Flame Rollout on Startup - How much is a problem?

So I just inspected a furnace that had a bit of rollout on the start up. I was hoping for a second opinion on if this alone is an issue. There is some rust that I will be writing up for further evaluation. Any thoughts would be appreciated.
Direct vent, Cat IV, Natural Gas, In-shot burners, Manufactured 2004.
Some rust at exchanger ports that seems to coincide with the flame rollout on start up. However there is also indications of condensation leaking from AC & Vent.

Condensate & Corrosion issues aside Is this amount of rollout acceptable? (See Video) Or is there an ignition timing issue or something else?

Evidence of deferred maintenance was observed. Recommend cleaning and / or general servicing of the heating unit by a licensed / qualified heating contractor prior to close.

Burn marks and evidence of a previous flame roll-out was observed at the burner cover / exterior face area. This could be or may have been a carbon monoxide concern. Recommend further evaluation by a qualified heating contractor prior to close.


Thanks Peter! Any thoughts on the rollout during startup in the video?

Rollout is caused by the combination of gases and flames present when the furnace is in operation. … The buildup of gases is largely due to the buildup of soot, which prevents the gases from leaving the chamber. As these inflammable gases build, they block the flame from the oxygen it requires.
This is the best I could find.


Okay, the answer is…

That is not flame rollout. That is a crossover tube that lights the right burners from the left burner after it ignites.

The rusting between the burners is from the crossover tube flame. Gas is corrosive and causes rust which is not a significant issue on the burner plate until it totally rots away in about 25 years.

The rusting below the condensate drain line is from condensation. The water inside the pipe is 40°F during normal operation and will condense water under high humidity ambient conditions. Sometimes they will insulate the first 4 feet of the condensate drain to prevent this from happening on the equipment which could drain into the control panel below which contains circuit boards and other delicate safety devices.

The rust spot inside the unit is difficult to see and diagnose but it appears it’s on the same side as the condensate drain from the evaporator coil above (leaking into the cabinet). That’s why they insulate the pipe.


Thanks David. So I could have done a better job explaining. I do understand the crossover flames and that was not what I was looking at (and calling rollout). Your description of this being the source of the rust and it not being a concern was very helpful BTW.
What I was referring to and trying to ask about is the initial burst of flame. Many furnaces I have inspected lately did not have as much as this one if any at all. And I have seen some where the flame, on start up reached inches out of the burner chamber before settling into a normal looking flame. From your & others responses I take it there is no issue with the initial burst on this one.
I am trying to determine a threshold for concern somewhere between this one and “I’ve just lost my eyebrows” Is there a rule of thumb for this?

There are two issues; #1 delayed ignition and #2 excessive gas pressure.

If either happens, you’ll know it. It will rattle your cage. End shot burners seldom have any issue that you should concern yourself with. So there is no rule of thumb to make up. Ignition is all electronically controlled, with a bunch of safeties programed in. What you more likely to see is the the furnace does not fire off on the first try and has to go through a purge cycle (which seems to last forever). Just let it do it’s thing. If it fails too many times (three) it will lock itself out. You have to reset everything to continue (which I would never recommend that you proceed any further but to call for service).

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Thank you David! I really appreciate your time and knowledge.

Yes watch the flame ignite with your head off to the side…report any anomaly.

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