Intermittent furnace light-off problem


  #1  
Old 01-04-12, 05:37 PM
B
Member
Thread Starter
Join Date: Jan 2012
Location: USA
Posts: 2
Upvotes: 0
Received 0 Upvotes on 0 Posts
Intermittent furnace light-off problem

Hello folks,
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 following: 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
 
  #2  
Old 01-04-12, 06:08 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
The most likely cause is that the burner input isn't set properly.

To do that you need the correct orifices and the proper manifold gas pressure.

Even installing the original burner orifices isn't a guarantee that they are the right ones, although having that as the problem would be a low probability.

Setting the input at 3.5" might be good enough for government work, but not adequate to set the input on a condensing gas furnace, which can be picky about such things.

Who converted the furnace back to natural gas? When I worked as a repairman for a gas utility, we converted the gas appliances for new customers. If the utility did that, I'd call them back and have them check again on the work they did.

Finding the correct set up for a furnace is relatively complicated. The best way is to contact the furnace manufacturer's technical services representatives and have them calculate the correct orifice size and manifold pressure for your furnace.

To do that they need the average BTU per cubic foot of gas, specific gravity of the gas, elevation, and probably some other stuff I can't remember offhand.

So who did the conversion back to NG?
 
  #3  
Old 01-06-12, 06:34 AM
B
Member
Thread Starter
Join Date: Jan 2012
Location: USA
Posts: 2
Upvotes: 0
Received 0 Upvotes on 0 Posts
The orifice size is 44 and the inlet and outlet gas pressure was set and confirmed by two different HVAC companys. The manufacture's install manual calls for 3.5" WC pressure for NG so that was what the HVAC Pros had to go on although while they were on site they had made calls out (I do not know if they were talking with a factory rep but it is a good idea) for specs/advice.
Taking the BTU measurements is also a good idea that has not been done yet. The conversion back to NG and initial troubleshooting was done by a highly-rated local HVAC residential/commercial company. I asked a second, also highly-rated HVAC company to step in after the first said they could find nothing causing the problem. Thanks for you insight, further comments/suggestions appreciated.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: