Potterton puma 100e - hot water OK, CH not igniting

[Shep]

I have a puma 100e which has just developed a fault - the hot water is working fine, but the central heating seems to be completely dead... when the timer calls for heating (or I switch to "constant"), I hear a faint click a second or so later (like a relay switching somewhere?), but nothing happens - pump doesn't start, boiler doesn't fire up... the machine is silent.

The pump and fan _do_ seem to be running fine when there's a demand for hot water, and the red diagnostic LED on the system board lights when the hot water is running and switches off as soon as the demand stops. The pump will run for a couple of minutes after the hot water is turned off, and I can feel the central heating pipes getting warm.

Possible cause: About a week ago, I discovered that the expansion vessel on the heating circuit was short of air (after observing some wild fluctuations on the system pressure guage as the system warmed up, and the occasional random venting of water). I topped up the expansion vessel to the recommended pressure using a car tyre pump. This seemed to work well - for a few days, the system pressure needle was absolutely stable and the heating seemed to be running better than it has for years... then, 2 days ago... totally dead.

To be honest, this boiler is a bit of a turkey... we inherited it when we bought the house, and over the last 5 years I've ended up replacing the diverter valve, pump, fan, system board... I'm kind of getting the message that it's time to stop throwing good money after bad and get a new one fitted, but if anybody could suggest something to keep the beast limping along for a little while longer, I'd be much obliged

 

[ChrisR]

If you don't have the servicing/fault finding instructions they'e at

http://www.partsarena.com/baxi/System/DATA/Dx/DS1/installation/2245/I04-2245/image1946.gif

Drill down from the http://www.partsarena.com/baxi - bit bigger.

You would check that the On signal is getting to the boiler and the front panel switch is working. But as you're hearing a click, they're probably OK.
Next is the CH temperature sensor which should measure 7.5 to 15k at room temp,
then the main pcb - again

If its the main pcb at least they're around cheap - I have several.

Pumas are good for blokes like me because they go wrong fairly often, but to be honest you've already changed most of the dodgy parts. Some people have no trouble at all from them!

 

[Shep]

If you don't have the servicing/fault finding instructions they'e at...

Thanks for the link - fortunately, I had a copy of the servicing instructions already. Unfortunately, I can follow the chart all the way down to the box that tells me that the system is working normally. Except it isn't

 

[Shep]

Just answering my own question (in case somebody comes and does a search on this fault in the future...)

Dry joint on the PCB - resoldered, and now working perfectly

 

[chrishutt]

How do you spot a dry joint on PCB? What does it look like?

 

[gazthepottertonengineer]

dry !!!!

also if this problem occurs again a dripping hot tap can cause this to happen as well

 

[chrishutt]

gazthepottertonengineer wrote

dry !!!

So a dry joint on a PCB looks dry.... does that mean not as shiny (wet?) as other joints? And should one confirm the fault with a continuity test between board and component (assuming no other route for current)?

 

[bradleybear]

A dry joint is dull, and gives poor electrical conductivity. Normally happens on the relays on these, excessive heat caused by resistors on PCB and vibration (clickety click) from relays makes it bad connection. Give the relays a gentle prod with an electricians screwdriver and you can normally find the dry joint.

 

[gazthepottertonengineer]

normally happens on neutral connection you mean top left always burns out normally burns pcb away as well

 

[bradleybear]

Everytime you go to one of these, total fail, cust says they can smell TCP from boiler, you know its that dodgy neutral soldering on the top left. The TCP gives it away I suppose! As well as the black scorching and smoke coming from the board!

 

[ChrisR]

The corner thing kills the whole boiler or makes it intermittent though. The relays don't last as long as they might, either. Sounds like this is not the usual joint!
I once went over a puma board with a microscope - found a couple of dozen cracked joints!

Dry joints are (loosely) where the solder never "whetted" the surfaces. So it's in physical contact, but doesn't remain so after things have vibrated a bit. If the joints move a lot (thermal expansion), or the holes are too big on the board (like most potty boards), the solder can run out of ability to stretch, and it cracks. Some call it high-strain fatigue. (Not like true fatigue, which typically takes millions of cycles). You end up with a crack, which eventually makes a dry joint. You can see them as slightly dark lines across the solder.
There is a break-type which often occurs around the leg of a component. Especially heavy on poorly supported ones. Sometimes if you wiggle a relay you'll see its leg move to and fro within the solder.

The b******d joint on the top left of a puma board appears to be because it has to carry too much current, so melts the solder.