Worcester 35CDi domestic hot water... odd fault.....pcb?

[farqueue]

GC number 4731139 (N.G.) Worcester 35 CDi

I am really scratching my head with this one! Firstly wherever I set the temp guage on the boiler for dhw it makes no difference not sure of the exact temp but it goes from hot to sometimes scalding hot - intermittent. If I try to run a bath the water starts to run cold after a while & then hot again in other words "Cycling" but I get more cold than hot!

I have replaced the DHW sensor - no change at all.

My theory is that the boiler is sensing an overheat situation on the dhw and that is why it is "Cycling" BUT is it doing that because a pot on the control board is stuck on full (Which would explain why moving the dhw temp dial has no effect)

Any thoughts before I shell out on a PCB? Grateful thanx in eager anticipation

P.S. I repair domestic appliances not boilers but this is my own :-(

 

[diamondgas]

The control knob spindle's are well known for snapping internally so that whatever position it snapped at is where it's stuck, the knob still turns as expected. It means removing the PCB to get the snapped bit out.
However that does not explain the overheating. Did you put any heat-sink compound on the stat when you replaced it? Is the stat sitting flat on it's seating.. You need a good contact with heat-sink to get a proper temp registering! Also poor water flow can give similar issues.
If not sure get someone in B4 you shell out on a PCB!

 

[farqueue]

I have taken the board off & checked the spindle - Just 'cos a similar thing happens on washing machine boards sometimes. Yes I did use heat sink compound & the stat is sat on the "Flat" of the pipe as good as I can see it will sit.
If it was down to poor water flow what would cause that? is it likely to be the primary or DHW heat exchanger?

Many thanks.

 

[diamondgas]

If it was down to poor water flow what would cause that? is it likely to be the primary or DHW heat exchanger?

Many thanks.

Quick and easy check is how long to fill 1 Litre jug of water. That'll give you a good idea to the flow rate. If it's too low the h/w stat can be too slow to recognise rapid temp rise so the boiler shuts down instead of modulating (reducing gas pressure).

If your flow rate is okay then the next stage is checking for modulation. That you need to be qualified to do. Check your manufacturers instructions for the expected flow rate. Probably 9-12ltr/min at a guess!

 

[diamondgas]

If it was down to poor water flow what would cause that? is it likely to be the primary or DHW heat exchanger?

Many thanks.

Regards cause, could be as simple as blocked inlet filter. But could be scaled plate heat exchanger, or restrictions down stream of boiler. Can also be issues with main heatex or flow through that! System sludge, under-performing pump .. Lots to check before you decide on replacing the PCB..

 

[farqueue]

Steve....Diamond Geezer! Just checked the flow rate......1 litre in 5 seconds in the kitchen which by my calcs makes it around 12 litres per minute max quoted flow rate is 12.6 l/min.

 

[farqueue]

Given the good flow rate am I right in thinking it is unlikely to be a blockage therefor either a gas problem Maybe a faulty valve) or control board? I know it is dificult advising a numpty white goods engineer who knows nothing about plumbing but hey .....you've started now ;-)

Seriously I am just trying to gather some info before I decide where to go next.

 

[diamondgas]

Steve....Diamond Geezer! Just checked the flow rate......1 litre in 5 seconds in the kitchen which by my calcs makes it around 12 litres per minute max quoted flow rate is 12.6 l/min.

Good flow! Can you check wtr temp B4 the boiler shuts down? Control stat Min and max if possible just to see if what you're saying is correct! Other possibility could be an issue with gas valve ... If the boiler is staying on full rate the power to the modureg needs checking, and as I said B4, burner pressures. That only GS engineer can mess with ...

 

[diamondgas]

Given the good flow rate am I right in thinking it is unlikely to be a blockage therefor either a gas problem Maybe a faulty valve) or control board? I know it is difficult advising a numpty white goods engineer who knows nothing about plumbing but hey .....you've started now ;-)

Seriously I am just trying to gather some info before I decide where to go next.

You seem to have good tech knowledge and your assumptions show good judgement ... I'd be inclined towards what you say but there are tests to do to verify. either component will cost you a fair bit wonga! And it may be something totally different! That's where experience comes in... Listening, touching, observing expectations .... that sort of stuff!!

 

[aggis]

If it's heating up dhw, then burner goes off for a perioid, could be the plate heat exchanger, blocked on heating side. It gets too hot too quickly

 

[farqueue]

Okay
The boiler is situated upstairs right next to a shower room Boiler set to max the dhw in the shower room (Basin) temp cycles 30 - 56 - 30.
set boiler dhw to min and dhw temp in the shwer room (Basin) is pretty much constant 52.

Downstairs bath and basin dhw set to max on boiler - temp at tap cycles 30 - 51 - 30
Downstairs bath and basin dhw set to min on boiler - temp at tap cycles 30 - 49 - 30 (Hence I can't get a hot bath!)

Kitchen (Next to bathroom) Boiler dhw temp set to max - temp at tap cycles 32 - 54 - 32
Kitchen (Next to bathroom) Boiler dhw temp set to min - temp at tap cycles 33 - 51 - 33


Incidently the shower in the bathroom downstairs constantly goes hot & cold (Runs off the boiler) Whereas the shower in the room upstairs which also runs off the boiler stays pretty much constant temperature!

It is a bit confusing.......to say the least!
Don't give up on me now guys!!

 

[Stu.roberts]

I had a similar fault on a 28 cdi, and it just turned out that the pump had been turned down to 2 from 3, gotta be worth a look!

 

[GrahamM]

How old is the boiler?
Was it working ok before & how long you been having trouble?

 

[farqueue]

How old is the boiler?
Was it working ok before & how long you been having trouble?

The boiler is ten years old it was moved for a loft conversion about five years ago I guess the problem started around one year ago! but having changed the stat I assumed it was likely to be a board fault, the fact that it hasn't stopped working has slowed me down I guess :-(

 

[GrahamM]

Has it progressively got worse?
Due to it's age, I'm inclined to agree with diamond gas and aggis about the plate heat exchanger.

 

[farqueue]

I think it was intermittent at first and I guess it has got worse - also (If it helps) the boiler used to shut down completely if you ran a lot of hot water i.e. a deep bath it was then okay if you hit the red reset button. It doesn't do that now maybe because there isn't constant hot water being drawn (It cycles).

I am leaning toward your expert opinion & am tempted to try the plate heat exchanger first (Not diy of course). As with all things it is really useful just to discuss a problem with people who have not only knowledge but experience in the field and I am very grateful for all your input.

 

[diamondgas]

Hi farqueue, It's looking more likely to be age/sludge related. Check out the colour of the water when it's drained. Most likely your system would benefit from a flush as well as the plate H-ex replacing. Most times you can get away with just replacing the plate h-ex and advisiting to replace boiler & flush when the problem returns! See how it goes...

 

[farqueue]

Hi farqueue, It's looking more likely to be age/sludge related. Check out the colour of the water when it's drained. Most likely your system would benefit from a flush as well as the plate H-ex replacing. Most times you can get away with just replacing the plate h-ex and advisiting to replace boiler & flush when the problem returns! See how it goes...

Thanks for that......The plate H-ex is that the DHW h-ex 87161019760 or am I looking at the primary h-ex 87161428040? part numbers quoted from install & service book

 

[farqueue]

I think I just answered my own question!
HEAT EXCHANGER 87161019760

 

[diamondgas]

I think I just answered my own question!
HEAT EXCHANGER 87161019760

That's the plate heat exchanger yep! It isn't an exact science mind you when it comes to your fault and would be helpful to check out what the gas pressures etc. are doing prior to replacing. However 9/10 it's where the issue lies! goodluck!

 

[farqueue]

Okay thanks to all contributors & I will report back when it is all done! I have a gas safe man to call.

))

 

[farqueue]

Well guys......finaly had the heat exchanger renewed and.......BINGO! (ish) hot water is now no longer cycling, I was amazed at how little sludge was in the exchanger but it did the job....mostly LOL.

I now have the same problem in that no matter where i put the temp selector for DHW on the boiler the water stays the same HOT temperature! My nephew (Gas safe) is gonna come back and test the gas valve & voltages on PCB I will again report back

 

[diamondgas]

Thanks for getting back farqueue .... Are you sure the control knob spindle hasn't snapped, you can see them turning the potentiometer on the back of the PCB! Just second checking! You replaced the hot water sensor yeh?

 

[farqueue]

Awww! well I DID check it BUT you have planted the dreaded seed of doubt now ......I know it will be okay, but I will check again and PROMISE to own up if it's broken hahaha

 

[farqueue]

And yes I have changed the hot water sensor ;-)