Potterton Lynx2 CH Overtemp Prob

[thenorm]

Hi All!

I've been reading the forum for a while now and find it great but this is my first post. I'd like to thank everyone in advance for any help given.

I have a Potterton Lynx 2 Combi boiler which is about 11 years old. Last week it just started showing the reset light all the time. I decided it must be the PCB so I fitted a new one. Everything seemed to work but then I noticed that the CH would not respond to the temp dial on the front of the boiler. When the CH is switched on the boiler burns at full flame until the CH overheat thermostat kicks in and cuts off the flame. This really confused me as the hot water temp dial works correctly and the flame changes between high and low and off to keep the hot water at the temp set. I suspected the CH temp sensor may be faulty so I got a new one but there is no difference. I then thought the new PCB may be faulty so I exchanged it for a replacement but again the problem is still there.

Apart from the CH overheating, everything works properly - the pump runs and the radiators get hot etc. One strange thing I noticed was that when the CH gets to a temp that I believe is above the temp set by the dial on the front, if you turn the dial to the middle and then down to minimum quickly, the flame momentarily becomes lower, then returns to full flame accompanied by a single click sound which sounds like one of the relays on the PCB. I thought at one point it may be the gas valve but then didn't understand why this would work for the hot water. I'm running out of ideas now - does anyone have any suggestions?

 

[thenorm]

I've tried something else today. I short circuited the CH temp sensor to see what would happen. When I did this, the PCB correctly detected that the temp was too high and switched the flame down to minimum, followed moments later by the flame being extinguished but the pump still circulating so it seems to me it was reacting correctly to being told the correct temp had been reached (or exceeded). This being so, I can't see the problem being anything other than the CH temp sensor being faulty or the CH temp dial on the PCB being incorrectly calibrated for the output of the sensor but I've already tried 2 new PCBs and a new temp sensor

Any ideas would be gratefully received.

 

[weargas]

have you tried running the ch and turning the hw stat down to see if it modulates? I'm not sure if there's microswitches in the diverter valve that alter which pot will be used on the pcb (maybe someone else will know). alternatively trace the wire back from the diverter to see if it's only 3 wires going to the molex plug.

 

[thenorm]

Thanks, I'll have a go at this and let you know.

 

[thenorm]

Right, I've tried turning both of the temp dials on the front of the boiler to minimum but the CH still doesn't modulate the burner. I've read my boiler manual and it refers to a "motorised three way valve" which I am assuming is a diverter and it only has three wires. I assume (I assume a lot ) these are live, neutral and earth as the colours are brown, blue and green/yellow and it has a sticker on the lead saying "this device must be earthed".

I'm still a bit flummoxed at this point (when was the last time you heard the word flummoxed eh?)

 

[weargas]

well there's no control microswitches in the motorised valve so that's that.
it's a long shot but you may want to check the wiring to make sure you've got a dead short between the pcb and the sensor. because these sensors are ntc sensors if you're getting a resistance down the wires theoretically it could give the pcb a false signal. so if you're not familiar with how to do this,

disconnect the wires from the sensor and the pcb. attach one mulitmeter probe to the wire at the thermostat end and the other multimeter probe to the pcb end of the same wire. your multimeter should be set to measure resistance. you should get a reading of 0. repeat with the other wire.

 

[thenorm]

Thanks Weargas, I'll try this. I don't have a multimeter myself but I am borrowing a friend's tomorrow so I'll see how this goes. I guess if I get resistance on the leads I'll need to change them somehow.

 

[thenorm]

I've tried using the multimeter but am unsure of the results. I connected it to either end of each of the leads going to the CH temp sensor in turn and got readings fluctuating from 100 - 300 with the multimeter set to 2000 ohms. I then tried this on the leads going to the hot water temp sensor and got much the same result. Funnily enough, when I reconnected everything the hot water temp control didn't work! I found that this was caused by me shoving the multimeter sensor in the connector end of the lead end bending the connector open so it wasn't making contact with the PCB pin. I bent the connector back into shape and the hot water worked again. I then hoped this was the problem with the CH so I tried bending the connector into a shape to get good contact but this didn't fix it. It does make me think that the signal from the CH temp sensor isn't getting to where it's supposed to be going so I'll try more investigation into this. I could try replacing the leads connecting the CH temp sensor to the PCB but as the connector at the PCB end has several wires in it I'm not sure how I would do this.

 

[AlanE]

Funny I repair boilers for a living, all makes and types, and relativly rare to replace a PCB so I never cease to be amazed when someone says 'I decided it must be the PCB so I fitted a new one'.

Why the PCB? Well everybody says they keep failing. They appear even more unreliable when they are changed willy nilly.

thenorm says the radiators APPEAR to be hotter. Get a thermometer and actually check.

It would be cheaper to pay a proper heating engineer in the long run! You can howveer take comfort that sooner or later as you 'decide' to replace your boiler components bit by bit you will eventually get to the faulty part. At what cost would be anybodies guess.

PCB's are a little bit more reliable than many seem to give them credit for.

Alan

 

[weargas]

PCB's are a little bit more reliable than many seem to give them credit for.

except those on a potterton puma

'I decided it must be the PCB so I fitted a new one'

you'd be surprised the amount of heating engineers who do that alan. my supplier had to stop taking pcb's back because people were taking them to try and then returning them when they found it didn't fix the fault. problem was they might have damaged it in the process. The amount of engineers to claim to be experts when they know very little about the actual working process is quite astounding.

 

[thenorm]

Thanks ALanE. I did some investigation and if you read my first post, the replacement PCB did cure the major problem (i.e. Reset light on all the time, nothing worked). The only thing confusing me now is the central heating being too hot. I could get a thermometer and check the temp of the CH but I know it's too hot because the boiler burns at full flame until it cuts out (the gas valve isn't modulating the burner) with the radiators hotter than they've ever been. The confusing thing is that the gas valve does work (i.e. modulates the burner) when used with hot water. You can see the flame on full, then reduce as the set temp is approached and then go out when temp reached. Then it will kick back in pretty quickly to maintain the temp. The other thing is, if you set the HW temp to minimum, wait for the temp to be reached (i.e. flame low or off), then turn the HW temp up higher the boiler reacts and the flame goes full again until the higher temp is reached. With the CH temp set to min if you wait for the boiler to cut out then turn the CH temp to full nothing happens (flame doesn't restart).

I won't be replacing any more parts as I don't see what else could be causing the problem so I'll get a local CH engineer in and see if he can throw any light on this (unless you would like to hazard a guess). I still think the new PCB has a minor fault but we'll see...