Hot Spot on Furnace Cabinet?

**Michael Chambers** · 09-24-2016, 02:59 PM

I ran my laser thermometer of the rear wall of an operating furnace yesterday, and found a spot that measured 145 deg. F. I'm one that would test the accuracy of a "Wet Paint" sign every time, but erring on the side of caution in this case, I chose to NOT actually touch this area to confirm what the thermometer was saying. Knowing however, that water at that temperature would scald someone in just a few seconds, this seems to be quite a potential hazard to some little kid playing hide & seek, and pointed it out to my client.

My question to you though, is how hot should such a spot be on a furnace before it might be indicating a problem with the furnace? I'm envisioning a hole or crack in a heat exchanger allowing full flame heat to be directed towards a spot on the inner side of the outer furnace cabinet. I've asked a couple of HVAC guys about this in the past, but they act like hot spots like this are no big deal, and haven't been able to give me a definitive answer to the same question.

Consequently, I'm bringing this question to the brain trust encompassed by this forum. What say you? Do you check for this, and at what temperature would you start to raise an eyebrow? Thanks a lot for your opinion on this!

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**Scott Patterson** · 09-25-2016, 07:02 AM

Originally Posted by Michael Chambers

I ran my laser thermometer of the rear wall of an operating furnace yesterday, and found a spot that measured 145 deg. F. I'm one that would test the accuracy of a "Wet Paint" sign every time, but erring on the side of caution in this case, I chose to NOT actually touch this area to confirm what the thermometer was saying. Knowing however, that water at that temperature would scald someone in just a few seconds, this seems to be quite a potential hazard to some little kid playing hide & seek, and pointed it out to my client.

My question to you though, is how hot should such a spot be on a furnace before it might be indicating a problem with the furnace? I'm envisioning a hole or crack in a heat exchanger allowing full flame heat to be directed towards a spot on the inner side of the outer furnace cabinet. I've asked a couple of HVAC guys about this in the past, but they act like hot spots like this are no big deal, and haven't been able to give me a definitive answer to the same question.

Consequently, I'm bringing this question to the brain trust encompassed by this forum. What say you? Do you check for this, and at what temperature would you start to raise an eyebrow? Thanks a lot for your opinion on this!

That is something I would not normally check unless something looked out of the normal. If you just had one spot on the furnace cabinet the was "hot", I would be suspect that the heat exchanger might have a leak. I would just indicate that I found a very hot spot on the cabinet which could be an indication of a burner or heat exchanger problem. You should have an HVAC contractor look at it and determine if any repairs or if replacement is needed.

**Jerry Peck** · 09-25-2016, 09:01 AM

Originally Posted by Michael Chambers

I ran my laser thermometer of the rear wall of an operating furnace yesterday, and found a spot that measured 145 deg. F. I'm one that would test the accuracy of a "Wet Paint" sign every time, ...

That just one of the uses where one of these IR cameras would be good to have in your pocket ... yes, in your "pocket": FLIR C2 Compact Thermal Imaging System

Moisture meters can cost as much as that IR camera.

And, if you are an apple a day person, then this costs even less: Explore FLIR ONE | FLIR ONE

Those IR cameras and their prices have brought IR for home inspector's down to the range of 'should be in every home inspectors tool bag' ... if not 'in every home inspector's pocket'.

In your case, the question of 'how large it the hot spot' would be quickly answered and you would have a photo to show the HVAC tech what you saw when you saw it.

From another 'is the paint STILL wet' tester.

**ROBERT YOUNG** · 09-25-2016, 09:26 AM

Reflectance can heat objects.
Although I commend you for your hypothesis, it would be beneficial to eliminate reflectance.

**Markus Keller** · 09-25-2016, 10:06 AM

Have to agree with Jerry on this one regarding the IR camera. I would have pulled it out to take a look. I probably would have also called the manufacturer Rheem, Goodman, whoever and asked them about it because I just wouldn't have been able to help myself.
I realize time is short for guys who do multiple inspections a day. Nonetheless it can be very helpful getting into the habit of calling manufacturers with questions. Sometimes you get not very worthwhile boilerplate info for a phone person and other times you hit the jackpot of info for a tech guy.
Been looking at the Flir C2 since I played with it a bit at last years AHR expo. Haven't bought one yet since my full size Testo works nicely for me. Will probably get it before the end of the year.
Was going to buy the C2 recently but then opted for the MR160 since I do a fair amount of water intrusion work. In this case the MR160 wouldn't have been the optimal tool though.
For anyone who does water work you might want to look into then MR160. Initial use in basements have gone very well. I think this is going to end up being a well used tool.

**Michael Chambers** · 09-25-2016, 10:50 AM

Originally Posted by Jerry Peck

..............Those IR cameras and their prices have brought IR for home inspector's down to the range of 'should be in every home inspectors tool bag' ... if not 'in every home inspector's pocket'.........

Yeah, I've already gotten pretty close to pulling the trigger on a Flir One for my Android Max. You can get one on Amazon for $211 now.

Originally Posted by Jerry Peck

..............In your case, the question of 'how large it the hot spot' would be quickly answered and you would have a photo to show the HVAC tech what you saw when you saw it...........

Even with my limited technology laser thermometer I was able to determine that the hot spot was very small, maybe one inch in diameter. Possibly all the more reason to suspect a problem with the heat exchanger.

Thanks!

**Dan Hagman** · 09-25-2016, 11:09 AM

What were the results of the CO test you did on the furnace heat exchanger? What was the temperature rise of the furnace? You could have a hot area due to a plugged evaporator and filter with not enough air flow. The inside of the furnace cabinet is insulated and there could be insulation that has come loose and or dropped down. The furnace has a zero clearance to combustibles and a hot area is not that uncommon. It would be more common on an older furnace with atmospheric burners, was it an 80% or a 90+ pictures would also be helpful of the location of the furnace. If you really think this was a concern have them call an HVAC professional to cleaned and service the furnace.

**Jerry Peck** · 09-25-2016, 11:23 AM

Originally Posted by Michael Chambers

Yeah, I've already gotten pretty close to pulling the trigger on a Flir One for my Android Max. You can get one on Amazon for $211 now.

I didn't know they had a FLIR One for Android, I'd only seen them for the Apple - good to know: FLIR ONE for Android

**Michael Chambers** · 09-25-2016, 01:13 PM

Originally Posted by Dan Hagman

What were the results of the CO test you did on the furnace heat exchanger? What was the temperature rise of the furnace? You could have a hot area due to a plugged evaporator and filter with not enough air flow. The inside of the furnace cabinet is insulated and there could be insulation that has come loose and or dropped down. The furnace has a zero clearance to combustibles and a hot area is not that uncommon. It would be more common on an older furnace with atmospheric burners, was it an 80% or a 90+ pictures would also be helpful of the location of the furnace. If you really think this was a concern have them call an HVAC professional to cleaned and service the furnace.

You are assuming that I did a CO test and measured the temperature rise. I don't have a CO tester, and the air coming out of the registers was about as warm as you would expect. Although I do check the T/D on the A/C, I don't normally check the furnace operation temperature rise. You have to draw the line somewhere.

The filter was a good quality one, and it was clean. The furnace was a 23 years old Nordyne, 80% efficient model, and yes, I did recommend that the unit be checked out by an HVAC contractor. The attached photo shows the approximate location and size of the hot spot.

Thanks!

**Jerry Peck** · 09-25-2016, 05:45 PM

Originally Posted by ROBERT YOUNG

If you are a thermographer you would consider reflectance.
Flir C2 is 80x60. low focal range.
Minimum focal range should be 240 x 180.
The MR160 is a moisture meter.
I own Tramex MRH II and Delmhorst Techcheck Plus for probing wall assemblies. It also does none destructive testing as well.

Lots of money for good equipment . Educate before hand.
I used the same approach you are using and scaled up by investing tens of thousands.
You still must eliminate reflectance, hot or cold with thermal imaging.

Best of luck to you.

Robert

Robert,

You are still "fairly new" to this forum, and Markus does not post a lot, but if you read back on what Markus posts you will find that ... well ... that you Markus knows tad bit more than what your post indicates you think he does ... just sayin' ...

**ROBERT YOUNG** · 09-25-2016, 06:27 PM

Originally Posted by Jerry Peck

Robert,

You are still "fairly new" to this forum, and Markus does not post a lot, but if you read back on what Markus posts you will find that ... well ... that you Markus knows tad bit more than what your post indicates you think he does ... just sayin' ...

I hope my post did not appear to lean that way and if it did it was not intentional.
That is why I wished him good luck.

**Raymond Wand** · 09-26-2016, 08:57 AM

At twenty three years of age, I would have likely recommended budgeting for a new furnace regardless of the hot spot.

**Bob Harper** · 09-26-2016, 05:49 PM

If you don't have a national standard or listing spec. on thae heat signature of that appliance nor the Standard of Care to check it, don't go looking for trouble. If there is an area of discoloration on the outer jacket of a heating appliance that is consistent with focused heat stress then call that out and a potential problem and refer it to the mfr.-done. Don't go tilting at windmills.

Now, I've seen many boilers and furnaces where the paint and burnt off the appliance in one focused point and even melted away in several. That is clearly a call to shut it down until it can inspected by a mfrs. rep. or just replaced out of hand. However, you cannot infer anything much of value by an unguided, uneducated spot check of a temperature on a heating appliance. Now, if there are other signs and symptoms to go along with the theory of a malfunction then fine. Signs of flame roll-out, melted plastic grommets, discoloration, rusted metal, etc. all coupled with other relevant facts, such as signs of venting failure, abnormal combustion analysis numbers, sooting, operational complaints, etc. would trigger further investigation by someone qualified to do so but in almost all cases, that person would not be a home inspector.

What would be of more relevance is if that side of the appliance failed to meet clearances to combustibles.