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How hot should the exhaust pipe be?

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How hot should the exhaust pipe be? mkirsch1 01-26-2007
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Posted by on January 26, 2007, 10:59 am


Okay, last stupid question about my stupid Amana furnace.

I keep forgetting to use my IR thermometer on the exhaust pipe, but it
seems awfully hot to the touch. You can't keep your hand on the pipe
for more than a second or so. I don't think it'd burn, but it is beyond
the human threshold of pain.

It's an 80% efficient furnace, so I'd expect the pipe to be somewhat
warm to the touch. But, it's got a power vent too. Exhaust goes up a
conventional chimney and out the roof, along with at least 20% of my
gas bill :(

Should it be that hot? Understanding that some is radiating off the
pipe into the basement, is there a way to reclaim more of that heat?

Thanks for all the info!


Posted by EXT on January 26, 2007, 11:40 am


I don't know what temperature your furnace flue pipe should run, but an 80%
efficiency furnace is not very efficient, so I would expect that the flue
pipe would run quite hot. You would have what is called a mid-efficiency
furnace, basically a regular non-efficient furnace that has a flue damper
and sometimes a power exhaust added to reduce some heat loss.

You cannot reclaim most of the heat from your furnace flue as it requires
the heat to create the draft that will lift the exhaust up the chimney and
out the house.

Furnaces that run cool exhausts are the high efficiency ones that run 92% to
98% efficiency. They have a heat exchanger then run the exhaust through a
condensing chamber then power exhaust through a 2 inch plastic pipe the
outside. They need a drain line to dump the acidic water that they condense
from the exhaust.


> Okay, last stupid question about my stupid Amana furnace.
>
> I keep forgetting to use my IR thermometer on the exhaust pipe, but it
> seems awfully hot to the touch. You can't keep your hand on the pipe
> for more than a second or so. I don't think it'd burn, but it is beyond
> the human threshold of pain.
>
> It's an 80% efficient furnace, so I'd expect the pipe to be somewhat
> warm to the touch. But, it's got a power vent too. Exhaust goes up a
> conventional chimney and out the roof, along with at least 20% of my
> gas bill :(
>
> Should it be that hot? Understanding that some is radiating off the
> pipe into the basement, is there a way to reclaim more of that heat?
>
> Thanks for all the info!
>



Posted by BobK207 on January 26, 2007, 11:50 am




On Jan 26, 7:59 am, mkirs...@rochester.rr.com wrote:
> Okay, last stupid question about my stupid Amana furnace.
>
> I keep forgetting to use my IR thermometer on the exhaust pipe, but it
> seems awfully hot to the touch. You can't keep your hand on the pipe
> for more than a second or so. I don't think it'd burn, but it is beyond
> the human threshold of pain.
>
> It's an 80% efficient furnace, so I'd expect the pipe to be somewhat
> warm to the touch. But, it's got a power vent too. Exhaust goes up a
> conventional chimney and out the roof, along with at least 20% of my
> gas bill :(
>
> Should it be that hot? Understanding that some is radiating off the
> pipe into the basement, is there a way to reclaim more of that heat?
>
> Thanks for all the info!

I assume the furnace "exhaust pipe" is metal?

Metals are very good heat condcutors & do not have to be very much
above skin temp to give you the sense "this thing is pretty hot, better
not touch it or it will burn me".

How delicate are your hands? Do you have low pain threshold? People's
tolerance to hot objects varies a lot person to person.

Yes you can reclaim some of the heat by adding "heat fins" to to "pipe"
but you need a certain amount of residual heat in the flow to get it to
rise through the chimney.

I wouldn't mess with it.

cheers
Bob


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