topic Re: Heating and cooling load calculations in Revit MEP in Revit MEP Forum

Heating and cooling load calculations in Revit MEP

Anonymous — Tue, 25 Feb 2014 03:35:45 GMT

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

Re: Heating and cooling load calculations in Revit MEP

info — Wed, 26 Feb 2014 16:58:18 GMT

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.

Re: Heating and cooling load calculations in Revit MEP

Anonymous — Thu, 27 Feb 2014 01:51:49 GMT

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

Re: Heating and cooling load calculations in Revit MEP

vitaliy5151 — Thu, 27 Feb 2014 08:00:34 GMT

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!

Re: Heating and cooling load calculations in Revit MEP

russellvee — Thu, 27 Feb 2014 14:36:39 GMT

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?

Re: Heating and cooling load calculations in Revit MEP

Anonymous — Fri, 28 Feb 2014 00:44:37 GMT

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?

Re: Heating and cooling load calculations in Revit MEP

vitaliy5151 — Fri, 28 Feb 2014 06:47:15 GMT

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.

Re: Heating and cooling load calculations in Revit MEP

vitaliy5151 — Fri, 28 Feb 2014 10:17:53 GMT

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

Re: Heating and cooling load calculations in Revit MEP

Anonymous — Tue, 04 Mar 2014 22:43:40 GMT

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?

Re: Heating and cooling load calculations in Revit MEP

Anonymous — Wed, 05 Mar 2014 07:18:07 GMT

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.

Re: Heating and cooling load calculations in Revit MEP

Anonymous — Wed, 05 Mar 2014 07:24:53 GMT

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!!!!

Re: Heating and cooling load calculations in Revit MEP

russellvee — Wed, 05 Mar 2014 13:33:29 GMT

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.

Re: Heating and cooling load calculations in Revit MEP

Anonymous — Wed, 05 Mar 2014 13:54:16 GMT

i just hope the revit people see this thread and adopt your solutions in next revit package

Re: Heating and cooling load calculations in Revit MEP

Anonymous — Thu, 13 Mar 2014 21:49:31 GMT

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.

Re: Heating and cooling load calculations in Revit MEP

russellvee — Thu, 13 Mar 2014 22:09:58 GMT

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.

Re: Heating and cooling load calculations in Revit MEP

Anonymous — Thu, 13 Mar 2014 22:38:57 GMT

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.

Re: Heating and cooling load calculations in Revit MEP

Anonymous — Fri, 14 Mar 2014 01:12:40 GMT

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

Re: Heating and cooling load calculations in Revit MEP

russellvee — Fri, 14 Mar 2014 02:52:17 GMT

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?

Re: Heating and cooling load calculations in Revit MEP

russellvee — Fri, 14 Mar 2014 03:47:37 GMT

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:

Re: Heating and cooling load calculations in Revit MEP

Anonymous — Fri, 14 Mar 2014 04:09:22 GMT

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