Intermittent furnace light-off problem

[bsmithen]

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,
Bob

[how]

While this has not been my professional experience, some other tech's have talked about hot surface igniters glowing but not always get hot enough to ignite the gas. This has been over conversations of OEM igniters verses copycats or carbide verses nitride.
Considering how much more flammable propane is over natural gas makes me wonder if your igniter is sometimes taking too long to ignite the less flammable natural gas which is allowing for an excessive build up.
I wonder if your successful ignitions with the pre heating of the igniter are the proof of this possibility.

[Freon]

You might also check the induced draft system. Since the furnace is 16 years old the inducer may not be generating the correct draft on a cold start. Check the flue to be sure it's clear. I don't know the air/fuel ratio for NG Vs propane. When you first converted to propane, did they also change the inducer?

[heatseeker]

there are several issues that need to be locked at with your furnace. Gas pressure , location of hsi, proper orifices, no clogs in burners, properly operating gas valve. In other words you need more help.

[bsmithen]

I did question the on-site HVAC Pro regarding the draft system. In a nutshell, he said that because the pressure switches were not halting the light-off sequence, we can assume the draft system was ok and one of the first things I did was to use a flashlight and mirror to inspect the outside portion of the inlet/outlet. I have not run across any info that indicates the draft system is fuel specific although i still consider anything possible.
Even though the HVAC Pro working on the furnace says the hsi is within resistance readings (cold), the plan is to have an OEM igniter put in as early as today. I also noticed that the igniter (PN 025-33421-000) installation instructions call for 0" to 1/8" clearance between the hsi shield and the burner, but it looks to be closer to 1/4". Looking at this type of igniter and the way it mounts in the combustion box, I don't see a good way of getting it any closer to the burner--meaning bending the mounting flange of the igniter or anything like that to get it closer to the burner would run a risk of cracking the ceramic etc.
We'll frist see what a new igniter will do. Thank you for your feedback--further comments are more than welcome. Bob

[bsmithen]

I am still having an intermittent light-off problem. Based on ongoing information from many HVAC Pros regarding this model of furnace being known for lighting problems when using NG, now I think it can be nothing but the HSI simply not getting hot enough. Can anyone give me some insight about HSIs? I have tried two new 025-33421-000 HSIs and the furnace continues to experience a nasty, delayed light off only when the furnace has been idle for a while and the burner box is cooled down. Are there hotter HSIs available? I have found very little information on correct current draw for an HSI and nothing about how hot they should actually get. Suggestions?
Thanks,
Bob

[how]

Not all furnaces use 120V for their igniters. Have you confirmed you have 120V for the igniter you are using?. Is the diffuser in the burner which faces the igniter clear and corrosion free (especially on the side that the igniter sits on)? What altitude is your furnace at to determine that 3.5'WC is appropriate? Did you determine 3.5"wc with a digital or water tube manometer? How does your temp rise fit with the temp rise on your rating plate? When it does fire up, do all the burner flame shapes look the same? Does the igniter bracket allow you any variance to angle the HSI slightly deeper into the main burner flame path that doesn't involve modifying it?

[bsmithen]

Yes I have checked that there is 120v to the igniter and each of the 3 burners have been cleaned--especially the diffuser ends, and put back in the original positions. we are at about 1100 ft ASL although I'm not sure if they took that into account when they were setting the WC pressure. Both times the WC pressure was checked they used a digital manometer. The subject of temp rise comparison to the rating has not come up at all yet so I am not sure--let me know if I should look into that further. Everyone (Pro techs included) says the same thing about the flame pattern/- "looks real good" and when lit, the flame appears to fully penetrate the shield of the igniter. The installation instructions for the "NORTON MINI HOT SURFACE IGNITER CONVERSION KIT" (installed when the furnace was new back in '96 and replaced the org PN 025-31801-000 igniter with a 025-33421-000) say when properly installed,"the clearance between the metal shield of the igniter and the burner should be between 0" and 1/8"". I have probably had the igniter in just about every positions (in 16th in increments) between 0 and 1/4" in an attempt to get a consistently smoother light-off with no success. I am considering cutting a slot in the shielding of the HSI where it lines up with diffuser just to better allow the gas to contact the glowing metal--i.e. getting desperate.

[how]

You've got a vexing problem for sure. Some stray thoughts about your situation...
A 3 - 5 sec delay on ignition is a pretty short time delay to bring about a concussive ignition.

Digital manometers are not as reliable as they get out of calibration whereas water tubes are self regulating but they are more cumbersome to use. I doubt though that the set gas valve pressure is the issue.

Forget about the temp rise. Doesn't apply to your situation.

Your inducer related presure switches MAY only be set up to deal with inducer failings that result from a reduced draft. This means that an increased draft may not trigger any pressure switch led code (that your tech's were depending on) but may cause difficulties with the gas igniting the way it was designed for. A gas flow that is sucked into the exchanger too fast to fluff into the sheilded igniter. A number of manufactures have restricter plates on the outlet of the ID assy to limit the flow but as these rust away (propane is worst for this) the flow increases beyond spec and causes a host of unusual problems. If your tech's are going to come out again, ask them before hand to find out what the ID assy port is supposed to measure in WC. (This is different than what the pressure switches respond to).

[bsmithen]

Your post is interesting because one of the Techs trouble shooting the problem was looking at the ID system (btw the restrictor plate is still in v good condition) and joked about how much they love these PBLU furnace lighting problems because of all the billable hours they can rack up screwing with the ID system to get a good light-off--not what I want to hear but at least he's being honest. That was almost 2 months ago and at the time, I was just hoping the problem would be something less expensive like the igniter or igniter position. I will call him to get a feel if they know what the ID assy port pressure is supposed to be and pursue that route if necessary.

Sticking with the ignition issue, I dont know why but back in '96 when the furnace was new, a NORTON MINI HOT SURFACE IGNITER CONVERSION KIT was installed which replaced the org PN 025-31801-000 igniter (an unshielded igniter I think??) with a 025-33421-000 (shielded) igniter. When the original 025-33421-000 failed several years ago, my local HVAC Pro sold me a Uni-Line 41-605 kit that, they say, is a direct replacement for the 025-33421-000. The 41-605 worked great during the propane days and on his advice, I have a new 41-605 installed right now. Until now though, the furnace had only been used with propane. Is it possible that with the harder-lighting NG I should be using the original PN 025-31801-000 (unshielded??) igniter? Is there a reason for shielded vs unshielded?
Thanks,
Bob