Laars LD400NL Low Nox Heater Will Not Stay On

[pattenpool]

I am working on a Laars LD400NL heater, low nox version with fan, of which there are very few out in the field. The problem is the heater will fire every time but shuts off after 2-3 seconds. I suspect the flame sensor, but not even Zodiac has the correct part number. They show the R0334300 which is a standard LD flame sensor integral to the burner. The LD400NL has a totally different burner and the flame sensor in this one was previously retrofitted with one that has the 45 deg bend in it. It appears a hole was drilled above the 2nd and 3rd burner to insert it.

When the heater shuts down, I am still getting 24V at the IND and TH terminals, gas pressure is around 7.5" and drops to 6" when valve opens. I am out of answers other than the flame sensor (also tried new R0480100 IGN Control) but I do not have a way of "jumping out" the flame sensor and I can't find a source for the correct flame sensor part number. Any help would be appreciated.

 

[danpik]

You don't need to replace the flame sensor, just clean it. Use a piece of steel wool and scrub it real well (the metal part). Also clean the metal surfaces near it so the rust does not insulate the electrical current. The sensor only provides a path to ground for a small electrical current. The flame provides the rest of the circuit. 35 years in the home heating business and I have only ever had to replace 1 sensor and that is because the home owner was messing with it and broke it.

 

[ps0303]

I've cleaned a many of them with maybe only a 5% success rate. I've cleaned them and have been called back after a few months with the same exact problem. I replaced the sensor and all was good with no call backs.

 

[danpik]

I've cleaned a many of them with maybe only a 5% success rate. I've cleaned them and have been called back after a few months with the same exact problem. I replaced the sensor and all was good with no call backs.

interesting...Wonder if it is a weather/humidity thing. What are you cleaning them with. Sandpaper is a big NO-NO.

 

[pattenpool]

Out of curiosity, why is sandpaper bad?

 

[danpik]

Out of curiosity, why is sandpaper bad?

Scratches in the flame rod material can be a source of continuing corrosion. Sandpaper also sometimes contains silicones. These will act as an insulator to the microamp current that gets rectified thru the rod. Remember, we are dealing with current in the 3-5 microamp range and it does not take much to insulate it. Every furnace tech class I have ever been to has been adamant about how to clean them. I have even used a folded dollar bill in a pinch to polish one