Hot duct?

[Joshua L. Frederick]

Noticed a duct/plenum in the basement (aprrox. 20' away from furnace) that was too hot to touch. Never noticed before, of course I don't usually touch them all either. I just happened to "lean" on this one to look at something else. Anyway, is this normal? What would cause temperature rise (heat exchanger problem?)? The thermostat was set at 80 to test furnace.

Thanks in advance.

[Charley L. Bottger]

Air temp in the duct can be as much as 130 or higher with furnace operating if the space the duct is located in is conditioned the duct is not required to be insulated. If the space is not conditioned most rectangular duct is insulated to the inside. My guess is your duct is not insulated and would feel hot to the touch. The furnace has a high temp limit switch that would shut the burner down if it was operating properly

[KEVIN WOOD]

You are near the boot of the supply vent. Heated air at this point is squeezed into the smaller duct which raises the temperature. This creates a hot spot from friction. This is normal and expected for 20 feet from the plenum.

[Charley L. Bottger]

Originally Posted by kwood

You are near the boot of the supply vent. Heated air at this point is squeezed into the smaller duct which raises the temperature. This creates a hot spot from friction. This is normal and expected for 20 feet from the plenum.

I think your thinking has dropped off the deep end

[KEVIN WOOD]

Only way I know how to explain this. Do you have a better way?
Example
When air passes through a big duct and gets squeezed it increases in temperature. Friction is not the best Phenomena but people understand it.
This only applies to a short distance away from heat source and high velocity.

[Michael R. Boyett]

Originally Posted by jfrederick

...too hot to touch.

90.6° is too hot to touch?

[Joshua L. Frederick]

Originally Posted by mboyett

90.6° is too hot to touch?

My thermometer is old & cheap and may not be 100% accurate, however, yes, it was hot enough that I couldn't keep my hand on it very long. Just doesn't seem right.

[Charley L. Bottger]

Originally Posted by kwood

Only way I know how to explain this. Do you have a better way?
Example
When air passes through a big duct and gets squeezed it increases in temperature. Friction is not the best Phenomena but people understand it.
This only applies to a short distance away from heat source and high velocity.

Well I have had a temp rise in a duct system due to its surrounding ambient but don't think I ever squeezed it and had a temp gain we are not talking high compression as in a air compressor.

[Charley L. Bottger]

Originally Posted by mboyett

90.6° is too hot to touch?

Sorry Mike I did not see the temp in the pic he has no problem its imaginary Josh you should Hug a duct more often

[KEVIN WOOD]

Charlie:

Do you have a picture of a heated ventilation trunk and boot take offs.
I would love to have one for the reference.

[Charley L. Bottger]

Originally Posted by kwood

Charlie:

Do you have a picture of a heated ventilation trunk and boot take offs.
I would love to have one for the reference.

Nope I can get you one this winter in IR if you want BTW I am just ragging your CMI in fun

[Robert J. OConnor]

Friction has absolutely nothing to do with it ... you probably couldn't even measure the increase with standard measuring devises. A standard efficiency furnace may have a temperature rise at the equipment of between 80°F - 90°F (data plates usually say anywhere between 70°F - 100°F rise). If you had say 65°F return air, the supply at the furnace discharge would be around 135°F - 155°F. So even 20 feet away may feel too hot to touch ...

[KEVIN WOOD]

Friction is not the best Phenomena but people understand it.
Why is the plenum always hottest at the throat and on turns to the boot.
Because maybe the air flow over the throat is moving at a higher velocity. It might have something to do with laminar flow but this also presents a problem. No measurements have been done to see if friction has a bearing on this temperature only observance can show this with IR.

[David A. Andersen]

Do you know what a Ductulator is?

You had better not have an increase in static pressure so high that the temperature goes up!

What you are talking about is about turbulence.
Non-laminar flow increases heat transfer.
In thermography we consider these effects associated with the "Boundary Layer".

That duct is not 90, look at the material you are testing.
No emissivity adjustment, high reflection. You may be measuring the inside of the meter at that range.

[Linas Dapkus]

Originally Posted by dandersen

Do you know what a Ductulator is?

You had better not have an increase in static pressure so high that the temperature goes up!

What you are talking about is about turbulence.
Non-laminar flow increases heat transfer.
In thermography we consider these effects associated with the "Boundary Layer".

That duct is not 90, look at the material you are testing.
No emissivity adjustment, high reflection. You may be measuring the inside of the meter at that range.

Maybe a Fluke 570 series thermometer is in order. You can set the emissivity on those.