Residue in burner area of furnace

Good evening everyone
I was inspecting a 34year old conventional furnace today well past its expected life expectancy but was stumped as to what this mysterious substance was under the burners.
Any ideas would be a great help?

I’m a relatively new inspector. But I’ve been in the HVAC biz for almost 40 years. I can tell you this, that white substance is the byproduct of natural gas. You could even call it soot. If you were to try and pick it up with your fingers it would break apart rather easily. It’s just a little like talc powder with more consistency. If you look around your picture with the pilot, you’ll see more of it on the thermocouple and surrounding area. By the way, that pilot orifice need to be cleaned out for a better looking flame. That furnace (at that age) needs a VERY THOROUGH inspection from an honest to goodness HVAC tech. If the heat exchanger cells are still in decent condition, which is still possible, a good brushing within would be in order to get all that residue (white power). Those burners tell me it’s a natural draft unit. I used to install those. Wow, I’m getting old.


Thank you for your input Edwin. It did have a talc like feel as it broke apart when picked up. Yes natural draft conventional furnace. I recommended a qualified heating specialist estimate remaining life and estimate repair costs. Budget for replacement of the unit.

Also the blower on this unit was short cycling on start up. I immediately thought fan limit switch but didn’t want to guess.
I leave that up to the specialist. We are the general practitioners. :+1:

Way to go pal. The fan and limit was a good guess. It might have been adjusted. Yes, your recommendation is in good order.

Thanks. They decided to go with a new high efficiency furnace. 65000 btu.
I was wondering about the fan air induction fan. I know it starts before the call for heat to bring clean air into the system. Is there another purpose for it?

The burners you had pictured were unlikely to have an induction (also called an inducer) assembly. That was a natural draft furnace right? But to answer your question, an inducer assembly was used to raise the efficiency of the unit. Also, the burners are shorter. Furnaces were then able to get out 80% plus more heat out of a cubic foot of natural gas (or so says the manufacturers). But really it only got up to about 77%. Still better than the old furnace you inspected.

Ok great i understand that it make the furnace more efficient but does the inducer fan run after high efficiency furnace fires up or just at the beginning?

I read through this post, Edwin I am impressed on how much HVAC information you spit out in such a short thread.

Thanks Greg. You’re too kind. My primary job has been HVAC for almost 40 years so I’ve seen the old gravity furnaces all the way to these 90% units.
But to get back to mjuri;
The inducer operates before. On a call for heat, a relay on the circuit board energizes the inducer. The pressure caused by the inducer activates a pressure diaphragm switch (sometimes two of them). That starts a digital timer to allow for a pre-purge of the heat exchanger before the igniter kicks in to light the burners. Then the blower (supply air) kicks on around 45 seconds after the heat exchanger has gotten hot. When the thermostat is satisfied, the burners quit and the blower will remain on for 90 seconds to get as much heat as possible while the heat exchanger is warm enough. Then the cycle begins again when called.

With that much knowledge I bet you could stay very busy about this time of year just inspecting furnaces, are you that advanced with AC units and heat pumps?

I wished I could have went on service calls with you for a year. HVAC is my weak point and I learn best watching and hands on and I could ask questions.

I’m pretty much the same way. I’ll be happy to help you with any further questions you have regarding this. Heat pumps are really just a/c’s only they have more components and the control wiring system is slightly more complicated. But if you can read a schematic (just like a road map) you’ll be able to get where you are going.

Edwin I really appreciate it, Funny thing is I just got a call today from someone who has no heat. If I knew what you knew I would have jumped on it but I told him I could run over there and make an educated guess but can’t guarantee anything, I just told him to call an HVAC tech. I can usually figure out the situation at my place or my rental house but not comfortable enough to work on somebody else’s units.

Yeah, that’s the best advice you can give. A qualified HVAC tech worth his/her salt should be able to find the problem.