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2751 Views, 32 Replies

02-24-2014 07:34 PM

Hi All,

Couple of questions with this?

- Do you trust Revit heating and cooling calculations? I mean after you modify all the building materials and weather information do you do your calcs with Revit or a third party laod calculation software?

- How about duct and pipe sizing? Most of consultants do those with other tools.

- I have been exporting revit spaces as gbXml to E-20 to do the load cals but I need to mannually input all those calcs back into Revit. Is there a faster way of doings that?

- And lastly most of the clients don't like the way Revit shows the calculations Report. Is there a way to customise those outputs and get them shown in the way I want?

Cheers,

Mehdi

Solved! Go to Solution.

Solved! by vitaliy5151. See the answer in context.

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!

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02-26-2014 08:58 AM in reply to:
Mehdi.Kardehi

I asked the same thing a few weeks ago and got no replies. I think we're on our own. If you are on subscription you can go to au.autodesk.com and watch this video: __MP2462 : Beyond 3D: Improve Efficiency by Using Revit MEP for Mechanical Calculations__

It has some pointers for doing load calculations as best as Revit can, which is questionable at best. I don't have E-20 or Trace so I have tried just about everything imaginable to get accurate load results with any sort of confidence factor. Its so close to being a useful calculation tool, but missing just enough features to make it a colossal waste of time and effort.

The ductwork is pretty much the same story. It would be so awesome to be able to use they system inspector, tag airflows, create color charts showing velocities, etc. if those features actually worked as advertiesed. Instead, you'll spend hours trying to figure out why your duct has lost all its connections once you try to segment and size the trunk.

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02-26-2014 05:51 PM in reply to:
info

hope these get fixed in 2015 version...so disappointing

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02-27-2014 12:00 AM in reply to:
Mehdi.Kardehi

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!

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02-27-2014 06:36 AM in reply to:
Mehdi.Kardehi

I would prepare yourself for disappointment. I have yet to see anyone from Autodesk acknowledge that the load calculation feature is a problem. The 2012 AU video linked above is from an Autodesk seminar and basically says the calculations are rubbish and not to trust them. Don't they know it's dangerous to put insufficient software tools in the hands of engineers, at the risk that they might actually use them? They improved the energy model GBS export feature, but if the energy conservation measures are based upon faulty calculations, what the heck is the point?

I am really suprised there isn't an API add-in that does hvac calcs right. I would think Carrier or Trane could easily develop something useful, or risk having a third party software company take a big bite out of HAP and Trace market share. What bothers me most is how simple the changes would be to make HVAC calcs useful. If they were to simply allow us access to a few more parameters in a space schedule (wall/window/roof/floor area, window solar gain), I could set my own U values for the envelope and calculate my own space loads in a schedule. If they gave access to a few parameters at the zone level (peak ventilation load, #/people) I could calculate my own system loads in a schedule. It would be so EASY! That's all I'm hoping for in 2015. Just a few parameters. Please?

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02-27-2014 04:42 PM in reply to:
vitaliy5151

Thanks Viraliy.... I did similar thing in E-20 and got to the same results as you mentioned...photo is attached....

I would be happy to hear the way that you get around this issue....

Have you done similar excersice in duct and pipe sizing calculations?...they look a bit strange too

I am sure Autodesk put in ther disclaimer somewhere that we dont guarentee that the calculated values are correct to cover their asses...this is too much deviation...

I am not a subscription user but can please someone with subscription open a case for this?

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02-27-2014 10:47 PM in reply to:
Mehdi.Kardehi

*Have you done similar excersice in duct and pipe sizing calculations?*

* *

Yes, I did. Coming soon with detailed report with screenshots, because, according to my feelings, my file attachments to the __message__ was not enough.

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02-28-2014 02:17 AM in reply to:
Mehdi.Kardehi

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

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03-04-2014 02:43 PM in reply to:
Mehdi.Kardehi

Getting correct R-values for walls is great, but how do you expect to get accurate loads with out the ability to add equipment loads (sens and lat.)? Especially in places like kitchens. Also does revit take into account external shading on windows?

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03-04-2014 11:18 PM in reply to:
mmcglone

yes U R correct, apart from that where or at what stage does the revit takes the fresh air requirment,,,,

but its good that atleast one issue is fixed, hope rest will get fixed in future.

but its good that atleast one issue is fixed, hope rest will get fixed in future.

Ajay K. HVAC Designer, BIM Modeler

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