Automatx

The unit runs in occupied mode with the supply and return fans on at constant speed. Each half second the controller reads the outdoor, return, mixed and supply air values, works out a supply air setpoint, and then decides how much of that it can get from outdoor air before using a coil. Heating and cooling never run together.

Supply air setpoint. With reset on, a proportional-integral loop on space temperature moves the supply air setpoint between 12 and 20 °C. A warm space lowers the setpoint, a cool space raises it. The loop stops integrating when it reaches either limit, so it doesn’t wind up and overshoot when the load changes. Switching reset off holds the fixed setpoint you choose, and switching it back on starts from the current value without a jump.

Order of control. The outdoor air damper is used first, between its minimum position and fully open. If mixing outdoor and return air can’t reach the setpoint, the heating or cooling coil makes up the difference. The heating coil is modelled with a 25 K rise at full output and the cooling coil with a 12 K drop.

The space. The room is a simple thermal model: internal heat that rises with occupancy, heat lost through the envelope to outdoors, and the supply air adding or removing heat. Return air runs 0.5 K warmer than the space. Time for CO₂ is compressed so changes appear in about a minute.

Alarms. Values turn red when the space is more than 1.5 K from setpoint, supply air is more than 2 K from its setpoint, mixed air drops below 5 °C (a freeze risk to the coils), a coil is at full output, outdoor air is below −25 °C or above 32 °C, return air humidity is below 25 % or above 60 %, the space is over its design occupancy, or return air CO₂ is more than 10 % above the level expected at design occupancy.

This is an illustration, not a control program. A real unit adds safeties such as freeze stats, smoke shutdown, filter alarms, fan status proving and occupancy scheduling, and its coils handle moisture as well as temperature.

An economizer uses outdoor air for cooling when it takes less energy to condition than return air. This unit uses comparative (differential) enthalpy: it calculates the enthalpy of the outdoor air and the return air from their temperature, humidity and the site pressure, and enables the economizer when outdoor air is at least 2 kJ/kg lower. It disables again when outdoor enthalpy rises above return. The gap between the two points stops the economizer cycling on and off when the values are close.

Enthalpy matters because humid air carries latent heat. Outdoor air at 24 °C and 80 % RH is cooler than a 25 °C return, but it holds far more energy, so bringing it in would load the cooling coil. A dry-bulb economizer would let it in; an enthalpy economizer keeps it out. The opposite also happens: warm but dry outdoor air can have lower enthalpy than return air, and then full outdoor air with mechanical cooling uses less energy than recirculating.

When enabled and outdoor air is cooler than the supply setpoint, the damper modulates to mix outdoor and return air to the setpoint, never closing below its minimum position. When outdoor air is warmer than the setpoint but still has lower enthalpy, the damper goes fully open and the cooling coil trims (an integrated economizer). When disabled, the damper sits at its minimum position.

Humidity sensors drift, and an enthalpy comparison is only as good as the two sensors behind it. Many current guidelines pair enthalpy control with a fixed dry-bulb high limit for that reason, and real units add a mixed air low limit to protect the coils in cold weather.

People breathe out CO₂ at a fairly steady rate, so the CO₂ level in a room tells you roughly how many people are in it compared with how much outdoor air it is getting. Demand control ventilation uses that to cut outdoor air when a space is lightly occupied, which saves the energy of heating, cooling and dehumidifying air nobody needs.

How this unit does it. A sensor in the return duct measures CO₂ and compares it with a setpoint. A proportional loop runs from 0 % at 200 ppm below setpoint to 100 % at setpoint, and its output moves the damper’s minimum position from the unoccupied position up to the design position, as ASHRAE Guideline 36 describes for single-zone units. Both positions are set at commissioning by measuring the outdoor airflow; the controller itself never measures airflow. The economizer can still open the damper further whenever it helps with cooling.

Where the setpoints come from. ASHRAE Standard 62.1 sets ventilation as a rate per person plus a rate per floor area (V_bz = R_p·P_z + R_a·A_z). For each space type the page takes the Table 6-1 rates and default occupant density, works out the steady-state CO₂ at design occupancy with 400 ppm outdoors and 0.0052 L/s of CO₂ per person, and uses 90 % of that as the setpoint, the method Guideline 36 uses. The unoccupied position supplies only the floor-area part, which 62.1 requires whenever the space is occupied.

Limits of a fixed setpoint. Because of the floor-area part, the outdoor air needed per person changes with the number of people, so one CO₂ setpoint is exact at only one occupancy. Damper position is also only a stand-in for airflow: the airflow at a given position changes with fan speed, filter loading and wind. Both are accepted trade-offs for a simple single-zone system without an airflow station.

Health care in Ontario. Hospitals, clinics and long-term care homes follow CSA Z317.2, which sets ventilation as air changes per hour of outdoor air and of total air for each type of room, and is generally stricter than ASHRAE 62.1. For those space types the page holds the damper at the design position and uses CO₂ only for monitoring. The air change rates shown are the CSA Z317.2:19 values quoted in public guidance from Ontario Society of Professional Engineers and College of Physicians and Surgeons of Ontario documents. The current edition is CSA Z317.2:24, so check the rates, and whether reduced ventilation is permitted at all, against that edition and the facility’s infection prevention and control requirements before using them on a project.

All results use the moist air equations in the ASHRAE Handbook of Fundamentals, worked in SI units and converted for IP display.

Pressure. Atmospheric pressure comes from altitude using the standard atmosphere: p = 101.325 × (1 − 2.25577 × 10⁻⁵ × Z)^5.2559 kPa, with Z in metres. Weather changes real pressure by a few percent, which shifts humidity ratio by about the same amount.

Saturation pressure. The saturation vapour pressure of water is calculated with the Hyland–Wexler equations, one over liquid water above 0 °C and one over ice below it.

Humidity ratio and relative humidity. Humidity ratio is W = 0.621945 × p_w / (p − p_w), where p_w is the partial pressure of water vapour. Relative humidity is p_w divided by the saturation pressure at the dry bulb temperature.

Enthalpy, volume and density. Enthalpy is h = 1.006 t + W (2501 + 1.86 t) kJ/kg of dry air, with zero at 0 °C dry air. In IP the reference is 0 °F dry air, h = 0.240 t + W (1061 + 0.444 t) Btu/lb, which is why IP and SI enthalpies don’t convert directly. Specific volume is v = 0.287042 (t + 273.15)(1 + 1.607858 W) / p m³/kg of dry air, and density is (1 + W) / v.

Dew point and wet bulb. Dew point is the temperature at which the saturation pressure equals the actual vapour pressure. Wet bulb is the thermodynamic wet bulb from the ASHRAE psychrometric equation, with the ice form used below 0 °C. Neither has a direct formula, so both are solved numerically by bisection to well within 0.01 K.

A sling psychrometer reads slightly differently from the thermodynamic wet bulb, and sensors in the field carry their own error, typically ±2 to 3 % RH for humidity sensors.