ASHRAE 62.1-2007

[GreatSmoky]

There is an equation in this standard for a primary outdoor air fraction: Zp=Voz/Vpz (eqn 6-5) which is pretty much a percentage of outside air where Voz is the outside airflow and Vpz is the primary airflow.

But then it shows in table 6-3 in the same standard that the higher the Zp value, the lower the ventilation efficiency (Ev). I would guess that the higher Zp (percentage outside air), the higher efficiency it should be.

This is a big mystery for me. Am I seeing things or interpreting it wrong? Anybody who has experience with this standard and can help would be greatly appreciated.

 

[tys90]

The 62.1 Users Manual doesn't say anything about Table 6-3 really except that it can give you low ventilation effectiveness values. I'm going to attempt to explain it how I understand it. As with anything about 62.1 this may be wrong.

Table 6-3 makes approximates of how effective a multizone system is at delivering outdoor air to each zone. If you have a high Zp that table ASSUMES you have a wide variety of individual Zps. This also assumes an average Zp of .15. Say you have a bunch of zones with a Zp of .15 and one with .55. Your average outdoor air percentage will actually be somewhere just over .15 but far less than the .55 percentage that one zone needs, thus ineffectively supplying that zone with it's required amount of outdoor air. I *think* that what that table is doing. That's also why they note it results in an artificially low value of Ev at high Zp numbers.

I've never used that table, I've always used the equations in Appendix A. I would suggest using the Appendix A calculations for multizone systems. TRACE700 has a nice feature where you can limit the Zp to an ideal number (through trial and error) that maximizes your ventilation effectiveness. It may increase your overall airflow but decreases your outdoor air requirements. Of course, you must make a judgement on what is the right choice because going too overboard on airflow just to reduce the outdoor air amount a little bit can result in large equipment and energy costs.

 

[GreatSmoky]

Thanks tys90! For the appendix A method, what are you using for the Er term if I just a have a conventional VAV system with no secondary return air? If I had a fan powered box, what value of Er would you use?

 

[tys90]

There isn't a quick and easy answer to that one. It depends on where you are recirculating the air from and how you are doing it. The 62.1 Users Manual does go into detail about determining Er. I would suggest getting a copy of the manual.