Air exchanges per hour override issues?

[Anonymous]

Hello!  I am modeling a single family house in Revit that has recently had a blow door test of 3.0.  I am wishing to model this 'air change per hour' value of 3 in Revit to analyze the resulting energy data in Green Building Studio (GBS).  I have tried adjusting the 'air changes per hour' in the 'outdoor air information' for each each zone AND I have tried adjusting the same value in the advanced energy settings under the analyze tab.  I have also adjusted the 'air changes per hour' in 'space type' settings.  None of these changes are impacting the GBS energy results.  Curious if there is a way to adjust the 'air changes per hour' in such a way that will impact building energy values?  Thanks in advance, H   

[yes_and_no]

I m not an engineer, but air change in residential typically 8 ACH (8 air change per hour). It relates to your amount of openable windows and room volume. When you cannot provide such minimum, then mechanical means shall applied, meaning HVAC supply air (only by system with outdoor air intake). The number "3" minimum value by air blow at the door is a y/n for prescriptive approach, not a performance approach (by Green studio, LEED or whatever you think will return a result after such input). Do correct if I m wrong.

[Anonymous]

@yes_and_no, Thanks for your input. Yes I am wanting to model, or rather show the effects on energy demand (i.e. the HVAC systems), between several different ACH values. The home that I am modeling is a highly efficient home with a low ACH number, supported by the blow door test. For some reason, Revit and subsequently, Green Building Studio is not reflecting a change in energy use/cost when I change the ACH value. No matter what value I input. It should, however, so I am wondering what I am missing. Appreciate your reply to my inquiry!

[yes_and_no · Accepted solution]

Like I pointed out, ACH is a minimum code requirement (in CA it is in the mechanical code). Blow door test is a y/n prescriptive requirement, not a Performance approach (meaning require very little calculation, or mostly just a y/n check mark), hence Rv will not response. You can expect RvMEP (not RvArch) to response if you have HVAC system modeled completely and calculated within Rv (even HVAC engineers don't do that in Rv).

If your highly efficient house has low ACH (sounds contradicting here, efficient house should have efficient ACH, not "low"), blow door test is not the answer unless the door is a latchless type.

[Anonymous]

Thanks again for helping me to understand the issue, @yes_and_no. I do not know the details of the blow door test, only that openings were sealed and the building was pressurized at 50 pascals. In regards to the low ACH value, I am basing that on the Dept. of Energy's rebate program that requires a low value from a blow door test (https://basc.pnnl.gov/information/infiltration-meets-ach50-requirements) . I do have a split heat pump system assigned to the model but it is possible I do not have it specified correctly. I was informed by an engineer that I would see an impact in the building energy modeling results if I adjusted the air changes per hour for the building.

[HVAC-Novice · Accepted solution]

That blower door test doesn't give you a number you can use in an energy simulation. Actual infiltration depends on actual pressure difference and that is not 50Pa most the time. dP depends on wind, and temperature, stack effect etc. Same for ACH (infiltration) , this is just an assumption of a given building and will vary as well. 

[Anonymous]

Thank you @HVAC-Novice for taking the time to explain. I have been trying to model a 'tight building envelope' for my graduate work and I thought that I was on the right track. I will have to rethink my approach, again.

[HVAC-Novice]

IIRC Trane Trace700 had a function to calculate infiltration for each hour based on the hourly TMY3 weather data (wind speed) and based on the elevation of each space above ground and if the surrounding are rural etc. So you got varying infiltration based on actual conditions (wind, and dT) for every hour. But it was very crude and you needed some baseline at zero dP and 0 dT, which is not what you have. I recall just using typical infiltration estimates, and T700 came up with huge infiltration during the design hour. I have the feeling it took a lot of babying to get something reasonable and you still have to guess the base infiltration. I don't know yet how they have implemented that in Trane Trace3D yet, but based on how immature T3D is, don't have much hope. 

For load calcs I use values and estimate if the building is leaky (0.25) , average (0.12), or improved (0.08 cfm/ft²) based on exterior wall area. but this only works for load and I have to estimate what leakage class the building has based on the typical construction. 

ACH doesn't work to begin with since the area that contributes to infiltration is the enclosure area only. So an ACH for a cube would be the same value like for a room with the same volume, but a very elaborate envelope. So any method you use should be based on exterior surface area, not volume. 

[Anonymous]

@HVAC-Novice Thank you for that information. It definitely helps me to know of other avenues. I had high hopes for Revit, with regards to building energy analysis with highly efficient building envelopes, but it is good to know that the software is not quite there yet. You made some good points, thanks again.

[HVAC-Novice]

Let me throw in another wrench: The actual U-values of construction also vary depending on specific design and execution due to thermal bridges. Not only for stud-walls, but also consider different methods to structurally attach a balcony or other features. Structural material typically is dense and transmits heat well. When you know how the building is built in real life, this is manageable and you can get better estimates. Better than for infiltration values. 

To my knowledge most energy simulation software doesn't have good features to account for thermal bridges to any good detail. that's why we call it an "estimate" 🙂

[yes_and_no]

I felt I confused you more than helping you. Sorry.

I took a look at GBS interface, and wonder how would you make a connection between that and Rv.

 

[Anonymous]

@yes_and_no. No worries at all! I appreciate all thoughts and input as they make me think. You were right in the end, that the ACH did not impact energy results. @HVAC-Novice confirmed that for me in a way that I understood. I am finishing graduate school and my work has been to create building information models of affordable housing units in the US in Revit and then upload them into Green Building Studio for analysis. (actually the latest version of Revit has a GBS interface). Revit does energy analysis too, but it is not as specific. So I am not sure what your question is exactly, but part of that process has been trying to model energy-efficient designs, in particular a tight building envelope. Unfortunately, while I have been able to demonstrate other how other energy-efficient strategies, like triple glaze windows, higher insulation values and heat pump HVAC systems improve building energy performance, I have not had as much luck modeling a 'tight' building envelope. So now I am trying to find a way to quantify what I can not model in Revit and GBS. There's the long answer! 😃