[EnergyPlus_Support] VAV reheaters and outdoor air reset for LEED / ASHRAE 90.1

["Jean marais" <jeannieboef@xxxxxxxxx>]

The last couple of posts regarding my replies on this topic were a little in disarray. I have tried to consolidate that with this post. Regards, Jean.

Background:
As an AHU typically supplies more than one zone, the situation may arise where the one zone requires heating whilst at the same time the other needs cooling. By insuring that the AHU delivers (zone cooling setpoint - 11 deg C) air to the terminal units all the time, you can insure that both heating and cooling can take place at the required delivery air temperatures. 
ASHRAE requires the "design supply air temperature" to the zone to be (zone cooling setpoint - 11 deg C) for when the zone needs cooling, and (zone heating setpoint + 11 deg C) for when the zone needs heating. "
"design" meaning for the design day or worst case. To me this is a maximum, not a constant. Therefore, the reheating coil should try and maintain the room temp, not the outlet coil temp, but the outlet coil temp may not exceed the maximum of the "design supply air temperature". This way you can avoid overheating a room that has very high ODA requirements.
Similarly, the cooling supply air may not be colder than (zone cooling setpoint - 11 deg C).

Application:
In the heating case (at â??designâ?? conditions) the air is raised from not lower than (zone cooling setpoint - 11 deg C) to not higher than (zone heating setpoint + 11 deg C) by the reheater. 
In the case where all the zones need heating, your AHU should still deliver not lower than (zone cooling setpoint - 11 deg C) to the terminal units as a â??preheatedâ?? temperature if ODA temp is below (zone cooling setpoint - 11 deg C).
The AHU (when in cooling mode and in heating mode) delivers air at lowest (Cooling Setpoint Temp - 11 deg C) to the terminal unit as per G3.1.2.8. If the zone requires heating, the rest of the heating is done by the reheater (delivery air temp @ Heating Setpoint Temp + 11 deg C). If the zone requires cooling, the reheater coil must not operate.

The Conundrum:
e+ terminal reheaters, by default, will try and bring the delivery air temperature to the same temparature as the zone heating setpoint temperature, if the zone requires cooling, i.e. the terminal unit is in â??cooling modeâ??. (This is because usually reheaters are reheating air that has been classically overcooled at the AHU to dehumidify the air in summer conditions, and, as it is terrible practice to blow such cold air onto people if no reheating has taken place, it reheats the air.) If the zone requires heating, the reheater will heat the air up to the maximum setting for the reheater which should be set to not exceed (zone heating setpoint + 11 deg C).

Therefore, "if the zone requires cooling, the reheater coil must not operate" presents a problem. Currently, the best way to do this is to schedule the availability of the electric or water heater to OFF. This can be a timed schedule, EMS controlled or the preferred AvailabilityManager:HighTemperatureTurnOff monitoring the zone exhaust node temperature so as not to exceed the zone heating setpoint + 0.5 deg C tolerance. (This means that if the zone exceeds the zone heating setpoint by 0.5, the reheater will turn off.)

Please note the earlier mentioned AHU delivers air at â??designâ?? not colder than (Cooling Setpoint Temp - 11 deg C) to the terminal unit as per G3.1.2.8., must include a OutdoorAirReset as per G3.1.3.12 for some system types 5 through 8, which is realized with SetpointManager:OutdoorAirReset.

This is because, ASHRAE assumes, as is often the case in real systems, that the outlet air condition at the cooling coil is constant, where e+ can allow this to fluctuate if the setpoint managers are properly defined, we will not be using this functionality for ASHRAE 90.1 simulations. Basically the outlet air condition is allowed to go warmer when the outdoor air is not so warm. So for LEED ASHRAE simulations it is better to stick to the norm and set a fixed temperature with SetpointManager:OutdoorAirReset object to set the temperature schedule.

By the way, this setpoint manager is also used for the water loops in ASHRAE simulationsâ?¦see G3.1.3.4 and G3.1.3.9 as it can be applied to both water and air fluid loops.
Note also G3.1.3.13 when setting terminal unit settings for minimum flow rates.

Note Temperature Setpointmanagers can't be applied to terminal unit reheaters.

The AHU without terminal reheaters:
Your control of AHU coils is an important part of getting this right. This topic has been experimented with by myself extensively and mentioned in detail in previous posts. In a nutshell, allowing the coils bandwidth result in best results.
It may be necessary to add an additional â??ductâ?? object. This will allow you to set a setpointmanager on the inlet of the duct and one on the outlet of the duct, so that they donâ??t overwrite each other.

I prefer setpointmanager:single(or multi)zone:heating with the lower limit set to (zone cooling setpoint - 11 deg C) and the upper limit set to (zone heating setpoint + 11 deg C) placed on the last node of the AHU in combination with setpoinamager for the oda mixing duct (I forget the name) to control the heating coil and mixing duct. Make sure to associate both heating and cooling coils to this setpoint manager using the list object for this.

The cooling coil setpoint manager can be set up as described earlier with a SetpointManager:OutdoorAirReset and assigned the outlet of the cooling coil.




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