Playbook: Simultaneous Heating & Cooling

Adapted from HVAC FDD Reference v1.0, Remediation Playbooks (pp. 153–154).

Step 1 — Verify the fault

1. Pull a 24-hour trend of the heating valve command and cooling valve command for the same AHU or FCU on one chart.
2. Confirm both commands are above the rule’s configured material thresholds for its configured duration. AHU-0016 and FCU-0007 intentionally use different library defaults; verify the deployed parameters before triage.
3. Check data quality flags — rule out bad sensor readings before acting.
4. Check for intentional cooling-plus-reheat/dehumidification, freeze protection, valve exercise, commissioning, or mode transition. These states are NO_EVAL for FCU-0007 unless the deployed sequence explicitly says otherwise.
5. If position feedback sensors exist, compare command vs. actual position to determine whether the problem is the control sequence (software) or a physical issue (stuck valve).
6. Do not price command overlap alone. Quantify only from verified valve position/flow and delivered heating/cooling energy during the overlap.

Step 2 — Remote fix (BAS changes, no site visit)

1. First confirm the unit is not designed for intentional cooling-plus-reheat; do not add an interlock that defeats humidity, freeze, or equipment protection.
2. Check the ordinary control sequence for a heating/cooling interlock:
   • The heating valve should close to 0% before the cooling valve may open, and vice versa. If no interlock exists, add one — the most common fix.
   • G36 §5.16 specifies a minimum deadband of 2.8 °C (5 °F) between heating and cooling loops.
3. Check for a deadband between the heating and cooling PID loops:
   • At least 1 °C (2 °F) between where heating stops and cooling starts; if the loops overlap, insert a deadband.
   • For single-duct VAV systems, verify the SAT setpoint is above the reheat lockout temperature before investigating individual zone interlocks.
4. Check for stuck manual overrides on either valve command (BACnet priority array). Release any override holding a valve open — among the most common retro-commissioning findings (PNNL-27338).
5. Check whether the heating coil has an OAT lockout; if outdoor temps are warm enough that heating shouldn’t run, enable one (typical: disable heating when OAT > 16 °C with 2 °C hysteresis).
6. If the AHU serves mixed interior/perimeter zones, verify perimeter reheat is not fighting the central cooling coil — SAT reset (see missing-reset) often resolves this.

Step 3 — On-site service (if remote fix doesn’t resolve)

Only qualified personnel may open panels or work on hydronic valves. Follow the site’s lockout/tagout procedure and never bypass freeze, condensate, pressure, or electrical safeties to clear an FDD finding.

1. Inspect the heating valve actuator: command to 0% from the BAS and physically verify full closure; check the mechanical linkage (disconnected linkages are common on older systems); verify normally-open vs. normally-closed — an NO valve opens whenever the actuator loses its signal.
2. Inspect the cooling valve actuator with the same checks.
3. On pneumatic-to-digital conversions: normally-open pneumatic valves need spring-return actuators; check for air leaks in the tubing — small leaks can prevent full closure.
4. If a valve cannot close mechanically, replace the actuator ($200–$800) or valve body ($500–$1,500).
5. Check for coil bypass: on older AHUs a face-and-bypass damper can mix hot and cold decks — inspect the bypass damper actuator and linkage.

Step 4 — Confirm resolution

1. Monitor both valve commands for 48 hours after the fix.
2. Verify both commands do not overlap above their configured thresholds during applicable modes; separately confirm intentional dehumidification still works.
3. The fault should clear automatically. FCU-0004/0005 may remain if physical valve leakage persists after the command conflict is fixed.
4. Recalculate impact from measured thermal input/output, not command percentage.