BOSCH 28 CDI going very hot and then cold

[sean33]

Hi Everyone

Can you help please. The DHW on a28CDI is going very hot, and then the boiler shuts off for 10-20 seconds, and then fires backup again. I have de-sludged the system (twice), replaced the heat exchanger with a new one, installed a new thermostat, cleaned out the diverter valve, re-greased it and installed a new diaphram. The pump is on setting level 3. It also get very hot. Could it be the PCB? Any ways of testing this ? thanks

 

[bennygas]

if gas rate/bp are ok, i think you need to be looking at how the boiler's dissipating the heat produced. were diverter and old heat ex badly fouled? maybe check vanes on pump aren't choked up with crud?

 

[diamondgas]

Hi sean33 & :welcome: to the forum

If your hot water is getting to hot then I'd initially think you're domestic flow rate through the appliance is pretty poor, would that be the case? Is the hot water flow/cold water supply poor? Or just hot water flow?

 

[glenno1]

is the diverter valve pin staying in contact with micro switch when calling for dhw

 

[sean33]

is the diverter valve pin staying in contact with micro switch when calling for dhw

Hi Yes, the Diverter valve pin is staying in contact with the micro switch. I have done some temperature sensor readings and i have the following maximum outputs off the Heat Exchanger, starting top left with the DH water pipe (with heat sensor connected) 59.3C, bottowm left CH flow pipe 45C, Cold Inlet 18C, and finally top right on CH return plastic manifold 47C. The DH water pipe is increddibly hot! I have set the control knob from setting 1 up to maximum, and this makes no difference, it still cuts out.

 

[sean33]

Hi Guys, When i opened up the diverter valve the diaprahm was completely shot, melted virtually. parts were wedged in the gauze mesh etc.. But whole valve system has been cleaned out completely ane left in desclarer for several hours. Gave this a thorough cleaning, and re-greased it, and installed new diaprahm which is operating great. thanks. How would i check vanes on pump? I have taken off pump inspection cover and checked that it turns easily. Pump does get very hot though!

 

[sean33]

Flow rate is hard to distinguish, as house only has mixer taps installed. Cold water takes 11 seconds to fill 1 litre jug up, and hot water flow takes 13 seconds.

 

[glenno1]

Are you getting a good flow through your hot taps(Have you measured this)

 

[glenno1]

Sorry didnt see your last post.Have you checked filters

 

[bennygas]

am i reading this right - 13 seconds for 1 litre? seems ridiculously slow. if that the case i wouldn't be surprised if this was affecting things. about the pump thing - forget i said it - thought you were talking about the primary side of things, only on re-reading did i see you meant DHW. don't know why i did that, sorry mate.

 

[sean33]

am i reading this right - 13 seconds for 1 litre? seems ridiculously slow. if that the case i wouldn't be surprised if this was affecting things. about the pump thing - forget i said it - thought you were talking about the primary side of things, only on re-reading did i see you meant DHW. don't know why i did that, sorry mate.

No Worries, like i said there is no seperate taps in the house... not sure how to get an accurate water flow reading. The outside cold tap only takes 5-6 seconds to fill up 1 litre.

 

[sean33]

filters are all clean. cant see any blockages anywhere. Like i said, i drained and flushed the system twice with Fernox F3, with a day or two between drains.

 

[diamondgas]

Flow rate is hard to distinguish, as house only has mixer taps installed. Cold water takes 11 seconds to fill 1 litre jug up, and hot water flow takes 13 seconds.

60/11+5.45 l/min ... 28CDi = 13.5l l/min 30[SUP]o[/SUP]C .... I think you have cold water supply issue personally Could be other things but such a poor supply will inherit the issues you describe IMHO

 

[sean33]

I even removed the flow regulator from the heat exchanger to speed up the flow through it. what bother me is the temp on the DH pipe with thermostat on. This gets upto 60C when boiler in use. Will re-test on outside tap cold tap again, but flow was 5-6 seconds a litre. I have noticed the mixer taps have cut off valve before htem, which are probably not full bore. Will changing these help?

 

[bennygas]

stop tap's wide open is it?