Understanding the Max Z ratio
Posted on: October 1, 2010
A common energy modeling question is: Why did my VAV minimum go up when I added max z ratio in Trace 700? And why is the heating ventilation greater when using std 62?
This is a common issue when running standard 62, as it is a natural consequence of the calculations. I think it is best to walk through an example:
Take room a room where the ventilation requirement is 1 person * 5 cfm/person + .06 cfm/sqft*203 sqft = 17 cfm of ventilation in cooling.
In heating, it requires 17/.8 = 21 cfm of ventilation (since the htg ez is set to 80% under the airflows tab).
The space load requires 105 cfm at the space cooling peak.
And the total system supply is 7337.
In order to calculate the rooms critical %OA, we need to consider it's minimum stop which is .3 *105 = 31.5 cfm. So, looking at 17/31.5 = 54% OA. HOWEVER- the maximum z ratio was set to 50% - which means that the %OA is limited to 50% for a given room. As a result, the VAV minimum is raised so that 17/(new vav Minimum) = 50%..... where the new vav minimum is then set to 34 cfm. So that answers the 2nd issue as to why the VAV minimum is different than 30%. Obviously, this is calculated on a room by room basis. This also answers part of the reason why the heating ventilation is greater (the 80% effectiveness on the htg ez versus 100% on clg ez).
Finally, there is the issue of the ventilation resulting in higher numbers on the room checksums: In the example room, the cooling ventilation is 44% of the supply. Since there are critical zones with a maximum %OA as 50%, and a mixed air system can only provide one %OA to the entire system, it needs to calculate the OA per the formula
Y = X / (1+ X - Z), where Y is the system percent OA,
Z = .5 (50% max z-ratio was specified)
and X = Nominal CFM OA/Supply air total = 2880/7337 = .393
SO: Y= .393/(1.393-.5) = 44.0% - the system level percent OA
which is the % OA that you will see on the first page of the standard 62 report. The checksums value will be similar but slightly different because of slight changes in the block airflow and individual spaces.
To learn more about Trace 700, please sign up for the Trace 700 online Training
A common energy modeling question is: Why did my VAV minimum go up when I added max z ratio in Trace 700? And why is the heating ventilation greater when using std 62?
This is a common issue when running standard 62, as it is a natural consequence of the calculations. I think it is best to walk through an example:
Take room a room where the ventilation requirement is 1 person * 5 cfm/person + .06 cfm/sqft*203 sqft = 17 cfm of ventilation in cooling.
In heating, it requires 17/.8 = 21 cfm of ventilation (since the htg ez is set to 80% under the airflows tab).
The space load requires 105 cfm at the space cooling peak.
And the total system supply is 7337.
In order to calculate the rooms critical %OA, we need to consider it's minimum stop which is .3 *105 = 31.5 cfm. So, looking at 17/31.5 = 54% OA. HOWEVER- the maximum z ratio was set to 50% - which means that the %OA is limited to 50% for a given room. As a result, the VAV minimum is raised so that 17/(new vav Minimum) = 50%..... where the new vav minimum is then set to 34 cfm. So that answers the 2nd issue as to why the VAV minimum is different than 30%. Obviously, this is calculated on a room by room basis. This also answers part of the reason why the heating ventilation is greater (the 80% effectiveness on the htg ez versus 100% on clg ez).
Finally, there is the issue of the ventilation resulting in higher numbers on the room checksums: In the example room, the cooling ventilation is 44% of the supply. Since there are critical zones with a maximum %OA as 50%, and a mixed air system can only provide one %OA to the entire system, it needs to calculate the OA per the formula
Y = X / (1+ X - Z), where Y is the system percent OA,
Z = .5 (50% max z-ratio was specified)
and X = Nominal CFM OA/Supply air total = 2880/7337 = .393
SO: Y= .393/(1.393-.5) = 44.0% - the system level percent OA
which is the % OA that you will see on the first page of the standard 62 report. The checksums value will be similar but slightly different because of slight changes in the block airflow and individual spaces.
To learn more about Trace 700, please sign up for the Trace 700 online Training
A common energy modeling question is: Why did my VAV minimum go up when I added max z ratio in Trace 700? And why is the heating ventilation greater when using std 62?
This is a common issue when running standard 62, as it is a natural consequence of the calculations. I think it is best to walk through an example:
Take room a room where the ventilation requirement is 1 person * 5 cfm/person + .06 cfm/sqft*203 sqft = 17 cfm of ventilation in cooling.
In heating, it requires 17/.8 = 21 cfm of ventilation (since the htg ez is set to 80% under the airflows tab).
The space load requires 105 cfm at the space cooling peak.
And the total system supply is 7337.
In order to calculate the rooms critical %OA, we need to consider it's minimum stop which is .3 *105 = 31.5 cfm. So, looking at 17/31.5 = 54% OA. HOWEVER- the maximum z ratio was set to 50% - which means that the %OA is limited to 50% for a given room. As a result, the VAV minimum is raised so that 17/(new vav Minimum) = 50%..... where the new vav minimum is then set to 34 cfm. So that answers the 2nd issue as to why the VAV minimum is different than 30%. Obviously, this is calculated on a room by room basis. This also answers part of the reason why the heating ventilation is greater (the 80% effectiveness on the htg ez versus 100% on clg ez).
Finally, there is the issue of the ventilation resulting in higher numbers on the room checksums: In the example room, the cooling ventilation is 44% of the supply. Since there are critical zones with a maximum %OA as 50%, and a mixed air system can only provide one %OA to the entire system, it needs to calculate the OA per the formula
Y = X / (1+ X - Z), where Y is the system percent OA,
Z = .5 (50% max z-ratio was specified)
and X = Nominal CFM OA/Supply air total = 2880/7337 = .393
SO: Y= .393/(1.393-.5) = 44.0% - the system level percent OA
which is the % OA that you will see on the first page of the standard 62 report. The checksums value will be similar but slightly different because of slight changes in the block airflow and individual spaces.
To learn more about Trace 700, please sign up for the Trace 700 online Training
A common energy modeling question is: Why did my VAV minimum go up when I added max z ratio in Trace 700? And why is the heating ventilation greater when using std 62?
This is a common issue when running standard 62, as it is a natural consequence of the calculations. I think it is best to walk through an example:
Take room a room where the ventilation requirement is 1 person * 5 cfm/person + .06 cfm/sqft*203 sqft = 17 cfm of ventilation in cooling.
In heating, it requires 17/.8 = 21 cfm of ventilation (since the htg ez is set to 80% under the airflows tab).
The space load requires 105 cfm at the space cooling peak.
And the total system supply is 7337.
In order to calculate the rooms critical %OA, we need to consider it's minimum stop which is .3 *105 = 31.5 cfm. So, looking at 17/31.5 = 54% OA. HOWEVER- the maximum z ratio was set to 50% - which means that the %OA is limited to 50% for a given room. As a result, the VAV minimum is raised so that 17/(new vav Minimum) = 50%..... where the new vav minimum is then set to 34 cfm. So that answers the 2nd issue as to why the VAV minimum is different than 30%. Obviously, this is calculated on a room by room basis. This also answers part of the reason why the heating ventilation is greater (the 80% effectiveness on the htg ez versus 100% on clg ez).
Finally, there is the issue of the ventilation resulting in higher numbers on the room checksums: In the example room, the cooling ventilation is 44% of the supply. Since there are critical zones with a maximum %OA as 50%, and a mixed air system can only provide one %OA to the entire system, it needs to calculate the OA per the formula
Y = X / (1+ X - Z), where Y is the system percent OA,
Z = .5 (50% max z-ratio was specified)
and X = Nominal CFM OA/Supply air total = 2880/7337 = .393
SO: Y= .393/(1.393-.5) = 44.0% - the system level percent OA
which is the % OA that you will see on the first page of the standard 62 report. The checksums value will be similar but slightly different because of slight changes in the block airflow and individual spaces.
To learn more about Trace 700, please sign up for the Trace 700 online Training
A common energy modeling question is: Why did my VAV minimum go up when I added max z ratio in Trace 700? And why is the heating ventilation greater when using std 62?
This is a common issue when running standard 62, as it is a natural consequence of the calculations. I think it is best to walk through an example:
Take room a room where the ventilation requirement is 1 person * 5 cfm/person + .06 cfm/sqft*203 sqft = 17 cfm of ventilation in cooling.
In heating, it requires 17/.8 = 21 cfm of ventilation (since the htg ez is set to 80% under the airflows tab).
The space load requires 105 cfm at the space cooling peak.
And the total system supply is 7337.
In order to calculate the rooms critical %OA, we need to consider it's minimum stop which is .3 *105 = 31.5 cfm. So, looking at 17/31.5 = 54% OA. HOWEVER- the maximum z ratio was set to 50% - which means that the %OA is limited to 50% for a given room. As a result, the VAV minimum is raised so that 17/(new vav Minimum) = 50%..... where the new vav minimum is then set to 34 cfm. So that answers the 2nd issue as to why the VAV minimum is different than 30%. Obviously, this is calculated on a room by room basis. This also answers part of the reason why the heating ventilation is greater (the 80% effectiveness on the htg ez versus 100% on clg ez).
Finally, there is the issue of the ventilation resulting in higher numbers on the room checksums: In the example room, the cooling ventilation is 44% of the supply. Since there are critical zones with a maximum %OA as 50%, and a mixed air system can only provide one %OA to the entire system, it needs to calculate the OA per the formula
Y = X / (1+ X - Z), where Y is the system percent OA,
Z = .5 (50% max z-ratio was specified)
and X = Nominal CFM OA/Supply air total = 2880/7337 = .393
SO: Y= .393/(1.393-.5) = 44.0% - the system level percent OA
which is the % OA that you will see on the first page of the standard 62 report. The checksums value will be similar but slightly different because of slight changes in the block airflow and individual spaces.
A common energy modeling question is: Why did my VAV minimum go up when I added max z ratio in Trace 700? And why is the heating ventilation greater when using std 62?
This is a common issue when running standard 62, as it is a natural consequence of the calculations. I think it is best to walk through an example:
Take room a room where the ventilation requirement is 1 person * 5 cfm/person + .06 cfm/sqft*203 sqft = 17 cfm of ventilation in cooling. In heating, it requires 17/.8 = 21 cfm of ventilation (since the htg ez is set to 80% under the airflows tab). The space load requires 105 cfm at the space cooling peak. And the total system supply is 7337.
In order to calculate the rooms critical %OA, we need to consider it's minimum stop which is .3 *105 = 31.5 cfm. So, looking at 17/31.5 = 54% OA. HOWEVER- the maximum z ratio was set to 50% - which means that the %OA is limited to 50% for a given room. As a result, the VAV minimum is raised so that 17/(new vav Minimum) = 50%..... where the new vav minimum is then set to 34 cfm. So that answers the 2nd issue as to why the VAV minimum is different than 30%. Obviously, this is calculated on a room by room basis. This also answers part of the reason why the heating ventilation is greater (the 80% effectiveness on the htg ez versus 100% on clg ez).
Finally, there is the issue of the ventilation resulting in higher numbers on the room checksums: In the example room, the cooling ventilation is 44% of the supply. Since there are critical zones with a maximum %OA as 50%, and a mixed air system can only provide one %OA to the entire system, it needs to calculate the OA per the formula
Y = X / (1+ X - Z), where Y is the system percent OA,
Z = .5 (50% max z-ratio was specified)
and X = Nominal CFM OA/Supply air total = 2880/7337 = .393
SO: Y= .393/(1.393-.5) = 44.0% - the system level percent OA
which is the % OA that you will see on the first page of the standard 62 report. The checksums value will be similar but slightly different because of slight changes in the block airflow and individual spaces.
To learn more about Trace 700, you may sign up for the Trace 700 online Training
Bob Fassbender graduated from the University of Wisconsin - Madison with a degree in Chemical-Engineering. Following graduation, he spent 3 years working as a Marketing Engineer for Trane C.D.S. In the C.D.S. group, Bob developed and supported design and analysis software, primarily TRACE 700™. In addition to his development work, Bob also traveled around the country as a TRACE 700™ and System Analyzer™ instructor. Bob is also an experienced user with eQUEST energy modeling software. Today, Bob continues training and energy modeling as a LEED accredited professional (with a focus on LEED EA credit 1).