HVAC troubleshooting: hot and cold complaints

Hot and cold complaints are the most common calls a facility team receives. They are easy to dismiss as personal preference, and sometimes that is part of the story. More often, though, a persistent complaint points to a real problem: a failed actuator, a mislocated sensor, a zone serving incompatible spaces, or a system that cannot meet the load.

Good troubleshooting is systematic. Instead of turning down the setpoint and hoping, work from the occupant outward, from the room to the terminal unit to the air handler to the central plant. This guide lays out that process and the most common causes found at each level.

Start with the complaint

Before touching equipment, collect information:

• Where exactly? One desk, one room, one side of the floor, or the whole building?
• When? Morning only, afternoons, Mondays, sunny days, cold snaps?
• Too hot or too cold, and how often? Constant discomfort points to a different cause than intermittent discomfort.
• Other symptoms? Drafts, stuffiness, noise, humidity, odours.
• What changed? New tenants, renovations, furniture moves, new equipment, schedule changes.

Patterns in time and location are powerful clues. Afternoon overheating on west-facing perimeter offices suggests solar gain. Monday morning cold complaints suggest a warm-up or optimal start problem. One cold office surrounded by comfortable ones suggests a terminal unit fault.

Measure the actual conditions

Occupants feel temperature, radiant effects, air movement and humidity together. ASHRAE Standard 55 describes how these factors combine. Measure, rather than relying on the thermostat display:

1. Air temperature at desk height in the occupied zone, compared with the zone sensor reading.
2. Surface temperatures of windows and exterior walls, using an infrared thermometer.
3. Air movement near the occupant, looking for drafts from diffusers or cold windows.
4. Relative humidity, if stuffiness or dryness is mentioned.

A discrepancy between the zone sensor and the occupied zone is common. See thermal comfort and ASHRAE Standard 55 for how these factors interact.

Zone-level causes

Most complaints originate at the zone.

Terminal unit checks

For VAV systems, the terminal unit is often the culprit. Verify that:

• The damper actuator responds to commands and the damper actually moves through its full stroke.
• The airflow sensor reads correctly. A dirty or damaged velocity sensor can make the controller think airflow is satisfied when it is not.
• Minimum and maximum airflow setpoints match the design and the air balance report.
• The reheat valve opens and closes fully, and hot water reaches the coil.

Our guide to VAV box types and selection explains how these components work.

Overrides and setpoints

Check the BAS for points left in manual override, temporary setpoint changes that became permanent, and schedules that no longer match occupancy. Overrides left in place after past troubleshooting are a frequent hidden cause.

System-level causes

If several zones served by the same air handler are uncomfortable, look upstream:

• Supply air temperature too warm in cooling or too cold in heating, perhaps due to an aggressive reset strategy or a failed valve.
• Duct static pressure too low for distant zones, especially with a static pressure reset that is not responding to zone requests.
• Economizer faults allowing too much cold outdoor air in winter or hot air in summer.
• Fan problems such as belt slip or a VFD limited to a low maximum speed.
• Ductwork issues like disconnected flexible duct, closed fire dampers or crushed ducts above ceilings.

Trend logs are invaluable at this level. Plotting zone temperatures, damper positions, supply air temperature and static pressure together often shows the problem clearly. See trend logging and using BAS data.

Plant-level causes

When many systems struggle, check the central plant: chilled water supply temperature and differential pressure, hot water temperature, boiler or chiller staging, and pump operation. A heating hot water reset that drops water temperature too far during a cold snap can leave perimeter zones cold across the building.

Design and envelope limitations

Sometimes everything works as designed, and the design does not fit the current use. Common examples:

• A conference room zoned with offices, so it overheats when full.
• An interior zone sensor controlling a perimeter space.
• A space converted to a server room or kitchen without added cooling.
• Drafty windows or poorly insulated walls producing cold radiant surfaces.
• Interior partitions added after construction, splitting a zone with one sensor.

These require changes to zoning, terminal units, airflow or the envelope. An engineering review can identify the most practical fix.

A troubleshooting procedure

1. Document the complaint, location, timing and recent changes.
2. Measure conditions in the occupied zone and compare with the zone sensor.
3. Review BAS data for the zone: setpoints, overrides, damper, valve and airflow trends.
4. Physically verify the terminal unit, actuator, airflow sensor and reheat.
5. Check air distribution: diffusers, ductwork and balancing dampers.
6. Review the air handler and plant if multiple zones are affected.
7. Correct the cause, then follow up with the occupant and confirm with trends over several days.

Recording each complaint, cause and fix in the CMMS builds a history that reveals recurring problems.

Frequently asked questions

Why are some people always cold when others are comfortable?

Clothing, activity, personal metabolism and location all affect comfort. ASHRAE Standard 55 recognizes that no single condition satisfies everyone. Local factors such as drafts from diffusers or cold windows often affect specific seats and can be corrected.

Does lowering the thermostat setpoint cool a room faster?

Not usually. Most systems deliver cooling at a fixed rate once the setpoint is below room temperature. A much lower setpoint just overcools the room later.

When is air balancing needed?

When airflow measurements at diffusers or terminal units do not match design, after renovations or tenant changes, or when complaints persist despite working controls. Air balancing re-establishes correct airflow throughout the system.

Key takeaways

• Treat persistent hot and cold complaints as diagnostic data, not just preference.
• Use timing and location patterns to narrow down causes before touching equipment.
• Most problems are at the zone: sensors, terminal unit actuators, airflow sensors, reheat valves and overrides.
• When multiple zones are affected, check supply air temperature, static pressure, economizers and the plant.
• Some complaints stem from zoning or envelope limitations that require design changes.