How Revit 2020 calculates U-values for estimating heating and cooling loads ?

[dyp4f]

Hi.
I am trying to figure out what exactly formula Revit uses to calculate heat transfer
for estimating heating and cooling loads.

As we know from the theory, the heat transfer via (let say) a wall, equals to:
U*A*dT , where
U is the overall heat transfer coefficient (W/m2K)
A is the wall area (m2)
dT is the temperature difference over wall (oC)

My question is exactly how Revit calculates the U-value, because this is the core of the
problem.

In general when we have multiple layers in the wall , U can be calculated as

1 / U = 1 / hci + Σ (sn / kn ) + 1 / hco , where

sn is the thickness of layer (n) (in m)
kn is the thermal conductivity of material in layer (n) W/(m K),
and
hci & hco = inside & outside wall convection heat transfer coefficient W/(m2 K)

If we have one layer, the formula is : 1 / U = 1 / hci + s / k + 1 / hco

The part: [ s / k ] of the formula represents "conductivity".
The part: [ 1 / hci + 1 / hco ] of the formula represents "convection".

We (usually) don't only have "conductivity", except in very rare cases.

We (almost always) have "conductivity" AND "convection". 

Can anybody please confirm that Revit calculates heat transfer according to the correct
FULL formula, I mean taking into account not only "conductivity" but "convection" also ?

[HVAC-Novice]

There are so many things wrong with the Revit load calculation, somewhat im-precise U-values are the least of your concern. 

Use a dedicated software and there you can enter the actual U-values (or that software has a way to calculate them).

They should just remove that "feature". If you actually design (like in you put a stamp on a design and a contractor will install based on that design), don't use Revit. May as well use a rule-of-thumb if you need something for schematic design. 

[dyp4f]

Someone from Autodesk can answer please, if the heat transfer calculations take into account "convection" along with "conductivity", or not ?

[RobDraw]

This is a user forum. They are your greatest resource.

[AutoCAT95 · Accepted solution]

On the example of a wall, as you said:

If you select a wall and go to the properities -> construction -> structure edit and add layers of the wall. Thickness is easy to set, and as you noticed it's important to have U value for thermal calculations. To set it correctly we edit material of each layer (... symbol near material name). You can see Identy, Graphics, Appearence, Physical and Thermal Tabs - if not you can add them. In thermal Tab you can edit thermal conductivity of material in layer.

As you also noticed we need inside & outside wall convection heat transfer coefficient for proper calculations. There is no such an option in revit to add tihs, but we can workaround here. Just add some thermal resistance (hci, hco as you call them) to other physic layer of the wall, to get propper U value.

That is the way to get the correct U value for your walls etc. I hope I described it clear.

Anyway further calculations of Heating and Cooling loads tool in Revit leave something to be desired ...

[dyp4f]

Of course Rob, you are right. Users are the most valuable resourse, indeed. But no user has access to the internal computational engine inside Revit, so in this case, only Autodesk could answer about how heat transfer calculations are actually executed.

[dyp4f]

Thank you AutoCAT95, your answer is meaningful and practical.
I am just wondering about the following:
1) if convection is not taken into account in the internal calculations, then your solution to add conductivity coefficient could indeed simulate the reality.
2) if convection is actually taken into account in the internal Revit calculations then by adding conductivity coefficient we are going to make a computational error.

[AutoCAT95 · Accepted solution]

I have checked it out and (what we call it in Poland) Rsi, Rse - is not included into calculation of U value. In further heating load calculations the U values for components are used - You can choose schematic types from analytical model or inside Revit libraries.

I hope it helps you.

[dyp4f]

Thank you again AutoCAT95. I was suspecting that convection is not included in Revit calcutations, and your definite answer make things clear.

[AutoCAT95]

If that answers leaves no doubt, please mark it as a solution. If you will encounter new problems in that subject i can try to help you.

[Anonymous]

Dear all,

I was also intrested on the calculation of heating and cooling loads in revit and I know there has already discussed many times on this topic. I am very sad to hear that even in the new version of Revit 2020 things have not changed.

Here there is an old link where they show the missing of the Rin and Rout values on the calculation.

https://forums.autodesk.com/t5/revit-mep-forum/heating-and-cooling-load-calculations-in-revit-mep/m-... 

I know there are other software to do this in a detailed way (we use solar computer or linear for example), but it sounds so stupid that there is a specific tool to do that directly in Revit but it doesn't work!

It would be nice just to be able to overwrite the U values (including already conductivity and convection) in to the settings below. Actually it looks like that is not possible to choose a different U values from the window list... 

[AutoCAT95]

Actually you can modify that Analytic construction list in notepad and add your custom names and values.

[Anonymous]

O really? Can you maybe show me how? which is the path to do that? Thank you very much for your help 

[AutoCAT95]

Sure, here you are:

https://knowledge.autodesk.com/support/revit-products/troubleshooting/caas/sfdcarticles/sfdcarticles...

[DARTEC]

i have the issu , i found that revit U value calculation are wrong cause they forget to add 1/hi+1/he the internal and external surface resistance  =0.05+0.12 =0.17 m²k /w this what cause the mistake in U value 

i have made the same wall calculation in Carrier HAP program and it confirm the Revit Calculation have missed 1/hi+ 1/he value