins went in first time - the right hand end also went in easily. Nowhere in the service manual is this tip documented, but really it cost me 2 hours of trying because I thought if I took the right hand end out I wouldn't be able to clean the joint and replace the O-ring without removing even more parts from the boiler.Obviously, I hadn't been able to clean out the seating faces of the hole in the pressure valve body, but the new O-ring seems to have sealed anyway.

The heat exchanger went in easily - it was the last easy part of the job.

The first attempt at putting the filling loop in place seemed to go OK, but pushing the unit rightwards to locate into the inlet valve was very stiff. The left hand end located up OK, and the water was turned on. The reason for the stiffness became apparent, the little O-ring on the right hand end had caught somehow, and broken, with the obvious result once water was applied.

So the water was turned off and what was left in the boiler drained again. The filling loop removed, O-ring replaced, and a second attempt made. This time the right hand end seemed to locate more easily. The left hand end screwed down, and the boiler filled. A few drips from the left hand end of the filling loop were stopped by tightening the screws, and all looked good. But the pressure didn't come up, and the boiler wouldn't turn on. Double flashing reds suggested no water pressure, but water had definitely flowed into the boiler, and the hot water taps would run, but obviously not heat the water. After much head scratching, and repeated turnings of the filling key - I eventually tried turning the little grey knob - water flowed into the boiler, and straight out of the left hand end of the filling loop mating surface!!! In the blind panic of having water all over the place, I dived underneath to turn off the valves again, and disloged the front panel from it's hinged down position. It fell floorwards, but I caught it before it pulled the pressure gauge pipe out, which was very fortunate.

Stripping the filling loop out again, I found the metal bracket hadn't located over the mating surface properly. Screwing the filling loop down had located it against the support bracket, but not fully against the actual mating surface on the diverter valve. This was corrected, the loop refitted, and water turned on again. This time, no immediate signs of serious leaks. A bit of drying up was required before I could turn the electrics back on, but eventually the switch was thrown.

Well, the pump would run, but still no heat. I could hear the igniter ticking, but no flames. I couldn't tell if this was because the gas wasn't being turned on, or there was no spark at the igniters. The flashing lights indicated an electrical sensor fault, and in investigating this the broken cables were found - remember I said the front panel had fallen.......

Now - I should be able to do electronics - but all the cables listed in the manual, and visible on your photos seemed to be in place. In fact it was the igniter cables that had pulled out (snapped actually!). So I had to get to the controller board and re-attach these cables, but where and to what I couldn't tell at this point.

I should probably point out that to get the controller board out, you have to remove the 3 control knobs. The manual does say "carefully", but they are very stiff. The hot water control knob came out easily enough, they are just a simple knob with about a 2.5 inch shaft. The central heating control was much tighter, but came eventually. The On/Off just didn't want to move. A heave with a pair of pliers brought it out though - but the shaft was only an inch long, and had an ominously jagged end. So, the on-off controll knob was broken, a length of it's shaft was still in the panel (so I couldn't use one of the water control ones in its place) and of course, the swich was set to the off position.

I'd really had enough of the job by this stage, so I decided to put this control knob issue off until later, and deal with the problem I had started to fix, i.e the reconnection of the electrode wires. I managed to get the controller board lifted, but the polarity of the connectors for these two wires is not listed in the manual. I took an educated guess, pressumed that polarity would not be an issue to the electrodes of the igniter probes and with heart in mouth took the soldering iron to the loose wires.

Now it really was anybody's guess if I'd soldered the igniter wires the correct way round (if they were polarised), so I could be about to send a spark to the gas with no reference to any logic. But I still couldn't try it out, because of the broken control knob. However, on close inspection, the shaft is basically round, with a flat edge to locate and key into the switch body, and there was at least a cm of shaft with the flat here - so I stuffed it in and hoped there would be enough to engage. And, low and behold there was - turning the knob did feel to be driving the switch.

Fortunately at this point, there was no-one else in the house to get blown up with me, so I threw the power on and turned the switch anyway. Still being alive, and the house still having all it's windows, I even braved turning on a tap - water flowed, and it was HOT!!!!. I set the timer up, the pump whirred, and the radiators started radiating. At this point, the cat wandered in, curled up infront of the kitchen radiator, and looked across with an "about time too" look on it's face. It soon had a "what was that for" look on it's face as my size 9 connected with an appropriate part of it's anatomy.

Hey - I now have a warm house, and a hot shower. And it's all thanks to you. I'm sure I'll forget, and eventually forgive you for making it look oh so easy.

Seriously, thankyou. I'm really happy to have been able to do this myself. I really hate paying others to do stuff I can do myself, and especially when the so called regulations say you have to use a "professional" despite the apparent knowledge and ability of some of these pros being suspect to say the least (that may sound rich coming from me but I'm not claiming to be a pro - this job was my first boiler repair). It seems that in part the primary intent of some of these regulations may well be to protect the commercial interests of the industry. Anyway, rant over and thanks again!

P....... B.......

There's got to be a moral to this story... (not quite sure what it is yet, but it may prove some comfort for you if things start to go not quite as you'd planned!)

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