Worcester Bosch Greenstar 35CDi wall RSF Combi problems (E9)

[ro80rob]

Hello
First post so here goes (and its rather long).

History
My boiler was installed new at the end of 2005 (around about the time WB were retiring it from their range it now seems). I did a lot of research and it seemed to be one of if not the best manufacturers at the time. Till now, I've had no problems.

So far
I have looked extensively on this site and I believe that I have some sort of blockage in the hot water side of the system resulting in E9 tripping out.

Detective work done so far includes......

1) Changing the overheat stat switch screwed into front of the primary gas to water heat exchanger (I checked with a meter that this was the switch that was tripping out and it was a 2 minute job).

2) Checking the pump works. I borrowed a pulse generator from work (electronic engineer u c) and simulated the pulses into the motor. I believe from my stethoscope that it has 8 speeds (inc stop), and I plotted them out in excel and got a straight line with respect to pulse width, so I guess it is spinning at different speeds, but I can't tell exactly what they are. I also note that when it is connected back to the control PCB, it gets pulses that are in the range I used. Anyway, I'm going to assume it is good.

3) Checking spot temperatures around the primary heat exchanger with my point and shoot infra red temp sensor (incidentally I had to stick some black PVC tape to the points I wished to measure, as the shiny aluminium has poor emissivity so the sensor doesn't see the temperature properly), I can see the heat exchanger getting upto 95C in the region of the overheat switch and maybe higher when it E9s and trips off.

4) I have checked the primary temp sensor and am happy that it agrees with reality (resistance wise)

5) I have checked the DHW temp sensor and am happy with it also

6) I have checked the pulses from the flow turbine using a scope. All fine

Just as a little anecdotal evidence....
I can get the system to stay on if I significantly drop the setting of the hot water temp knob. I also note that if I am running hot water and turn this knob up, the boiler responds and increases output (well the fan revs up and the pump pulses respond to demand higher pump speed). These two facts give me high confidence that the control PCB is OK. I will add also that I have noted a slow reduction in hot tap water temperature over the past 12 months or so in my bathroom sink tap (smaller flow tap). It has still been possible during that time to get sufficiently hot water from my bath tap, which has a higher flow capability.

All the above leads me to think I have a blockage in the hot water circuit somewhere within the boiler. I have read extensively about problems with the DHW plate heat exchanger, but also of blocked pumps and even the main gas to water heat exchanger, which seems to be gold plated if you can believe the cost of many. Oh, and I live in a soft water area.

Any opinions on my fault would be very useful before I plunge into getting repairs.

What would also be really useful would be a full parts list.

I would really like to find out the cost of parts before I start paying out in progressively expensive chunks to get parts replaced. Also, there are gaskets and O-rings on everything, so it would be useful to know these sizes, oh and to be sure I am looking at the right primary heat exchanger if I needed. WB don't list that in their 'short parts list', and there does seem to be many different ones with differing prices.

Cheers.... Rob.

 

[nickso]

my suggestion is to stop fannying about and get worcester out to repair it. for a fixed price they will have all the parts in the van, are likely to know that fault already and if they don't fix it first time they come back and try again.

if you really want to persevere why not remove and inspect the domestic H/E, if it only trips on HW then thats a likely culprit.

the primary H/E is not really within your scope to remove and replace.

 

[ro80rob]

my suggestion is to stop fannying about and get worcester out to repair it. for a fixed price they will have all the parts in the van, are likely to know that fault already and if they don't fix it first time they come back and try again.

if you really want to persevere why not remove and inspect the domestic H/E, if it only trips on HW then thats a likely culprit.

the primary H/E is not really within your scope to remove and replace.

Hello Nickso
Thanks for your reply.
You should understand where I am coming from with this problem. I have read up considerably on this site and elsewhere on the interweb about this type of problem and was getting concerned about the number of times people have had their boiler issues unresolved by repairers. I am therefore nervous with (it seems) good foundation.

I accept that it may seem as 'fannying about', but I have found the excercise very interesting (do I sound a bit like a train enthusiast here?). To find brushless motor speed control incorporated in the modest central heating pump was impressive. We have large (MW) frequency controlled demin water pumped cooling systems at work, but to see it in the humble boiler was a suprise.

I think (with the groundwork I have done) that I agree with you in that it is most likely the DHW plate heat exchanger. We have very large ones of these at work too, but these can be separated into their individual plates so that they can be pressure washed clean. I don't know how one of these 'sealed' items could be adequately cleaned.

I am still perplexed as to why this has blocked up. I live in a soft water area, and the central heating sealed system is treated with inhibitor. My previous combi, a Myson Midas Si (one of the most unreliable boilers there was it seems) worked almost without fault for 12 years, with the exception of having to change the DHW temp probe every couple of years through its life. There was never any blockage question, and the water always looked clear as drinking water out of the sealed system when I bled the rads.

As to removing/inspecting the plate heat exchanger, it looks to be easily within my ability, but I would be at a loss I guess if it was that part at fault, and I had not the replacement part on hand to fit. Never a good idea with the wife. Things just have to be right first time.

You are correct about the main heat exchanger. Apart from competency issues, I don't think I want the responsibility of that going pear shaped (you've met the wife), and the cost of it as a part alone is more than enough of a deterrent.

I didn't realise that Worcester had their own engineers and did a fixed cost repair. I thought it was British gas or someone else who didn't really 'know' the boiler thoroughly. Having the right parts in the van is a big plus too.

I shall give them a phone and find out.

Cheers... Rob.

 

[penfold46]

£197 for a fixed price repair (last time I advised a custard who needed a new ev on an old 35cdi). Absolute bargain for the job needed on that boiler. No room to fit a remote on that one.

P46.

 

[ro80rob]

£197 for a fixed price repair (last time I advised a custard who needed a new ev on an old 35cdi). Absolute bargain for the job needed on that boiler. No room to fit a remote on that one.

P46.

Thought I'd give an update since all experience helps others.
I did as I was told and booked the WB fixed price repair service. £210 inc of VAT. Got the service next day after booking too. No waiting.

The service engineer was most helpful and listened to all my findings and did a few additional checks himself, which included plugging his laptop into the PCB to check fault codes etc.

I demonstrated the fault a couple of times. Isn't it nice when you don't have an intermittant fault, but a 'hard fault' that you can demonstrate.
The engineer could not understand how the boiler was tripping on a limit stat, but the water temperature leaving the boiler was only 85 C (cross checked by a separate probe). He was convinced that it wasn't the DHW water heat exchanger from experience as if that was blocked, it would trip out on a different fault, and should never hit the limit stat. He decided to 'phone a friend' to discuss it, and came back from his van with a new gas to water heat exchanger block. i.e the main bit, costing over £600 to buy retail.

He went on to explain my fault as a rare one (maybe he would see one every 2 or 3 years). Apparently, it is possible for the gas burner itself to have a pinhole in it, resulting in a mis-directed flame jet. This can play on the aluminium fins inside the heat exchanger, and can slowly burn them away in a specific region. This then allows the flames to heat directly (and locally) the aluminium of the exchanger. It was this effect that was allowing the flame to sufficiently heat the exchanger near the overtemp stat to cause it to trip.

So

After a bit of a struggle (I had to help) to disconnect the pipe that feeds into the bottom of the exchanger (it was a very tight fit in its O-ring seal[the worst one the engineer had ever done!]), my heat exchanger block was removed complete, and the new complete exchanger installed and connected up.
After a few rudimentary leak checks and some gas pressure checks, the system water pressure was restored and bled. Water samples were taken to check for acidity (and sent off... I'll hear about these if there is a problem), and 2 hours later I had a working system again. By the way, my heat exchanger block will be sent off to WB for a post mortem to see how/why it has failed so that action can be taken on future products to prevent problems. Excellent!

Result!. I got a £600 part fitted for £210. What is more, its got a gaurantee, its a major component and all in all it was a good experience ( a first experience for me).

Just want to say thanks for the help.

Cheers.... Rob.