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Exceedingly hot supply-air


Bain
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A house I checked out this afternoon had a 20-year-old furnace with a cracked heat-exchanger that wasn't visible until the chambers heated up. The supply-air temperature---incredibly---was over 200 degrees as you can see from the photo. Is the extreme temperature attributable to the flue gases comingling with the conditioned air? Or might there be another explanation? The blower was operating normally, and clearly the limit switch needs checking, but I was wondering if anyone had seen this before. I think the temperature would have kept climbing had I not shut the furnace down.

I called all the principles, so it's out of my hands now, but still, I wonder if there's anything else I should have been aware of. Oh, the furnace was a 75,000 BTU Sears and Roebuck.

John

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Mike, you'd be surprised by how often I see that here, especially in old houses. Maybe it stems from complaints about getting chilly while bathing. Every once in a while, when I stick my lighter up to the register, I discover that it's a return, distributing all that steam and yuck-smell throughout the house.

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Originally posted by Bain

A house I checked out this afternoon had a 20-year-old furnace with a cracked heat-exchanger that wasn't visible until the chambers heated up. The supply-air temperature---incredibly---was over 200 degrees as you can see from the photo. Is the extreme temperature attributable to the flue gases comingling with the conditioned air? Or might there be another explanation?

The other explanation would be that the blower was set to the wrong speed and the high-limit was either set too high or broken. (or the filter was very dirty or there was some other type of obstruction.) Maybe that's *why* the heat exchanger cracked.

I've seen furnaces with holes in the heat exchanger the size of my fist and the supply air wasn't unusually hot.

The blower was operating normally, and clearly the limit switch needs checking, but I was wondering if anyone had seen this before. I think the temperature would have kept climbing had I not shut the furnace down.

Cool! It might have been interesting to see how far it would go. Maybe you could've soft boiled an egg over one of the registers.

- Jim Katen, Oregon

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I doubt the cracked heat (.00001% probability)exchanger was the cause of high temps. It's far more likely that the exchanger was symptomatic of the high temps. Metal doesn't like going through heating/cooling cycles because it moves around when it does and when it moves around it eventually fatigues and breaks. Two factors determine when it will break. 1) number of cycles 2) tempertaure extremes from cold to hot and then back again.

The perfect design would be to start the furnace in the fall and have it run continuously until spring creating one long cycle. That's one of the significant reasons that cars that are driven long distances routinely go many more miles than cars that are "short cycled". The miles mean virtually nothing, the cycles are paramount.

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I ran across an old floor furnace once, in a 20's house. It was at 186 degrees and still climbing slowly when I took my thermostat off of it (plastic case digital, I got worried). I told 'em to have it checked out, and don't stand on it in sneakers or barefoot. [:-bigmout

Brian G.

When You're Hot, You're Hot

(Remember Jerry Reed?)

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On older furnaces, with a combination fan/high limit switch, the settings should be 100° for fan off, 150° for fan on and 200° to shut the whole furnace down.

If the fan is running the discharge temperature should never hit 200° unless it is A)malfunctioning or B)someone has tampered with the settings.

This condition would rate very high on my "fix it now" list.

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Originally posted by Bain

I wasn't aware of the precise settings you mentioned, Terence, but I knew something was out of whack and phoned the seller immediately. What about settings for a newer furnace? I'm not up to speed on those, either.

John

The newer furnaces have multiple thermal limit controls placed in various locations to sense abnormal conditions such as flame roll out etc. These are preset, no adjustment can be made they are, electrically speaking, open or closed.

The blower is typically on a time delay relay. When there is a call for heat the time delay relay becomes energized and after a preset time the blower comes on. No adjustment can be made to these either.

Hope this helps.

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  • 2 months later...

If the register is right above the heat exchanger higher temps can be encountered. Normally temperature rise across the furnace is measured where the temperature probe does not have line of sight with the heat exchanger. Normally in a trunk duct 6 - 12 inches from the main plenum. Higher temperatures can also be the result of dirty air conditioning evaporator coils, dirty blower wheel blades, incorrect blower rpm, undersized ductwork both supply and return, oversized furnace for the buildings heat loss. Using high merv furnace filters on furnaces incapable of ever producing the required air flow for the additional air flow restriction of the filter. Overfiring the furnace. On 90+ percent efficiency furnaces dirty secondary heat exchanger coil.

I have not encountered but one of my service tech friends had one furnace tripping limit because of a crack in the heat exchanger in the area of the limit.

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