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clearance between flue and duct


Ken Meyer
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I don't have the definitive answer but it doesn't seem that the circulated air, and therefore the duct, could be any hotter than the gasses being exhausted through the flue, even in a 90+ furnace.

I think the circulated air could very well be hotter than the exhaust temps but neither would be above the deformation point of the PVC.

I don't think there's a spec and it's a self correcting problem if it does get too hot.

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Originally posted by Chad Fabry

I don't have the definitive answer but it doesn't seem that the circulated air, and therefore the duct, could be any hotter than the gasses being exhausted through the flue, even in a 90+ furnace.

I think the circulated air could very well be hotter than the exhaust temps ....

Chad, I'm trying to picture that, but keep coming up short in this old head. [:-dunce] Wouldn't that require a heat exchanger to operate at over 100% efficiency? I didn't think that was possible no matter how convoluted the exchanger. I'm seriously curious...could you (or someone else) explain?

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If 95% of the heat is extracted from the combustion process the combustion by-product will be very cool; almost all the energy (heat content) was removed by the heat exchanger and transferred to the circulated air. Tempering the remaining heat content with unused combustion air creates a scenario where circulated air could be hotter than exhaust.

The dew point of the exhaust gas is dependent on excess combustion air supplied, but it's usually around 180 degrees. So, before the exhaust leaves the furnace it's below the dew point and it just keeps getting cooler from there.

When exhaust leaves an engine it's around 1400 degrees. By the time it exits the tail pipe it's usually less than 150.

The self correcting bit is the flue will deform away from the ducting if it gets too hot.

It won't ever get that hot though. I've done a fair amount of PVC heating and bending. At 300 degrees PVC is malleable, but not melting or sloppy.

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Originally posted by Chad Fabry

If 95% of the heat is extracted from the combustion process the combustion by-product will be very cool; almost all the energy (heat content) was removed by the heat exchanger and transferred to the circulated air. If the fan speed were slow enough I'm sure you could get air temps much higher than exhaust temps.

My problem with that is that if the circulated air were hotter than the cumbustion gases at some point in the heat exchanger, then wouldn't heat want to transfer the other way...ie the circ air would actually heat the combustion gasses. If the circ blower speed was reduced, then I would expect both the circ air and combustion gasses to increase in temp, not just the circ air. I could see equalibrium being reached, but I'm still having a hard time with actually hotter.

No big deal. I'm probably missing something basic here and just having a senior moment.

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Richard,

I don't think Chad is saying the circulated air temperature would be higher than the combustion gases where the two are passing each other inside the heat exchanger. He's saying that on a high efficiency furnace the exhaust gasses will quickly cool to a temperature below the circulated air temp after this point.

Brandon

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