Air fuel ratio issue on Navien NCB-240.

AviP

A customer of mine has a persistent issue that seems hard to diagnose. It seems that every time that there is a significant time interval between the last call and the current call for heat or hot water, the boiler fails to restart and throws the code E003 (Ignition Failure). The usual suspects have all been checked: power is on, gas line is on, flame rod ignitor has been replaced, water pressure is good.

Navien tech support has obviously been called and after going over all the options, one suggestion was to manually "choke" the air inlet. Interestingly, this works consistently at restarting the boiler but the problem might resurface again the next day. Apparently, this has to be done on a frequent basis by the customer now that winter is upon us.

Effectively, this solution is a stop-gap measure as the customer cannot take vacations as the boiler might not restart and the house will freeze over, burst pipes, you get the idea. The house is located at 500 ft altitude, and the boiler dip switch settings are appropriate (Front panel DIP Switch set of 10: 4-OFF, 5-OFF). The boiler runs on propane and the dip switch is set right (Front panel DIP switch set of 2: 2-ON). Boiler was installed in Dec 2014 and ran error free until Jan 2019. This problem has persisted since Jan 2019.

So my question is: What can be done to correct the air-fuel ratio for a cold start?

mattmia2

Is the equivalent pipe length of the vent and air intake more than the minimum? Is there a screen that has fallen off the inlet or outlet? The rate that the fan and gas valve modulate together seems to take in to account the effects of the supply and vent piping.

AviP

It's about 70" on the intake and 45" on the exhaust. Both have exterior screens that are still good.

Jamie Hall

While you're at it, check the regulator on the tank. If it's sticking or failing (not unheard of) it could behave when the thing has been run frequently, but hang up when it hasn't.

mattmia2

Probably wouldn't hurt to clean the inducer fan either, dirt might make it perform a lot differently than expected as the speed is varied.

AviP

Jamie: Are you referring to the regulator for the propane tank or something internal to the boiler? Propane tank regulator looks quite rusty but it was installed at the same time in Dec 2014.

Mattmia2: I'll work on the fan. It's a PITA to access.

I'm leaning towards backdraft issues so I'm also going to open both vents as well and look around, especially the exhaust. Does this unit have an exhaust valve that opens and closes as needed or is the exhaust always open.

DZoro

What is your gas piping length from secondary reg to the boiler, what size, and type of gas pipe.
Incoming standing pressure and operating pressure?
Suggest so far is changing the secondary regulator, possibly also the first stage.
Have they done a rpm test on the inducer?
Rpm test on the fan will tell if the damper or inducer is a issue.

I have removed the internal damper against naviens wishes, but have had 100% success with the units having a damper / fan rpm issues and or high wind situations.
D

Jamie Hall

I'm looking at the regulator on the propane tank. Not saying it's a problem -- but don't forget to check it.

AviP

I will check out the pipe and regulator next time I'm in.