Revit MEP Forum
Welcome to Autodesk’s Revit MEP Forums. Share your knowledge, ask questions, and explore popular Revit MEP topics.
cancel
Showing results for 
Show  only  | Search instead for 
Did you mean: 

Heating and cooling load calculations in Revit MEP

60 REPLIES 60
SOLVED
Reply
Message 1 of 61
Mehdi.Kardehi
27397 Views, 60 Replies

Heating and cooling load calculations in Revit MEP

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

60 REPLIES 60
Message 2 of 61
info
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.

Message 3 of 61
Mehdi.Kardehi
in reply to: info

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

Message 4 of 61
vitaliy5151
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:

 

Concrete


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:

 

Revit analysis of heat gain and heat load


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:

 

Concrete in Rehau Rauwin


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:

 

Concrete in Rehau Rauwin with no Rin and Rout


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:

 

Rehau RAUWIN calculation


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!

 

Message 5 of 61
russellvee
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?

Message 6 of 61
Mehdi.Kardehi
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? 

 

 

 

Message 7 of 61
vitaliy5151
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.

Message 8 of 61
vitaliy5151
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:

 

Storages data

 

 

Storage # 1 concrete wall (by default):

Concrete 200 with U=5.2

 

Storage # 2 concrete wall (corrected). Let's make Rin

Resistance heat absorption

 

Now Let's make Rout

Heat transfer resistance

 

And after all the manipulations we get it here:

 

Concrete 200 with U=2.76.jpg

 

Hope this helps!

Vitaliy

Message 9 of 61
mmcglone
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?  

Message 10 of 61
ajax05k
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.
Ajay K. HVAC Designer, BIM Modeler
Message 11 of 61

thanks mate

 

I am really hoping that Autodesk fix this ASAP...

 

any developer knows that the product need to be tested before putting it out for sale!!!!

Message 12 of 61
russellvee
in reply to: mmcglone

The internal gains is more of a concern for me as well since I use the analytic constructions for the envelope, which can be edited in XML. At present the only way to add latent load to a space is by adding people. I have tried adding just one person per space and then setting the sensible and latent loads (per person) to equal the total load I need for the space.

 

For example, ASHRAE fundamentals tells us that the latent gain for a residential space (in BTU/h) is q = 68 + 0.07(Area) + 41(People). I can calculate this with a separate variable and plug that into the latent gain per person for the 1 person assigned to my space. This seems like a pretty ridiculous workaround though.

Message 13 of 61
ajax05k
in reply to: russellvee

i just hope the revit people see this thread and adopt your solutions in next revit package
Ajay K. HVAC Designer, BIM Modeler
Message 14 of 61
jostorres
in reply to: Mehdi.Kardehi

Hello to all

 

Im actually study the precision of Revit for cooling load calculation. For compare the calculus i use Elite Chvac this program can make the calculus with RTS method like Revit. Basically the major differences in calculus are for:

1- walls for the reason previosly explain but can fixed whit the method previoly explained too. Can edit any wall for Revit take de R value from ASHRAE tables will be formidable.

2- Roof, the roof configuration for U value o use default values. 

2-Windows are difference are like more then 100% of calculated by Elite Chvac, thats very high diferencce. Actually i dont now the reason 

3-Partition load, this point i really dont understand why dot work, this is a easy calculos but im not sure if revit dont take de interior walls like partitions or wall. In both cases the load calculated it´s not the same, diference are more the 90%. 

4-Ventilation load its lees in 10%, acceptable percentage.

5-Fab heat, i dont how o were can configure this. Someone now how to modified this?

 

Like all i hope Autodesk make something this, deveploment this aplication.

 

Message 15 of 61
russellvee
in reply to: Mehdi.Kardehi

Nothing seems to make mathematical sense with their calculations. If I specify 1 person in a space with 250 Btu/h sensible and 200 Btu/h latent, the load report will tell me I have 393 Btu/h of gain from people and 190 of that is latent. Why???

 

This forum really needs the emoticon where the smiley face is banging his head on his desk, because that's how Revit usually makes me feel.

Message 16 of 61
jostorres
in reply to: russellvee

Can trie verifying in panel propierties of the space, especified the heat gain per person.

 

revit.png

 

Make sure this changes has saved and calculate. If the problem continue check de parameter in the space type.

 

Message 17 of 61
Mehdi.Kardehi
in reply to: jostorres

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 Smiley Indifferent

Message 18 of 61
russellvee
in reply to: jostorres

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?

 

Capture.JPG

 

 

 

Message 19 of 61
russellvee
in reply to: russellvee

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:

Capture.JPG

 

 

Message 20 of 61
jostorres
in reply to: Mehdi.Kardehi

Check the profile operation, tomorrow morning (for me) actualize with a image. But chek de profile operation of building and the tipe space

Can't find what you're looking for? Ask the community or share your knowledge.

Post to forums  

Rail Community


Autodesk Design & Make Report