Lynx fault.

[andsam]

Had probs with a Lynx today. When cold it will fire no probs for ch or dhw but after 20 secs it starts knocking off and firing back up. All the pcbs have been changed and the sensing electrode. I have changed the air flow sensor ( second hand ,getting a reading of approx 2.2 resistance) but it still does it .If i remove the sensor and bridge it it works OK. This does sound like it is the air flow sensor I know but i feel i may be missing something. Also does anyone know if i can still get these sensors.
Cheers to anyone who can help.

 

[ollski]

There's a recent post on this and I think the fault is the wires to the stat (yellow I think) and no, the airflow sensor apparantly is long gone.

 

[ChrisR]

reading of approx 2.2 resistance

2.2 Ohms is OK afaik - they quote about 1.5 cold.

 

[Agile]

A few tips!

Potty deleted the spare air flow sensors about november 2004. T&T still had a few left.

Check the resistance at the disconnected plug to the PCB ( check but 6+7 I think ).

Tighten the spade connectors to the switch. As the resistance is so low its a very critical connection.

Others have suggested the leads can "break down". Since its a fibreglass outer sheath I find this hard to immagine and I dont know what this is meant to mean either.

The resistance measured at the PCB plug is about 2.2 ohms at a 20*C room temperature. With the fan on drawing in external air at 6*C this falls to about 1.95 ohms.

The boiler looks for a "low" resistance meaning the fan is drawing in external air. As the sensor ages it probably increases its resistance.

As a test experiment, start the boiler and link the sensor connections on the PCB and see if that sorts out your fault.

You can replace the element in the sensor with a couple of turns of 1 kW electric fire element wire but its difficult to solder as its NiCr wire.

Should I offer a recon sensor service ???

Tony Glazier

 

[andsam]

Thanks for the replies .I am going back tomorrow and have managed to get a new air flow sensor ,i know they are like rocking horse s**t. If it dosent sort it i will remove my spares and through the boiler out of the window (2nd floor flat) it wont sort the boiler but i will feel a little bit better.
Thanks again.

 

[ollski]

The leads are fine when cold but as they heat up with the boiler on they lose continuity and knock the boiler off....don't ask me how or why, it's just a fact. Continuity of the air flow sensor should be 1.5 to 1.8 ohms.

 

[Agile]

I am left wondering if this is really the spade connectors?

Operating in a harsh environment they are not really suitable for a connection where consistant low contact resistance is essential.

I always severely tighten up both the female spades to prevent contact resistance causing a problem.

Tony

 

[ChrisR]

You can replace the element in the sensor with a couple of turns of 1 kW electric fire element wire but its difficult to solder as its NiCr wire.

Should I offer a recon sensor service ???

Agile says you can botch your boiler safety components with bits of wire. Hmmm!

 

[andsam]

For anyone who is interested been back to the job today. Replaced air flow sensor still faulty . Replaced yellow wires all the way back to the molex plugs connected to existing flow sensor still faulty. Replaced flow sensor and connected to new yellow wires , working perfect. Boiler will be in the back garden if it goes down again. Thanks for everyones input.

 

[mmalone]

For anyone who is interested been back to the job today. Replaced air flow sensor still faulty . Replaced yellow wires all the way back to the molex plugs connected to existing flow sensor still faulty. Replaced flow sensor and connected to new yellow wires , working perfect. Boiler will be in the back garden if it goes down again. Thanks for everyones input.

Am I reading this right ??? it only worked with the NEW FLOW SENSOR AND WIRES ?? Not wires or NEW FLOW SENSOR seperately ??? Strange if correct.....

 

[andsam]

This is correct both componants had to be replaced. I have had it on these boilers where the fan wont start unless the combustion cover is on can anyone explain this .

 

[Agile]

I can only assume that both leads and sensor were "faulty". Or perhaps as I highlighted above the spade connectors were not making a low resistance connection.

If the cover is off then the fan draws air in from the room and NOT through the flow sensor so the element is not cooled as required.

I am not encouraging anyone to replace the sensing element! All I said is that it CAN be done! With supplies of the sensor no longer officially supported by the manufacturer some people might be tempted to do just that! We still have one in stock!

Tony