Maximum Leaving Air Temperature

[sauba]

What is generally the maximum leaving air temperature for VAV system with ceiling diffusers? I have seen temperatures ranging from 85 to 110 degrees. What is the most commonly accepted method for lowering this temperature and achieve desired heating? Are minimum airflows generally increased until acceptable leaving air temps are achieved?

Thanks.

 

[tombmech]

I'm not sure I understand your last statement - lower supply temperatures will need more airflow to heat the same load.

As for the rest, what you ask is essentially a very subjective issue: human comfort. Supply temperatures are generally governed by comfort tolerances. For instance, residential heat pumps supply air for heating on the lower scale of the numbers you cite. Often, comfort is sacrificed as the lower temperature, higher airflow causes a "drafty," cool sensation compared to higher temperature alternatives.

As long as the ratios of airflow vs. temperature are kept the same, the same heating capacity will result. However, there is a wide divergence on the sensation of comfort.

 

[walkes]

This depends on the system configuration. Do you have reheat coils in the air stream? Your sequence of control may be to reduce your airflow to the minimum position when the space no longer requires cooling. If the space then requires additional heat the reheat coil would pick up the load, it may be sized at this minimum position or at a greater volume. The reheat would be sized to allow whatever airflow to pick up from the AHU supply temp to the space room temp plus any heat loss from the space. So depending on your airflow volume your supply temperature discharge would vary (as tombmech stated earlier).

 

[sauba]

Tombtech - My last statement I intended to propose that I increase airflow to lower supply temperature.

Walkes - My system is hot water reheat vav in an office environment.

I guess I am just looking for a ballpark figure/ROT. Do you generally try to stay under 90 F? This seems to be the most common.

My follow up was that if I need heat to a certain BTU but this will yield a temp of say 130F what is the most common way to lower this to around 90 and still achieve the necessary heating (ie increase the minimum airflow on the box so that the temp goes down, airflow goes up)?

 

[walkes]

The discharge air temperature would then depend on airflow. Increase airflow to reduce discharge temperature. Your design should verify that you have adequate throw at minimum position. You may find that 130°F air at minimum causes hot spots, but by increasing the airflow the space temperature evens out.

85°F to 90°F supply should be fine. Here is a link to check for further design information.

http://www.energy.ca.gov/reports/2003-11-17_500-03-082_A-11.PDF

 

[cme]

I design all my systems for 105 deg F LAT.

 

[tombmech]

As a general rule, I respectfully disagree that 85 deg.F. supply is fine. Depending on localized air currents, you can get a wind-chill effect resulting in a physical sensation that the air supply feels cold. "Pull down" is almost non-existent, and infiltration becomes a serious complaint.

CME's minimum standard is one that works.

Other than that, walkes and myself are in agreement, too.