Top node hotter thanothers?

Robert G. Brown rgb at
Fri Jul 25 14:15:40 EDT 2003

On Fri, 25 Jul 2003, David Mathog wrote:

> We have a 20 x 2U rack and I've noticed that the
> top node is always a step hotter than the other nodes.
> Why?
> There is a slight gradient going up the rack (see
> below, 01 is on the bottom, 20 on the top) but it
> doesn't explain the jump at the top node.  At first
> I thought it might be due to hot air moving from
> the back of the rack, over the top of the highest
> node, and being sucked in by it.
> However no temperature change resulted when all
> side vents were blocked and cardboard pasted up
> the front of the rack so that only the same cold
> air as the other nodes could enter.  The only other
> difference between this node and the others is
> that there's hot air above 20 (two empty rack slots),
> but another node above all the others. So maybe all
> that hot air heats the top node's case and that
> couples the heat in?  I don't have an insulating
> panel handy to test that hypothesis.

What happens if the top node is turned off?  Does the second from the
top become the hot node?  What happens when the top node is swapped with
the bottom node?  It could just be that the top node's CPU cooler fan
has a piece of lint stuck on it and is running hotter, or even that its
sensor itsn't calibrated right.

It could be some sort of loopback of heated air as you describe, but if
you put a small fan and set it to blow across the top node you should
break up the circulation pattern if any such pattern exists.  I don't
have as much faith in cardboard used to block vents, since that can also
heat up the node by impeding circulation.


> node case    cpu
> 01   +34°C   +43°C 
> 02   +35°C   +44°C 
> 03   +37°C   +48°C 
> 04   +42°C   +50°C 
> 05   +38°C   +48°C 
> 06   +37°C   +50°C 
> 07   +36°C   +45°C 
> 08   +38°C   +48°C 
> 09   +38°C   +48°C 
> 10   +38°C   +48°C 
> 11   +36°C   +44°C 
> 12   +38°C   +48°C 
> 13   +38°C   +48°C 
> 14   +40°C   +49°C 
> 15   +38°C   +46°C 
> 16   +36°C   +46°C 
> 17   +39°C   +51°C 
> 18   +39°C   +48°C 
> 19   +39°C   +49°C 
> 20   +44°C   +54°C
> Temperatures were measured using "sensors" on these
> tyan S2466 motherboards (1 CPU on each currently.)
> The case value is the temperature reading by the
> diode under the socket of the absent 2nd CPU.
> The temperatures jump around a degree or two.
> Regards,
> David Mathog
> mathog at
> Manager, Sequence Analysis Facility, Biology Division, Caltech
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Robert G. Brown	             
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at

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