Controls, Energy and Troubleshooting
Thermostats and control loops, economisers, variable air volume, balance point and degree days, and a table of common problems.
What a control system does
The loads of Heating and Cooling Load Calculations are peak values that occur a few hours a year. The rest of the time the plant runs at part load, and the control system is what makes it follow: it measures the room, compares it with the setpoint, and moves a valve, damper or speed. Good control is the difference between a building that is comfortable and efficient and one that heats and cools at the same time.
Sensors, valves and control loops
A control loop has a sensor (temperature, humidity, CO₂, pressure, flow), a controller and an actuator (valve, damper, variable-speed drive). The most common algorithm is PI (proportional plus integral; see the PID Tuner if you want to tune one): the output is Kp × error plus the accumulated error, and it holds the setpoint without a steady offset. For example with a proportional gain of 4 percent per kelvin and a room 1.5 K below setpoint, the proportional part alone opens the heating valve by 6 percent.
| Device | Notes |
|---|---|
| Two-way control valve | varies flow; the loop becomes variable-flow, so the pump should follow the pressure |
| Three-way valve | diverts flow (constant total); simple but wastes pump energy |
| Damper | opposed blades for modulating; its authority (the share of the pressure drop it takes) should be above 0.3 |
| Variable-speed drive | follows duct or pipe pressure; the cube law makes it the best saving |
| Sensor placement | away from sun, diffusers and exhaust; calibrate each year |
A valve should be sized so that it takes a fair part of the circuit pressure: a valve that is too large only moves over the first 10 percent of its stroke, and the loop hunts and wears the actuator.
Control strategies that save energy
- Economiser (free cooling): when outdoor air is cooler or drier than the return air, use it instead of mechanical cooling. Control on enthalpy (better) or temperature.
- Setback and optimum start: lower the heating setpoint (or raise the cooling one) when the building is empty, and start early enough to reach comfort at opening time. A 4 K setback for 12 hours saves about 13 percent of the heating energy at an average outdoor temperature of 5 °C.
- Reset: raise the chilled water or supply air temperature when loads are light, and lower the heating water temperature in mild weather (weather compensation).
- Demand-controlled ventilation: vary the outdoor air with the CO₂ or occupancy (see Ventilation and Indoor Air Quality).
- Variable flow: variable-speed fans and pumps, VAV boxes, two-way valves.
- Avoid simultaneous heating and cooling, the most common waste: reheat of overcooled air, fighting thermostats, a heating valve that leaks while the cooling runs.
- Setpoint discipline: each kelvin of wider deadband saves roughly 6 percent of the energy (typical).
Worked example: economiser and VAV
Worked example: free cooling and a turned-down fan
- Economiser. The return air is 24 °C and 50 percent (h = 47.8 kJ/kg). Outdoor air is 15 °C and 60 percent (h = 31.1 kJ/kg), so every kg/s of outside air brings 16.7 kJ/kg of free cooling. At 0.5 kg/s that is 8.3 kW with the compressor off. At 30 °C and 50 percent the outdoor air has h = 64.2 kJ/kg, more than the return, so the economiser closes.
- VAV. At design the office needs 425 L/s. A typical minimum of 30 percent is 128 L/s: higher than the 55 L/s of ventilation air, so ventilation is safe at minimum flow. By the fan laws the fan power at minimum flow (30 percent) is about 2.7 percent of full power, if the duct pressure setpoint is reset.
- The catch: a VAV system on minimum flow in a hot room gives poor air distribution, and a high minimum needs reheat. Choose the minimum carefully.
Degree days and the annual energy
The energy a building uses for heating over a season is roughly its heat-loss coefficient times the heating degree days (HDD): the sum over the year of (base temperature − daily mean outdoor temperature) when that is positive. E = UA × HDD × 24 hours. Cooling degree days work the same way. Annual energy is not peak load times hours: the load varies with the weather, and the internal gains bring the base temperature down to a balance point below the room setpoint.
Worked example: the office’s annual heating energy
- Heat-loss coefficient = envelope 46.4 W/K (walls and glass) + ventilation 66.4 W/K (55 L/s) = 112.8 W/K.
- With 2 800 heating degree days (K·day) the energy is 112.8 × 2 800 × 24 ÷ 1 000 = 7 583 kWh of heat, or 8 425 kWh of fuel at 90 percent boiler efficiency (about 674 at 0.08 per kWh).
- A 70 percent heat-recovery unit on the ventilation air cuts the loss to 66.4 W/K and saves 3 123 kWh (41 percent). Demand-controlled ventilation that halves the average outdoor air saves about 2 231 kWh.
- This estimate ignores internal and solar gains and occupancy; use it to compare options, not to size equipment.
Troubleshooting
| Symptom | Likely causes | What to check |
|---|---|---|
| Room too hot or too cold | wrong setpoint; sensor in the wrong place; valve or damper stuck; low flow | measure supply temperature and flow; check the actuator moves; compare sensor with a handheld |
| Clammy, humid, but cool | oversized cooling with short cycling; high latent load; leaking ducts; return-air problems | check dew point; the coil should run long cycles; seal and insulate ducts |
| Stuffy, headaches, high CO₂ | low ventilation; dampers closed or blocked intakes; too many people | measure outdoor air flow and CO₂; check the minimum damper position |
| Cold air at the diffuser but a hot room | poor distribution, short circuit to the return, a duct leak | check throw and mixing; smoke test; measure airflow at the diffuser |
| Frozen cooling coil | low airflow (dirty filter), low load, refrigerant undercharge, a failed fan | check the filter, fan and airflow first, then the refrigerant |
| Noise or whistling in ducts | velocity too high; damper nearly closed; fan off its curve | measure the pressure; open dampers and slow the fan |
| Chiller trips on high head pressure | dirty condenser or tubes, tower fan or water problem, non-condensables | check the condenser approach, the tower fan, the water flow and the treatment |
| High energy bills | simultaneous heating and cooling; failed economiser; operating out of hours; overcooling | trend the data; check the schedule and setpoints; test dampers and valves |
| Condensation on windows or ducts | surface below the dew point; humidity too high; missing insulation or vapor barrier | compare surface temperature with the dew point (Moist Air and the Psychrometric Chart) |
Commissioning and maintenance
A large share of the energy waste in buildings comes from controls that were never commissioned: wrong sensor calibration, a damper wired backwards, schedules left at default. Test every point (command, feedback, range), run the sequences in all modes, trend the main values for a month, and fix what the trends show. Maintenance is mostly filters, coil and condenser cleaning, drain pans, belt and bearing checks, refrigerant and water treatment records, and recalibration. Record the results; the next person will need them.
Related
- Previous: Hydronic Heating and Cooling. Back to the overview: HVAC and Psychrometrics.
- PID Tuner and PLC and Logic Control for the control side.
- Sensors and Field Devices.
FAQ
Why does the room overshoot?
The controller gain is too high, or the integral action too fast, or the valve is oversized. Reduce gain and increase the integral time.
What is a good economiser changeover?
Compare the enthalpy of outdoor and return air (best), or use a dry-bulb changeover around 18 to 21 °C in dry climates and lower in humid ones.
How do I find simultaneous heating and cooling?
Trend the heating valve, cooling valve and damper positions; both open together in the same zone is the sign.
How often should the system be re-commissioned?
Every few years or after any change; continuous commissioning with trend analysis is better.
References
- ASHRAE, Handbook – HVAC Applications (building automation and control), Guideline 36 (high-performance sequences of operation for HVAC systems) and Guideline 0 (commissioning).
- CIBSE Guide H (building control systems) and TM 17; EN 15500 (control for HVAC); EN ISO 16484 (building automation).
- US DOE, Building Commissioning: the Practical Guide; Seem, J. E., on economiser control.
Last reviewed: October 2026. Savings figures are typical and depend on climate and building.