Potterton 100 with DHW but no CH

[ftgpmb]

My boiler doesn't fire up the full burner when I request CH.

Read couple of posts on here and followed the trouble shoot in the manual. HW works fine, pump spinning, fan on/off after HW. Thought I'd isolated it to the CH temperature sensor. Borrowed my neighbour's Ohm meter and get 3.3 k Ohms accross the CH temp sensor (I get 10 k Ohms accross the DHW sensor). Trouble shooting guide says I should have >7 k Ohms at room temperature so not working right? but CH is a bit warm because the DHW had been firing up, so perhaps its not the CH temperature sensor after all.

Any ideas how I can rule it in our out? I'm worried it's a PCB problem.

 

[ftgpmb]

So now I've let it cool down a bit, resistance has gone up to 7.6 k ohms - so probably not the sensor. When I connect it back up, I hear a relay somewhere on the board go click - which I presume is a good sign?

so what could the problem be? what should I check next. The trouble shoot guide says CHECK APPLICANCE WIRING TO MODULATOR BOARD (21-18867) REPLACE ELECTRONIC BOARD IF WIRING IS OK

 

[Gasguru]

Is this a Puma 100 or a Combi 100? Does the boiler work OK on hot water?

First thing to rule out are problems with the room thermostat or time clock. Bridge these out to test.

On the above boilers broken joints on the board(s) are a very common problem. Sensors are normally reliable (except when water leaks on the seals corrode the connections).

Ignore troubleshooting guides; they are notoriously unreliable and full of mistakes - you could end up swapping half the boiler parts using them (which is what the manufacturers want you to do) .

 

[ftgpmb]

did some further research on this problem on this forum and seems plenty of others have had this and it was the PCB. Sure enough, removed it, turned it over and there were the blackened joints.

Going to attempt the solder repair recommended here: //www.diynot.com/forums/viewtopic.php?t=42964

wish me luck! by the way, there is a very chunky looking capacitor on this board. should I discharge it through a resistor before I start working? anyone?

 

[ChrisR]

No need, it's low voltage.
Your board will have several dozen bad joints if it's typical. and blackened relay contacts visible through the plastic box(es). Resoldering may well make it better for a while, but...... !

 

[baxpoti]

My boiler doesn't fire up the full burner when I request CH.

More likely to be the wax capsule or pump or both!

 

[ftgpmb]

I can see that pins 1,2 and 8 on PL8 look cracked or scorched. Actually the relay contacts look quite shiney. I'll give it a go and see where we get to.

But if I end up needing a new PCB, given that the boiler is about 9 years old, is it time for a boiler rather than spend 120 pounds on a new modulation board? If so is there a good replacement with perhaps a bit more CH power (we've added an additional radiator and this boiler struggles a bit with them all on - when it's working obviously!)

 

[baxpoti]

This boiler will run 20 large radiators (5000btus) easily if piped up correcttly

 

[ftgpmb]

resoldering the dry joints worked great. We're back in business again.