Potterton Profile 40e boiler problem - turns itself on

[RodneyJ]

I have a Potterton Profile 40e boiler in a straightforward fully pumped system with two 2-way diverter valves, one for hot water and one for central heating.

There are two problems, both intermittent, though the first happens much more than the second.

1. When the boiler stops or is turned off, the pump continues to run for a long time. I know the pump must run for a short time to disperse the residual heat in the boiler, but it can continue to run for an hour or more.
The pipework is all under the floor or otherwise inaccessible, so I can't check the pipework layout to see whether there is a cooling circuit loop which is not shut off by the diverter valve. However, when hot water only has been on (no central heating) the bathroom radiator does not get hot, so I deduce that the bathroom radiator is part of the diverter-controlled central heating circuit, and not in a separate cooling loop.

2. The second problem occurred several years ago, then stopped, and has now started occurring again. The boiler turns on even when all the programmer controls are set to off. The only way to turn off the boiler is to turn off the power at the mains switch (there is a separate switched plug socket for electrical power to the entire heating system). When the power is turned on again, the boiler promptly fires again.

The programmer is a Sunvic SP50. I thought it was a programmer fault, so I have just replaced the SP50 with a new Sunvic Select 207XLS. However the new programmer made no difference.

The only thing I can think of now is that there is a fault in the PCB in the boiler.

Can anyone offer any help or advice about this please?
(1) is the problem caused by a fault in the PCB in the boiler?
(2) if so, how easy is it to replace the PCB?
(3) a new PCB seems to cost anywhere between £120 and £155 (+ carriage), depending where you buy it. Having just spent over £30 on a programmer which seems to have been unnecessary, I don't want to spend a lot more on a PCB which might be unnecessary; does anyone know of any way I can get a replacement PCB and try it, then get a refund if it is not a PCB fault? Is there a source of cheap used-but-working PCBs?

Many thanks for any help anyone can give with this.

 

[twgas]

Not likely to be the PCB. The most likely cause of the boiler firing when it shouldn't is one of the end switches in one of the motorised valves.
The pump overrun problem is likely to be the by-pass not adjusted correctly or partially (almost completely) blocked, or pump 'stat is faulty.

 

[powell30]

Your second problem is almost certainly a faulty zone valve, check which one and change.

 

[RodneyJ]

Thanks Mick,
I put in a new pump a few years ago, and the pump overrun problem occurred with the previous pump, so I guess it's not the pump 'stat. Damned if I know how to check or adjust or even find the by-pass, because all the pipework is hidden and virtually impossible to get at. I'll explore and see what I can come up with. Thanks again for your help.

Thanks to both of you for the comment on the zone valve: that's something I can probably handle.

Rodney

 

[twgas]

The pump 'stat is on the boiler not the pump so it can cause the problem whatever, or however many,pumps you change. If it is the pump 'stat you will need a RGI to change it as it involves removal of the sealed part of the boiler.
Can you post some pics of pump, valves, cylinder and pipes etc. we may just be able to point out a few things if they are visible.

 

[RodneyJ]

Thanks again Mick. I'll try to do a few pics and try to learn how to post them. It will be a few days before I can do this.

 

[RodneyJ]

Hi again,

I've done a few photographs of the central heating arrangement. All the controls are in a cupboard with the DHW cylinder. I have also done a schematic of the pipework (not the electrics) in the cupboard, which might help as it is more clearly laid out than the photographs. I deduced the direction of water-flow by turning things on and off and feeling which bits of pipe got hot and in what sequence.

Photos and illustrations are:
overall view pipework schematic
photo - overall view 1
photo - overall view 2 closer
photo - overall view 3 close-up
photo - Danfoss Constant Flow Regulator
photo - DHW motorised valve (CH valve also visible)
photo - unidentified valve on DHW tank return

There are two items (shown in the schematic and specific photographs) which I do not understand or cannot identify.

1. There is a device called a "Danfoss constant flow regulator type AVDO" with which I am unfamiliar (red in the schematic, also shown close-up in a photograph). A slider on top of the device is set at the extreme end, i.e. "minimum" position.
At first I thought that this might control a by-pass circuit and thus be the cause of the pump over-run problem (reasoning: minimum = closed = no water flow) but now I am not sure, because it is clear that water flows through it. It also seems to incorporate a one-way valve. I have not tried to adjust it, preferring to wait for more expert advice before I do something which I might regret. If I need to adjust it, (1) do I just push the slider along or do I turn the screw in the end of the device to move the slider, and (2) what position should I adjust it to: mid-point or maximum or a bit at a time and try-it-and-see?
I do not understand the purpose of this regulator. If it is causing the pump over-run problem, how does it do it? Even if the regulator is fully open, the hot water in the boiler circuit cannot go anywhere extra to cool if both the motorised valves are closed, so what is the regulator for?

2. On the return pipe from the DHW cylinder heat exchanger is something that looks like a gate valve without a handle (green in the schematic). I do not know what this is, but I assume it is simply a valve to isolate the DHW tank if necessary.

If there is a problem with one of the motorised valves, as suggested, how do I check them and find out which has the problem? They are both Danfoss Type HPA 2, and both are set to "automatic".

Very many thanks for your continued help and advice.

Rodney