Calculating Attic Air Temperature in Winter From Thermal Resistances of Construction Materials

[DCEngr1]

Engineers:

If the inside surface temperatures of wall construction of an interior wall and an exterior wall can be determined from resistances of the materials and interior/exterior temperature differentials, can the temperature of the air in the space in a pipe chase (between the walls) in an attic be determined with reasonable accuracy?

If so, how? Do the calculated surface temperatures of the wall areas exposed to the pipe chase assist in determining the air temperature in the space? If so, how?

Thank you,

DCEngr1

 

[IRstuff]

"If the inside surface temperatures of wall construction of an interior wall and an exterior wall can be determined from resistances of the materials and interior/exterior temperature differentials, can the temperature of the air in the space in a pipe chase (between the walls) in an attic be determined with reasonable accuracy?

If so, how? Do the calculated surface temperatures of the wall areas exposed to the pipe chase assist in determining the air temperature in the space? If so, how? "

Not quite sure why you didn't post in the HVAC forum.

The temperature in attic is highly dependent on how much air circulates due to the louvers and intentionally constructed leakiness of the attic, as well as the thermal leakage to/through the ceiling of the floor below.

Define "reasonable"

Your second paragraph doesn't make much sense. Thermal equilibrium requires the temperatures to balance heat flows, so the temperature of a chase has to to dependent on the temperatures around it, since the only other possible heat source/sink is the pipe itself.


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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[Dougt115]

If you know the material of the inside wall and the materials of the outside walls and the temperature of one side of each. Assuming a constant heat flow you can use T1:T2a and T2b:T3 and solve for T2a and T2b.
T1 is temperature of the room.
T2a is temperature of the chase on the room side
T3 is exterior temperature.
T2b is the temperature of the chase on the exterior wall side.

T1:T2a is dependent on the materials of the inside wall.
T2b:T3 is dependent on the materials of the exterior wall.

This is only approximate.

 

[racookpe1978]

Well, you could go a bit more detail: T_room would be mid-level (thermostat) air temperature.
T_air (immediately under the ceiling) will be 3-4 degrees hotter.

But, without knowing air flow through the attic, you know nothing. (As stated above.) You could make lots of estimates and guesses, but you have to start somewhere - get the air temperature of the attic near the pipe chase over at least 3x 24-hour periods. That's what you really want to know.

Now, it's mid-March, your attic may not be -15 deg F as in mid-Dec or January, but you have to start somewhere.

 

[dvd]

My advice is to save the time and money it costs to calculate this and spend it on heat-tracing, then forget about it. Pipes don't have to assume anything to freeze, whereas your assumptions to keep the pipes from freezing may not be effectively communicated to the pipes on every day of the year. Also it might be nice to insulate the interior wall so that the sound of the plumbing isn't a requirement to prevent freezing.

 

[ZDR1985]

At steady state, heat coming in from adjacent spaces (hotter space) should be equal to the heat going out to the adjacent spaces (colder space).

Heat flow in = Heat flow out

H = U A dT

U = heat transfer coefficient of wall / roof / floor

A = Area of wall / roof / floor

dt = temperature difference

Let say outside is hotter than inside, so inside will gain heat.

H (from outside) = H (to inside)
Uo x Ao x (To - Tx) = Ui x Ai x (Tx - Ti)

"o" means outside
"i" means inside

if you have more than one wall / roof / floor exposed, just add, and work on your algebra.

[(Uo1 x Ao1) + (Uo2 x Ao2) + ....][To-Tx] = [(Ui1 x Ai1) + (Ui2 x Ai2) + ....][Tx-Ti]

You are looking for Tx

Red flag if I am doing it wrong.