Can VE Reset Temp on Zone Reheat Valve Call?
Yes, this is possible with the VE. How we do this depends on what we are actually trying to do.
First, bear in mind that there are no reheat valves, per se, but conditions in the zone for which controllers generate signals for heating and cooling coils to rais and lower supply temperatures.
If we are resetting the supply temperature upward as permitted by the warmest zone on the system that is in cooling mode, then the following applies: Where a single AHU is serving multiple zones with terminal reheat, each zone in the multiplex has a vote as to what the off coil air temperature at the cooling coil should be. In cooling mode, when extra cooling is required, the airflow rate will first be increased in an attempt to maintain the cooling set point in each VAV zone. If the maximum airflow rate cannot maintain the room set point for a particular zone, the cooling SAT controller for that zone will vote for the off coil temperature from the cooling coil in the AHU to be reduced until some lower limit is reached (e.g., 55 F).
The off-coil temperature will be determined by the lowest temperature that is required by any zone in the multiplex. In each of the other zones, the VAV airflow will first be reduced as needed to avoid overcooling. Once the minimum airflow is reached, the terminal reheat coil will heat the air as required to prevent each particular zone being overcooled. Prototype HVAC systems five through nice are set up to model supply temperature reset in this manner. The attached document gives some more information on supply temperature reset.
If we are intending to further adjust (raise) the leaving air temperature at the air handler when all zones on the system are in heating mode (none are in cooling mode), we would first need to add ON-OFF set-point control in the cooling LAT reset controllers, such that the cooling coil would cease to operate when none of the zones are in cooling mode. We would then add similar controls (like the cooling SAT controls) to the heating coil in the air handler, such that LAT from this coil would be increased above the typical minimum of 55 F to something more useful for heating. (We can and probably would want to leave the standard min-SAT controller on the heating coil in the AHU.) The system-wide heating SAT would then be determined by the added heating SAT controllers voting (colder rooms would vote for increasingly warmer SAT), such that the zone reheat would have a lighter load or none.
Use caution, however: The added heating SAT controllers would need to have ON-OFF set-point controls to prevent overlap with the cooling SAT controls. Also, whichever room is coldest is going to win the vote for the warmest permissible heating SAT. If we are using dual-max VAV control (as the prototype VAV systems are set up for by default), any zone receiving warmer air than needed will reduce its airflow in an attempt to avoid being overheated; however, once a zone reaches its minimum airflow, if it’s getting warmer supply air than needed, it will overheat, as there is (not usually) a re-cooling coil. If we want to set up re-cooling, that’s basically the mirror image of re-heating.
