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High Efficiency gas furnace - return air temperature bubbabubbs 01-19-2007
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Posted by -zero on January 23, 2007, 6:28 pm

>
> nicksanspam@ece.villanova.edu wrote:
>>
> And what's the crap about poor air film conductance on both sides of a
> heat exchanger in a modern high efficiency furnace. If it's so damn
> poor, how come these furnaces are 93%+ efficient? Could it be that
> manufacturers know how to make heat exchangers that are efficient,
> including using thinner metal and proper air flow techniques?

Scrubbing surfaces for better heat transfer IS the industries
proverbial _Let's Build a Better Mouse Trap_. Combustion efficiency
design and integration is already well established and has many choices
to meat a criteria.

What was interesting to watch is the Discovery Channel's Lance Armstrong
saga. Specifically, detailing the interaction of air to the surface of his
clothing.

It went from researching golf ball dimples to mother natures design of a
Tuna!
ISTR, dimpled and scaly surfaces were the focal points.

As relating to heat transfer, a couple of years ago there was a program
showing the advances of ancient peoples, and how their levels of
achievement ranked to modern times.
How interesting that a properly hammered Wok was shown to have the best
heat transfer of all other kinds of modern designed woks.

My point is there's room for improvement.

-zero





Posted by Abby Normal on January 26, 2007, 8:31 am


On Jan 23, 3:38 am, nicksans...@ece.villanova.edu wrote:
> >mm wrote:
> >> >I am told by experts in the field that one of several ways that
> >> >high-efficiency furnaces squeeze more BTU's from their fuels is by
> >> >using thinner and thinner heat exchangers
>
> >> This seems like one of the false efficiencies...
>
> >That simply isn't true. The heat is going to transfer more effectively
> >across the thinner material. The thickness of the metal provides
> >a resistance to heat flow, just as thicker insulation, wood, or
> >anything else would.But metals are such good conductors that making the metal
thinner won't
> help much, given high resistance air layers on both sides, and thicker
> metal will spread out hot spots and increase efficiency.
>
> Nick

Well how high is the air film resistances in forced convection?

You have a lot of radiant heat transfer initially on the inside of the
HX, then perhaps in the tubular HXs , the tubes tend to taper down once
past the line of sight of the flame to enhance convection by making
the products of combustion more turbulent.

You then have one of your favourite issues, thermal capacitance, heat
needed to get the HX hot enough in the first place to start driving
heat into the air.

Old octopii are still in existance, utilizing this capacitance and some
old heavy duty clamshells still around too. The forced air clam shells
worked off of a fan/limit control, and would take some time before the
fan would engage.

Newer, thin wall HX's the fans are timing on maybe in 30 seconds tops
after ignition, and the heat stored in the HX s is gone in a short
period of time as well. Not like a heat anticiaptor shuts the heat off
5 minutes before the home is up to temperature.


So, I seriously doubt you ever desiogned a forced air furnace in your
life, so perhaps you could go stay at a Holiday Inn Express and then
after a good night's sleep, you can derive non-intinsic PI yet one more
time, and wow us with all the calculations involved in this simple heat
transfer problem.

It will most likely end up like your futile attempts to model a cooling
gain on a home as a simple "Conductance" based on indoor and outdoor
temperatures, and be 'completely out to lunch', yet one more time.


Posted by on January 24, 2007, 9:55 am
Well, the verdict is in. I called a Goodman authorized HVAC
dealer/contractor (in Denver Metro), and their technician told me that
the reason that return air temp must not be less than 55F is because of
the possibility of excess condensation.

BTW, I first called the Goodman hotline, but they told me that, for
liability reasons, they didn't provide tech support directly to
individuals. However, they told me to call one of their authorized
dealers with any questions. They gave me three names, and I called one
of them.

Cheers.


On Jan 18, 10:12 pm, bubbabu...@yahoo.com wrote:
> The unit in question is Goodman GMV9509050XBA gas furnace (95% eff.,
> 90,000 BTU.)
> It says in the Installation Instructions (page 7, Location Requirements
> & Considerations) that the following must be observed:
>
> "The temperature of the return air entering the furnace is between 55F
> and 100F when the furnace is heating."
>
> I am curious why the 55F requirement. I mean, when I'm not in the
> house, I would like to set the temp as low as possible in order to save
> on my heating bill. I think I could otherwise set it as low as 45-50F
> and still keep the water pipes from freezing. But I wonder why I'm not
> supposed to go below 55F. What could happen? Could the unit get damaged
> and why?
>
> Thanks
>
> P.S. I'm in the Denver, CO area - 5,300 ft altitude, if that matters.


Posted by Bubba on January 24, 2007, 5:57 pm
On 24 Jan 2007 06:55:45 -0800, bubbabubbs@yahoo.com wrote:

>Well, the verdict is in. I called a Goodman authorized HVAC
>dealer/contractor (in Denver Metro), and their technician told me that
>the reason that return air temp must not be less than 55F is because of
>the possibility of excess condensation.
>
Gee, where did I hear that answer about a week ago when you posted
your question?
Oh, thats right. It was from me!
Bubba

>BTW, I first called the Goodman hotline, but they told me that, for
>liability reasons, they didn't provide tech support directly to
>individuals. However, they told me to call one of their authorized
>dealers with any questions. They gave me three names, and I called one
>of them.
>
>Cheers.
>
>
>On Jan 18, 10:12 pm, bubbabu...@yahoo.com wrote:
>> The unit in question is Goodman GMV9509050XBA gas furnace (95% eff.,
>> 90,000 BTU.)
>> It says in the Installation Instructions (page 7, Location Requirements
>> & Considerations) that the following must be observed:
>>
>> "The temperature of the return air entering the furnace is between 55F
>> and 100F when the furnace is heating."
>>
>> I am curious why the 55F requirement. I mean, when I'm not in the
>> house, I would like to set the temp as low as possible in order to save
>> on my heating bill. I think I could otherwise set it as low as 45-50F
>> and still keep the water pipes from freezing. But I wonder why I'm not
>> supposed to go below 55F. What could happen? Could the unit get damaged
>> and why?
>>
>> Thanks
>>
>> P.S. I'm in the Denver, CO area - 5,300 ft altitude, if that matters.

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