How long should DHW Pump run after no demand? Boiler Lockout

[steved123]

I have a WBosch CD53i (Mark 1) which has recently started to Lock Out after running Hot water demand for a long time (e.g. Filling the bath). Short bursts of demand are ok. According to the installation / servicing manual, the fail point (G) indicates to check the CH pump is working, then the DHW pump, and then a few voltage checks on the control board.

The DHW pumps would appear to be working (it makes some noise when demand is on), and the after only 10seconds or so after the demand ceases, the pump stops.

My questions are:

1) How to be sure the pump is rotating?
2) How long after demans should the pump run?
3) Surely the DHW pipe work is not a loop like the CH pipework, so what good does it do to run the Pump after the demand has finished - surely there is no where for the water to disperse?

 

[COG]

First off never herd of a CD35i , you are describeing a 35 CDi mk 1. It has two pumps instead of a diverter valve so the dhw pump will run on for a short will just to displace some heat but its maily cooled down with the fan. The timeings etc you will find in the manual at the back in the flow charts not in the fault finding charts. Your boiler sounds as if you may have a partial restriction in the plate heat exchanger .

 

[corgiman]

I am sure that its a 350

 

[steved123]

Sorry, major typo in my original mail: It is a Worcester Bosch 35CDi Mark 1 - yes the one with two pumps in.

How does one get a restriction between the plates?

 

[Agile]

It could also be lime scaling in the primary HE.

Does it make any kettling sound? ( Thats like a kettle boiling! )

Tony