West Zones Overheat, Interior Zones Freeze: Diagnosing a VAV System
A 4-story, 120,000 sq ft office building has a single VAV air-handling unit with hot-water reheat coils at every zone. After the first winter, the west perimeter zones reach 78°F by mid-afternoon while the interior zones complain of being cold. The BAS trends show: supply air at a constant 55°F; every west zone box at 100% damper for four hours; duct static pressure at the end of the west run at 0.45 in. w.c. against a fan setpoint of 1.5 in. w.c.; every interior box sitting at its 30% minimum with the reheat valve 100% open from 11 a.m. to 4 p.m.; the economizer at minimum position; the AHU fan at full speed. Design occupancy is one person per 150 sq ft. The owner asks whether to add a chiller, add a boiler, or just fix the controls.
1. Diagnose the west zone overheating. What are the competing explanations, and what would you measure to discriminate between them?
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The damper position is not the airflow. A box at 100% damper with 0.45 in. w.c. at the end of the run against a 1.5 in. w.c. setpoint is starving. The damper can be wide open and still deliver far less than design. That single trend reading invalidates the premise that the west zones are getting all the air the design says they should.
Measure in this order:
- Traverse or read the flow ring at two or three west boxes and compare to design. A 40 to 50 percent shortfall confirms a distribution problem.
- Walk the west duct run: a failed fire/smoke damper, a closed manual balancing damper left over from TAB, collapsed flex duct, or an undersized run serving the longest path.
- Check the fan curve and total static - a fan at full speed delivering low static is either short-cycling air somewhere else or operating far off its curve.
Then, and only then, check the load side. West glass in the afternoon is the worst solar case; verify whether the design cooling load took the exposure into account and whether the specified shading was installed or value-engineered out. A west zone can be simultaneously under-supplied and over-loaded.
Do not accept 'the boxes are at max, so the chiller is undersized.' You cannot size central equipment from a damper position.
2. The interior zones are at 30% minimum with reheat wide open all afternoon. Is 30% the right minimum? What do the energy and ventilation codes require, and why does this matter?
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30% is a rule of thumb, not a code number. The zone minimum must be the ventilation minimum airflow, calculated per ASHRAE 62.1 using the breathing-zone outdoor airflow (people component plus area component) divided by the zone air distribution effectiveness, then adjusted for system ventilation efficiency. For an office at one person per 150 sq ft and 55°F supply air, that number is frequently well below 30% of design airflow. Setting every box to a flat 30% overcools the interior and forces reheat.
The energy code prohibits what is happening. ASHRAE 90.1 limits reheat so that a zone is not reheated above roughly 30% of its peak design airflow, and it requires that zone minimums be set to the ventilation minimum rather than to an arbitrary percentage. Cooling air to 55°F at the AHU and then reheating it to satisfy a zone that is already at its minimum setpoint is simultaneous heating and cooling - exactly the penalty the code is written to prevent. It is also the single largest energy waste visible in these trends.
Practical consequence. The interior complaints are not a boiler problem. They are the predictable result of over-set minimums plus a fixed 55°F supply temperature with no reset. Fixing that will cut reheat energy and stop the complaints without touching the boiler.
3. Answer the owner: add a chiller, add a boiler, or fix the controls? Give a recommendation and the sequence you would fund.
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Neither new central equipment is justified by these trends, and 'fix the controls' is right in direction but wrong as a scope. Reprogramming the BAS without rebalancing will simply move the problem around.
Fund in this order:
- Air balance and static survey on the west run. Find and correct the restriction. This is the cheapest fix and it directly addresses the 100% damper / 0.45 in. w.c. contradiction. Rebalance the whole system after.
- Recalculate every zone minimum per 62.1 with the actual supply temperature, and reset the boxes. Lock out reheat unless the zone is at minimum flow and below setpoint.
- Rewrite the sequence of operation: supply-air temperature reset driven by the worst-case zone (which may well be the west zone), economizer verification, fan static reset to the actual index box rather than a fixed 1.5 in. w.c.
- Solar control on the west glass if, after the airflow is restored, the load still exceeds design.
Tell the owner honestly: the central plant capacity has not yet been demonstrated to be inadequate, and spending capital on a chiller before step 1 would be paying to hide a distribution defect. If a load calculation after step 1 shows the plant is short, that is a separate, defensible capital request.