worcester bosch 28cdi hot water temperature not constant

[KSAdevelopments]

Hi all

I'm new to this forum and wondered whether anyone could help me with my conundrum.

I have a Worcester Bosch 28cdi boiler which is situated on the ground floor of my house. Currently the hot water fires up, runs for 30 seconds to a minute and the burner switches off and then fires up again despite continued demand for hot water.

I have checked the diverter valve and the micro switch is activated and remains activated all the while the tap is on. Both heat exchangers have been cleansed and descaled. DHW and CH thermisters have been replaced plus the overheat thermostat. My installer was flummoxed as am i. The gas valve seems to modulate for the heating and if you turn the hot water temperature down on the boiler to minimum it modulates at that stage too. therefore we decided it was the pcb, so having replaced that too, at £200, we thought it was good to go! alas no! Spoke to Worcester bosch theirselves and he said that it was likely to be a peforated diaphram in the diverter valve, but as you can see from earlier text, that doesn't appear to be the problem either.

The fan runs on demand from the ch and dhw

So any help or other ideas that anyone an give would be much appreciated!

thanks

kev

 

[scatmanjohn]

Hi...How have you descaled the dhw heat ex. Its sounds like that could be the problem.
Has your engineer measured the relevant temperatures which would give a big clue to the fault.

 

[Agile]

He said "both" heat exchangers.

I would have little faith in this fellow if he cannot diagnose such a simple fault.

Since I suspect it lies in CORGI engineer territory, get him ( or better someone more capable ) to loosen the combustion chamber cover and see if it runs with this partially open.

Tony

 

[scatmanjohn]

Agile wrote:

He said "both" heat exchangers

I know.

 

[KSAdevelopments]

thanks for your replies.

the DHW heat exchanger has been cleaned 3 times with spirit of salts. I don't know if he measured temperatures as unfortunately I was at work and didn't see. What temperatures should he have measured?

As for running the boiler with the front cover slightly open, this has been done but doesn't solve the problem

kev

 

[scatmanjohn]

You should measure the temperature of the flow out of the main heat exchanger. Its best to use an electronic thermometer that can be clamped to the pipe within the combustion chamber close the the CH sensor. If its getting up to 80*C turning off then cooling down and relighting i would be replacing the plate heat exchanger.

 

[ChrisR]

Blockage somewhere else on the DHW loop? Only takes a flake of "rust". Thermometers should point to it.

 

[Agile]

Has the air pressure switch operation been electrically monitored at the point that the boiler switches off?

Has the pump been removed and the impeller been inspected? Has the torque of the pump been checked?

Tony

 

[KSAdevelopments]

Thanks for those I will try them in the morning. Although I don't think the air pressure switch and pump were checked, i assume because the heating side seems to be working fine, if anything it seems to be hotter than it was before.


no doubt i'll be back, if the past couple of weeks have been anything to go by

kev

 

[Agile]

I am not asking or encouraging you to open the combustion chamber. Doing that would disturb the operating conditions.

You could, if you are competent to do so, monitor the voltages at the PCB end of the cable from the APS. Use the CH parameters to compare the operation with a DHW demand.

If you dont understand how a combi boiler works then you could make mistaken assumptions about DHW and CH operating parameters. The CH requirement is normally far less than the DHW if the boiler is set up correctly and so the combustion conditions are very different. It could work fine on CH but not at all on DHW.

The pump impeller can be inspected for debris and by momentarily running it out of place a finger placed on the smooth surface should be able to stop it turning with an index finger but hardly with a small finger.

Tony

 

[KSAdevelopments]

Hi all

The boiler tends to shut down between 75 and 80, according to my thermometer at the place that scatman suggested. It cools off and re ignites at about 55. I also measured the temperature at the pump and the DHW exchanger, and they matched the temparature of the water out of the main exchanger.

hot water output from the exchanger would rise to circa 40 c from a 19c start.

The voltage from the aps at the pcb is 5.15 while the boiler is not meeting a demand. When demand is activated the voltage goes to 0 in either mode.

The pump does let out some dirty water, when you open the inspection screw. I assume that I would have to disconnect the pump from the system to run the test you suggest, tony.


thanks

kev

 

[KSAdevelopments]

Hi again

just re reading the notes you guys have posted, i realised that I was supposed to measure the voltage when the boiler shuts off the burner. I did that for the hot water and there is no voltage to the aps while the burner is alight. voltage returns to the aps position on the pcb once the fan has stopped prior to re igniting the boiler. Is that how things should work?

thanks

kev

 

[scatmanjohn]

Hi... if its getting up to 80*C and it is still on high gas, then, my money is on the dhw heat ex.

 

[spider778899]

qwert

 

[simond]

I expect he has been waiting the last 3 yrs, just for that fascinating insight.

Lets hope you are right, otherwise it could be another three years without a bath.