Old Potterton Kingfisher MF Over Run Issue?

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OK let me give some background in the hope someone can help me with this old system:

S plan fully pumped CH & HW controlled via 2 single actuator valves.

Boiler installed 1998 and given pretty much trouble free service with the exception of the gas valve replaced around 2005, the Drayton SCR receiver failed and was replaced about 3 years ago and the fan packed up last year but was an easy swap. I maintain the system pretty well with an annual flush, inhibitor and bleed and know it inside out.

Earlier this year I noticed that when the heating turned off the pump would pulse on/off for around 30 seconds and give up but worked perfectly at all other times.

As I am pretty confident with electronics and plumbing I decided to diagnose myself and worked out that it was probably a bad PCB but as a few items were dated I decided to replace the actuator heads myself first, they are all working fine and did not solve the issue.

I then bought a brand new like for like PCB and fitted in the boiler and this solved the problem instantly but...

I have noticed that when the CH or HW turns off the pump stops immediately and then restarts about 10 seconds later and only runs for about 30 seconds.

There is no other issues at all like lock out etc. and HW and CH works well.

Is this normal over run behavior on these boilers?

I would imagine that pump over run should last longer than 30 seconds?

From the boiler manual I cannot see that the over run is triggered or stopped thermally and it is a very simple boiler but there may be some logic built in that the over run is only triggered if the temperature in the boiler is X degrees?

Have I got an issue or is this perfectly normal?
 
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I have been thinking about this and notice that the original PCB was part number 407779 and the replacement part is 407779-03.

The new part has only one fuse holder and a hole where the other should be and according to the boiler manual the removed fuse is for pump overrun protection.

Can anyone advise if the overrun function was removed from these boards by design in the 03 revision or if the design has been modified in some way or have I got another faulty PCB?
 
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@ texecom,

On the Kingfisher MF boiler, the pump overrun on the PCB is a 'timer' rather than a thermal control and as you rightly suspect there should be a fuse where yours has none (F1)

You need a PCB with this facility : two fuses F1 and F2.

The pump overrun timer will only work if the pump is wired directly to the PCB connection, it will run for several minutes after normal controlled shut off.

You have two single zone actuator valves, so naturally with any pump overrun there must be no closed pipe circuits downstream of the pump so a bypass of some kind will be needed for the excess heated water to dissipate during overrun.

Fuse F1 protects the general supply and pump output on overrun.
 
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Thank you for your comments MrTherm!

My system is as you describe with a bypass valve fitted and all working fine with the exception of no overrun.

It seems odd that the overrun timer is removed from the 407779-03 version of this board as this is indicated as a like for like replacement for the 407779 unless in later boilers this board is used in Potterton switched over to a thermal triggered overrun system or similar.

I was hopping that the overrun circuitry is on the board and activated by inserting a fuse from the old board in the space but I may be clutching at straws with that and I cannot take the boiler apart to have a look for a few days so to avoid any issues I have managed to find a brand new genuine Potterton 407779 board which should be with me in a couple of days and cure the issue.
 
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you are mixing your systems up , you say you have gravity HW then also say you have 2 zone valves and that the pump runs when the HW only is on, this is an S plan fully pumped not gravity HW
 
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Sorry yes, you're correct...

I've edited my earlier post!

Thanks.
 
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Further to my recent posts, I had a conversation with an engineer at Baxi/Potterton today so I'll update this thread with my understanding of his explanation for anybody else who may be searching the part numbers or boiler mentioned.

The original Potterton Kingfisher MF PCB marked 407779 had 2 fuses and a timed pump overrun which activated the pump for a few minutes after normal shut off.

The updated Potterton Kingfisher MF PCB marked 407779-03 is backwards compatible with the 407779 BUT has the timed overrun removed in place of a temperature triggered overrun which triggers at 70 degrees C based on the reading from the thermister. This was a power saving feature added to avoid endless overruns of the pump when not necessary.

I guess it is down to personal preference as to which is better but each has it's merits.
 
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