I have a 1996 Luxaire condensing high efficiency model PBLU-LD10N060A. It was a natural gas furnace but when we installed it in 1996 we only had propane so the professional installer converted it. Since then, it has been trouble free and the only thing that has ever been replaced was the hot surface igniter. Recently we installed a natural gas line to our home and had the furnace converted back to natural gas using the original NG jets (number 44) and the original regulator spring. After an adjustment of the WC pressure to 3.5, the furnace seemed start and work fine. A few day later I noticed the furnace was having a periodic ignition problem—every so often it would have about a 3-5 second delayed light-off and make a nasty loud “woofff” noise as it lit off. It is bad enough that you can see the sheet metal exterior “oil can” outward and it sometimes even blows the flame out. I have had two professional HVAC companies look at it, cleaning burners, checking in and out gas pressures, igniter resistance, timing sequences, etc. and they can find nothing wrong—usually when the service people are here the ignition and everything else it works fine.
I found that the delayed ignition problem usually occurs only after the furnace has been sitting idle for more than 20-30 minutes or so. I also found a way to force the furnace to always start normally after it has been idle. To “force” a normal start I do the - 1) Begin with a normal call for heat which starts the draft inducer fan. 2) A few seconds later watch the igniter begin to glow. 3) The igniter glows bright orange for about 17 seconds then 4) Listen for the gas valve “click” open. 5) Shortly after the “click” and BEFORE it has a chance to light, shut off the furnace power switch. 6) Wait about a minute. 7) Switch furnace power back on watch the normal start sequence as above. 8) The light off is quiet and normal every time after this procedure is done.
Any ideas why the recently converted NG furnace would have a light off problem like this only after it has been sitting idle for 20-30 minutes?
Thank you in advance for any ideas,