A psychrometric chart set up for the questions this unit asks of it. Load the weather file your DesignBuilder model uses and every hour of the year lands on the chart. Read those hours for the climate, then for the building, then for the air-handling unit, and see where the answers part.
Start from a capital city on tab 1, or load your own building's weather file (.epw). Everything happens in your browser: a file you load is read on your computer and is not uploaded anywhere.
The tutorial uses no tab: the chart work lives in the lecture. Nothing here is submitted or marked.
First the climate: where each hour of the year sits against the comfort band.
Any two properties fix a state of air, so one point on this chart is one hour of weather, and a weather file is 8,760 of them. The comfort band is the box of temperature and moisture the building is meant to hold (set it below). Each hour is read against it in plain words: cold (left of the band), warm (right of it), humid (more water in the air than the band allows, whatever the temperature), dry (less water than it allows, when the band has a lower limit) or comfortable (inside it).
Moisture is read first: a humid hour stays humid however much it is warmed. Every hour of the year counts here, because the weather does not keep office hours. The monthly bars give the seasonal pattern; with the design conditions on tab 2, that is the climate a building description needs.
Then the building: a cold hour outside is not always a cold hour inside.
People, lights, equipment and sun keep adding heat indoors, so an office stays warm enough on its own until the weather drops below its balance point: the outdoor temperature at which the heat it gains equals the heat it loses. That sits well below the heating setpoint. In the cold hours below the line the building needs heating; in the cold hours above it, the building is shedding heat and needs cooling, although the air outside is cold.
The balance point here is a guess about the building, not a calculation: it is a line to move. More people and equipment, or a better-insulated envelope, move it down. Your building's real figure is what a load simulation settles, which is DesignBuilder's job. Only the hours the building is in use count from here on (set them below).
Last the air-handling unit: what it has to do to each hour of outside air. What the outside air needs is not what your building needs.
An air-handling unit (AHU) takes in outside air and brings it to the band. Each hour is now named by the process that does that: heating (right on the chart), sensible cooling (left), cooling and dehumidifying (down and left, through a coil colder than the air's dew point) or humidifying (up). These are the climate's hours with new names: cold hours need heating, warm ones sensible cooling, humid ones cooling and dehumidifying, dry ones humidifying.
The building's answer still splits the cold ones. Above the balance point the air needs heating, yet the building needs cooling, and that cool outside air can provide it: free cooling. More internal heat moves the balance point down and turns more of the cold hours into free cooling.
Each city is the newest typical-year file (TMYx 2011–2025) for its airport station, the same file CBE Clima serves for that location.
CBE Clima: clima.cbe.berkeley.edu (opens in a new tab; this page itself makes no network requests).
The band is a design decision: the heating and cooling setpoints set its edges, the gap between them is the dead band, and the humidity limit sizes the coil. Set the band your brief would set.
Below the balance point the building needs heat; above it, the heat from people, lights, equipment and sun is more than the building loses, so it needs cooling. A load simulation settles the real figure. Leave it empty to hide the line.
Count only the hours the building is in use. An office that opens early meets cooler mornings; a building in use all night meets the coldest hours of all.
Arrow keys move the cross-hair by half a degree or half a gram per kilogram; hold Shift for steps of five.
Load a weather file to count its hours. The band and the regions are drawn already, so the chart works without one.
Every property comes from the ASHRAE Handbook: Fundamentals, chapter 1, using the perfect-gas relations, which stay within 0.7% of the full real-gas formulation between −50 and 50 °C.
One chart, one pressure. The chart is drawn at the standard pressure for the weather station's elevation, read from the file (101.325 kPa before a file is loaded). Each hour is placed by its dry bulb and dew point. The dew point fixes how much water the air holds whatever the pressure, which is why it is the second coordinate carried from the file. Plotting each hour at its own barometric pressure would put some points above the saturation curve drawn here.
How an hour is sorted. Moisture first, because only a coil colder than the air's dew point can take water out. If the hour holds more water than the wettest air in the band, it needs cooling and dehumidifying. If it holds less than the driest air in the band, it needs humidifying (and usually heating). Otherwise its moisture is already acceptable and a horizontal move brings it in: heating if it sits left of the band at that humidity ratio, sensible cooling if right. With a relative-humidity limit the band's edges lean, so the test uses the band's width at the hour's own humidity ratio rather than the setpoints alone.
The balance point. The outdoor temperature at which the heat a building gains from people, lights, equipment and sun equals what it loses through its envelope and its outside air. Below it the building needs heating; above it, cooling. Here it is an illustrative vertical line: hours that the outside air needs heated, but that lie above the line, are counted as free cooling for the building. No load is calculated.
Three readings of the same hours. Every hour is sorted once, by the rule above, and the three readings name the result differently. Read for the climate, cold is the hours the air-handling unit reading calls heating, warm is sensible cooling, humid is cooling and dehumidifying and dry is humidifying, so those two readings count the same hours in the same places. The building reading splits the cold hours at the balance point, and the air-handling unit reading keeps that split as heating and free cooling. The climate reading counts all 8,760 hours of the year; the hours the building is in use apply from the building reading on, so with a window set the climate reading still shows the whole year.
Operating hours. A weather file labels each hour by its end: file hour 1 is 00:00 to 01:00, file hour 9 is 08:00 to 09:00, and file hour 24 is 23:00 to midnight. A window from 07:00 to 19:00 therefore counts file hours 8 to 19, twelve hours a day. A window that starts later than it stops runs overnight: 22:00 to 06:00 counts file hours 23, 24 and 1 to 6. Weekdays only uses each hour's own date from the file's calendar, so a shift that starts on Friday night keeps its Friday hours and drops the Saturday ones. Public holidays are not removed.
The outside condition the plant is sized for, and what the plant has to do to it.
A design day is not the hottest hour in the file. It is a percentile: the 0.4% cooling condition is exceeded in about 35 hours of a typical year, so the plant copes with every other hour and falls short in those few. The arrow from that outside state to the edge of the band is the job the cooling plant is sized for. Across is heat to remove (the sensible leg); down is water to condense (the latent leg).
Two sets of numbers sit side by side here. The ASHRAE design conditions come from many years of records at the station and travel in the weather file's header; simulation tools build their design days from tables like these. The same rule applied to the file's own 8,760 hours gives a second answer, because a typical year is stitched together from typical months rather than being the long record.
Load a weather file on tab 1 first. This tab reads its design conditions and its hours.
From the hours. Sort the year's dry bulbs from hottest down; the 0.4% value is the 35th (0.4% of 8,760). Its coincident wet bulb is the mean wet bulb of every hour within half a degree of that dry bulb. The dehumidifying condition does the same with dew point, carrying the mean coincident dry bulb. The 99.6% heating value is the 35th coldest hour. These follow the definitions of the ASHRAE Fundamentals climatic design tables, applied to one typical year instead of the multi-year record.
The arrow. It runs from the outside design state to the band's corner at the cooling setpoint and the upper humidity limit, or straight across if the outside air is already drier than the limit. The share of the work that is drying is the enthalpy drop across the latent leg over the drop across the whole arrow, using h = 1.006 t + W (2501 + 1.86 t). It is per kilogram of outside air: how much outside air there is, and what the room adds, is what a load calculation settles.
What an air-handling unit does to the air, one state at a time.
Return air from the room and outside air meet at the mixing dampers, and the mixed state sits on the straight line between them, closer to whichever stream there is more of. The cooling coil then moves that air down and left, toward its apparatus dew point, the saturated state its surface behaves as if it were at; the air leaves a little short of it because some passes between the fins. To take water out, the coil has to cool the air below its dew point, and if the room needs drier air than it needs cold air, the air leaves colder than the room asked for and is warmed back up: the reheat leg.
A dedicated outdoor air system (DOAS) splits the two jobs. Its coil dries only the outside air, and units in each zone cool the room air without going below its dew point, so nothing is overcooled to dry it and the reheat leg goes. The chart shows states and temperatures only; how much energy each leg costs is a question for the load calculation.
Mixing. The mixed state is the mass-weighted average of the two streams' humidity ratio and enthalpy, which puts it on the straight line between them; the outside-air share here is by mass of dry air.
The coil. The air moves on a straight line from the on-coil state toward the apparatus dew point (ADP) on the saturation curve, and leaves a bypass fraction of the way back: off-coil = ADP + bypass × (on-coil − ADP), in both dry bulb and humidity ratio. The ADP is set by whichever target is harder to meet: the supply moisture the room needs, or the supply temperature. When the moisture sets it, the air leaves colder than the room asked for, and the difference is the reheat. A real coil's path curves along the saturation curve; the straight line is a good stand-in except when the on-coil air is already close to saturated, where it would leave the air wetter than saturated. There the off-coil state is held on the saturation curve. If the mixed air is already dry enough, the coil only cools; if it is colder than the supply temperature, it is heated instead.
DOAS. The unit treats outside air only and dries it to the supply moisture; the zone units cool room air from the room temperature toward the supply temperature at constant moisture. If that supply temperature is at or below the room's dew point, the zone coil would condense, which is what the DOAS exists to prevent, and the tool says so.