I want to build a small 1x1x1 m insulated (wooden) box, with the purpose of testing an internal coating (the idea is to buy 5 square meters of it instead of 60 for a full scale room). My idea was to place an electrical heating element and a small fan inside, and raise the temperature from approximately 20 to 40 degrees Celsius. Given the air conditions inside, I used the Carrier diagram to find the thermal power I would need to heat up the humid air volume in 0.5 hours. I then tried to consider transient heat losses through the walls of the chamber, of which I know the transmittance: to do so, given the relatively small temperature differences between inside (from 20 to 40 C) and outside (20 C), I figured I would reduce it to a steady state case with a constant internal temperature of 30 C (arithmetic mean between initial and final temperature). The thermal power lost through the walls and calculated this way is then, in my ideas, to be summed to the power needed to heat up the air volume, thus obtaining the power I need from the heating element.

My questions are:

1- Is that of the average temperature a good assumption?

2- Apart from that, am I doing anything wrong?

I reckon this is quite basic stuff, I'm sorry, but I never faced this kind of issues and second opinions would be of great help.

  • $\begingroup$ There are 6 internal faces, so you will need 6 sq. metres not 5... $\endgroup$ – Solar Mike May 30 '17 at 13:50
  • $\begingroup$ The bottom will be more insulated, that's why I said 5, plus the aim is to simulate a room, so i would only go as far as the vertical walls an the ceiling with the thin insulation I would apply, but that is not the point. The focal point of this thing is the approximations on the thermal transient... $\endgroup$ – smntnz May 30 '17 at 14:41
  • $\begingroup$ When we designed our house, we made sure to have good insulation underneath as well as around and on top.... $\endgroup$ – Solar Mike May 30 '17 at 15:31
  • $\begingroup$ guys could we please stick to the subject? I get it, I can make it 6 if that makes you happy, but please refrain from commenting on marginal stuff if you have nothing relevant on my real issue. $\endgroup$ – smntnz May 30 '17 at 18:49

I'm not sure what you're trying to figure out, but if you want to know how well your insulation is doing, I suggest that you add a thermostat and measure the total amount of power supplied to your heating element to maintain a constant temperature inside the box.

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  • $\begingroup$ I agree. To elaborate on your answer slightly: Although OP mentions "transient" in the title, steady state conditions are significantly easier to measure, and transient calculations don't seem relevant here (unless I've misunderstood the OP's intent). Either supply a constant power, and measure the steady state temperature with and without insulation, or use a thermostat to get to a constant temperature (e.g. a simple space heater from walmart) and use a power meter (also available cheaply) to measure the power supplied with and without insulation. $\endgroup$ – Daniel K Jul 20 '17 at 1:32

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