Unmet Hours

Baseline AHU system level sizing - LAT

asked 2016-09-14 06:07:50 -0500

markos
65 ●3

updated 2017-05-08 15:42:42 -0500

__AmirRoth__

4436 ●5 ●17 http://bleedinggreenna...

Hi everyone!

Can somobedy explain me please how should be AHU heating coil leaving air temepratures correctly set? Especially for system 5-7 there is packaged VAV with reheat and I wonder what temeperatures should be used for AHU heating coil and zone heating coil (reheat coil). I would say that reheat coil should be set with supply-air-to-room-air temperature difference of 20°F because of design airflow rates, so if room heating setpoint is for instance 72°F then reheat coil is set at 92°F. But my question is what about AHU heating coil? Should it be set the same as reheat coil? Or quite opposite = at 52°F and it would be something like preheat coil? Is it prescribed somewhere in App.G?

Thanks for any advice in advance!

Martin

answered 2016-09-15 09:02:13 -0500

Annie Marston
4297 ●4 ●31 http://www.resustain.com

The temperature coming out of your AHU for system 5-7 should be the temperature required for cooling in the space, which is generally 20F lower than setpoint temperature according to G3.1.2.8 Design airflow rates in appendix G. So in order for your room to meet setpoint in cooling mode the heating coil in the AHU should be set to the same temperature, otherwise you have simultaneous heating and cooling in your AHU. The reheat coils will then take care of meeting temperature when heating is required.

There is one more rule to be aware of in Appendix G G3.1.3.12 Supply air Temperature reset: "the air temperature for cooling shall be reset higher by 5F under the minimum cooling load conditions" and so if you are setting your heating coil at the same temperature as your cooling coil, during minimum load these temperatures need to be raised by 5F

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Thank you very much! This is what I needed to know. Just for sure: Am I right with the reheat coil reset = 72F to 92F? I hope so...

markos ( 2016-09-15 09:36:17 -0500 )edit

yes, although in OS and EnergyPlus you don't need to put a setpoint on the reheat coil. In zone sizing if you put in "temperature difference" and set it to 20F then that takes care of everything.