Heating and cooling load calculations in Revit MEP
Accepted solution
You asked: "- Do you trust Revit heating and cooling calculations?". Answer — No, I don't and that's why.
The answer to this question lies in the method of calculation of the coefficient of heat transfer resistance (R).
(Here I translate my message from the Russian-language forum)
As you know, heat resistance Ro = Rin + R + Rout where:
Ro — heat resistance [m2 * K / W].
Rin — resistance heat absorption (inner wall surface ) is about 0.13 [m2 * K / W].
Rout — heat transfer resistance ( the outer surface of the wall) is about 0.04 [m2 * K / W].
R — thermal resistance of the wall.
Thus calculated heat transfer coefficient (call him as in Revit, the letter "U"), ie U = 1/Ro.
So, it seems that Revit simply ignores the value Rin + Rout and calculate heat loss (heat gain calculation has not yet been tested) only based on the value of the thermal resistance of the wall, ie R.
For example, a wall consisting of concrete, 200 mm thick, the thermal conductivity K = 1.04 [ W/m2 * K] (the value of 1.04 is taken as an example , although it is stated in the thermal properties of the material for concrete default).
Create a wall in Revit with these characteristics and we've got this:
Thermal resistance R = 0.1923 [ m2 * K / W].
Heat transfer coefficient (U) = 5.200 [ W/m2 * K] and calculated as 1/R.
Here is a screen of what happened:
Like until all true ...
Now, to verify the calculations let's create a space 10 by 10 meters with floor and roof. And let's take concrete wall 200 mm, which had just been created. The internal temperature of 5C degrees. Outdoor temperature -19C.
Doing Revit analysis of heat gain and heat load and the output is this:
Naturally assume that everything is OK. But the first check in any other program such as Rehau RAUWIN or calculation in just Excel will show that Revit ignores the value Rin and Rout.
Now show the results of calculations in the program, which is used for a long time — Rehau RAUWIN. Here again, the screen, which shows the composition of the wall and the coefficients Rin and Rout. Their values are sometimes other, here are the average:
Here it is seen that the value of heat transfer coefficient K (or "U" in Revit) eventually equals 2.96 [ W/m2 * K], which is much lower than in Revit! It happens because of proper accounting coefficients Rout and Rin. If we'll set them to the zero value in Rehau RAUWIN, we get the following result:
Which corresponds to the WRONG value of the coefficient, which we got in Revit, which once again proves that these factors Rin and Rout in Revit are excluded. And we got no form to set them right in Revit!
If we'll set in Rehau RAUWIN program a similar space, which is already calculated in Revit, we obtain the following values:
From this it is clear that a significant difference in the calculations.
What may be possible to solve this problem? Is there anything in Revit ability to specify these factors in the settings ? Is it true I described the problem and whether it exists at all?
(end of translation from forum). Original text in russian here.
By the way, a little later I found a way to set these coefficients fraudulently. Then all the calculations are correct. But if this will be interesting, will talk about later.
Cheers!
So, here's a way that you can use to create a wall with the correct value of the heat transfer coefficient.
For a visual comparison, I created two storage #1 and #2 with the same dimensions and parameters.
Storage # 1 have the concrete wall, which creates Revit default, i.e. excluding Rin and Rout.
Storage # 2 have the concrete wall too, but I adjusted right R factor by adding coefficients Rin and Rout in my own way.
I created them by setting the appropriate values of thermal conductivity in the material properties. Thus Revit will think that it takes into account factors Rin and Rout in the calculation of heat loss.
Take a look:
Storage # 1 concrete wall (by default):
Storage # 2 concrete wall (corrected). Let's make Rin
Now Let's make Rout
And after all the manipulations we get it here:
Hope this helps!
Vitaliy
Can trie verifying in panel propierties of the space, especified the heat gain per person.
Make sure this changes has saved and calculate. If the problem continue check de parameter in the space type.
Again ...could some one with subscription please open a case for this and maybe refrence them this thread.
It is really disappointing and time consuming 
It does this with even the most basic model made from the default mechanical template (attached). Do you guys get different results?!? What am I doing wrong?
I thought perhaps there was some bizarre rule that you have to subtract 5-10% of your people load, but I've looked back and forth through the ASHRAE handbook and I don't see anything like that. Besides, why would you specify 200 if you only intend to calculate 190? It definitely doesn't do this in an equivalent Trace calculation:
