So I called this as “There’s apparent heat damage on the grille right at the air intake, perhaps indicating a blocked or semi-blocked flue pipe.” with an appropriate referral.
Your thoughts?
This is a gas wall furnace, Williams, less than 10 years old. It vents (supposedly) directly up to the roof.
The damaged paint is right where the flue is open to admit additional air, as part of the vent stack.
It is possible I suppose. Definitely looks like it could be heat damage. I think in this case, I personally would not have included an attempted diagnosis of the cause (blocked flue pipe in your narrative) in the report.
No, it does not!
I know nothing about wall furnaces, I’ve never seen one. But I do know one thing, you cannot mix combustion gas with indoor air!
There are two vents on a wall furnace, one for air to come into the furnace and one to discharge it from the furnace after it’s been heated.
The combustion air vent is located behind this internal duct, which goes from the furnace air intake to the roof. This is not like big furnaces that have barometric dampers to add air to the flu draft, which may be confusing you.
Excessive heat in the indoor air circuit is generally related to an adequate airflow, allowing excessive temperatures to occur within the system.
If you are in the winter season you could run the unit and get a temperature rise through the unit with a thermometer, which would give us something to work with.
Best thing is to just have someone check it because the blistering paint is not a normal occurrence. It’s not your job to analyze, but it is to know how the systems work, so you can test them.
Never ever is combustion air allowed to connect with the indoor air.
Right behind that is a part the vendor calls a “draft hood”.
They also warn"
If your furnace is in an open area unconfined space)*
the air that leaks through the cracks around doors
and windows may be enough for combustion and
ventilation air. The doors should not fit tightly. The
cracks around windows should not be caulked or
weather stripped.
From the US patent for a similar model
“To this end, typical gravity flow wall furnaces include draft hoods at the top of the exchangers. The draft hoods collect flue gases and allow them to expand. The expansion in the draft hood results in some decrease in temperature of the flue gases, thereby decreasing the velocity of the flue gases and increasing their pressure. Thus, the risk of a down draft from the exterior is minimized.”
US5368012A - Wall furnace with side vented draft hood - Google Patents
The traditional front draft hood (just behind the damaged area) is discussed here, in a patent for an improved draft hood:
The draft hood, however, needs to be vented to provide further cooling and to provide a secondary outlet for flue gases in the event the flue is blocked by an obstruction. Current wall furnaces locate the draft hood vents on the front of the draft hood. This vent location reduces the efficiency of the furnace by allowing some already heated room air to enter the vent and be discharged through the flue without heating the room.
The more modern version of the subject furnace has an additional sensor — WHICH I OFTEN FIND WAS NEVER HOOKED UP — that is meant to shut the gas supply, if too much heat spills out the draft hood.
Now that I’m thinking about it after the inspection, I suspect is the flue is indeed partially blocked, and this heater was made prior to the introduction of the high limit sensor.
A side venting draft hood is also disclosed.
The flue break or draft hood should never be vented inside of the Indoor Supply Air duct system.
Note: Blockage is only one potential cause.
Back drafting can be caused by interior/exterior pressure differentials created by other appliances in the building, the furnace itself, which is forced convection, or natural convection from building stack effect, or exterior wind pressures (building envelope openings).
Looking again at your initial photograph, you zoomed in so closely to this vent that it appeared to be the supply air register to me. My bad!
Yes, you have a back drafting problem. Just consider what I posted here as well. It’s probably not a good idea to write in your report the potential causes unless you write them all. Your client may just have someone look at the flu to see if it’s blocked with a bird nest etc. and walk away. This is a carbon monoxide problem, and it’s not something we want to fool around with. It’s best just to report that there are indications of flu back drafting and let the appropriate contractor analyze and repair it.
I was doing a home inspection when the neighbor across the street came by and asked me to come take a quick look at his house because his family wasn’t feeling too good. I found a 75 gallon gas-fired water heater with no vent installed through the building envelope. All five members of this family had carbon monoxide poisoning! You can’t even imagine how that played out! Building contractor lost his license over that!
Apparent heat damage was at about the 7/8 mark toward the top of the heater:
In this case I was able to proactively ensure the heater was shut down, and the appropriate professional engaged. If I’m able to get a report back on what was wrong I will post here.
Glad I never had the opportunity to meet one of these things! 
Thanks, but no thanks!
Update from the OP:
I just got word back that the heater technician found the heater unrepairable, and will replace it Friday. Good thing too: overnight low temperatures here could drop into the high 50’s by the end of the week.
