Air Speed Calc. for Letter Template

The template is looking for the airspeed experienced by the space occupants. If there is wide variation I would either use the maximum, or split a space into two different space types.

Your mechanical engineer is correct. For this credit, we care about air movement, as it has a direct impact on comfort. So air speed, not volume.

The engineer should be able to estimate air speed, based on diffuser selection.

So for an open office it would be ok to use airspeed at one interior diffusser and one at perimter and both at seating level or standing level in the office.

Aplogies my comment was supposed to be a Question if one can use 2 diffuers and average the velocity

I think y'all are overthinking this one. ASHRAE 55 allows "elevated air speed" to be used as a way to reduce the "operative temperature". In other words, you can tolerate higher temperatures by increasing air speed. But practically no effect occurs until you are in the 100 + FPM range.

Diffuser manufacturers usually show the "throw" of the diffuser at different air speeds. Unless, you are trying to use elevated air speed to shift the comfort zone, just make sure you select the diffusers to keep the air speeds where people are nice and low, then enter a nice round number in the table - something below 50 FPM. At that range it doesn't really matter if it is actually 20, or 30, or 42.8. As long as the air speed is low enough to have no effect on the comfort zone, it doesn't much matter for this credit.

Clear as mud?

Crystallclear and it should help yr mechancial engineer a lot although It seems he was not that confused but it was more the CFM/person thing that was not related to this credit...........

Your Mechancial Engineer might just have done an approach typical for an Engineer and put forward some credit options for the other project team members to reviev. Engineers normally try to write up specifications that leave little room for misinterpreation but sometimes the depth of the specifications may not be easily understood by non-engineers and maybe a lack of RFI started to get the credit intend confused. So it should be up to the other project team members to get back to the Engineer with questions on the options if there are issues that seem not so clear. Obvioulsy the Engineer may have a recommendation on the options he would consider is the best overall option, but again this would be from his point of view. The final credit option should obvioulsy be a consensus decision at the Project Team level.

If there is a difficulty in reaching a consensus at the Project Team Level, the final option selected could also be 2 options combined into one. In such case the Engineer job should only be to merge the 2 credits into one. It is however not the Engineer that should make that decision.

In the case of IEQ 7.1 should we not use the term velocity instead of speed ? - "Speed is the rate of motion but does not have a direction, velocity is the rate of motion but HAS a direction"

If there is a difficulty in reaching a consensus at the Project Team Level, the final option selected could also be 2 or more options combined into one.

I'm still very confused about this point, and how we show compliance. We are currently designing a 175,000 Courthouse builidng, with mostly priviate offices, and large courtrooms. Very little open office space. If we have to show compliance for all occupants, then don't we have to calculate the air fpm at the occupant for every diffuser? This seems very similar to me to the point back in v2.2 where we had to do this, and it was so labor intensive on the documentation that people quit going after it. Maybe we are overthinking this. Also, i thought every diffuser had to provide 40 fpm to the occupant in order to meet this point? Is there a resource that can walk us step by step through the calculations and documentation for this credit? We are just having a tough time with the air speed and documentation part of this credit. the LEED reference guide states no calculations are required for this credit, but it seems to me that a lot of calculations are required. Thanks!

You are creating work for yourself. The documentation is asking you to verify that high air speeds are not going to affect comfort. It doesn't require that you document every single air outlet in the project.

We usually just put a single line in the table for different activity types. For example - "office". One line in the table for every office in the building. We list the design criteria, and make sure it is less than 50 fpm.

The air speed portion of this is a real confusing part of this credit. I can estimate a speed based on diffuser throw along the path of the throw but there are so many variables that would change this value: temp, flowrate, diffuser placement, position measured, obstructions etc. It may be 40fpm at the end of its throw envelope (say throw is along ceiling and down wall) but, in the center of the room where the person's desk is may be much lower and who knows the effect of the desk they may be working at?

At our typical design conditions of 75F and 50%RH (for load calcs) and using an office environment (1.0 met) and summer (0.54 clo) you cannot get compliance at any air speed. At 40fpm speed the temperature needs to be 78.2F. But just because our system is designed to maintain 75F in summer doesn't mean the user could not choose to keep their space at the higher temps (thermostat control). Can we define a space temp as long as it is within the possibilities of the system and does this need to be on the contract documents?

Thanks,
Mike

With the conditions you describe you should be able to demonstrate credit compliance using the graphical method (Figure 5.2.1.1). This method may be used for metabolic rates between 1.0 and 1.3 met, clothing allowances between 0.5 and 1.0 clo, and airspeeds of 40 fpm or less. The summer and winter design conditions should be shown on a copy of the figure and included in the supporting documentation. As furniture can be rearranged, it is not necessary to be concerned about the airflow disruptions, but permanent obstructions such as soffits, columns, etc. should be considered in the diffuser placement.