At a glance
What this guide walks through
HVAC design is a chain: heat moves through the building envelope, humidity changes what that heat "feels" like and whether condensation risks appear, loads get estimated from both, ventilation adds a fresh-air requirement on top, and cooling equipment is selected to remove the total. This guide follows that exact chain, in order, with the reasoning at each step.
A practical comfort/moisture-management range, not a universal code-compliance band.
Q(Btu/h) = 1.08 × cfm × ΔT — the standard air-side sensible heat shortcut.
A unit of cooling capacity; it says nothing by itself about the building area that capacity can serve.
Reduce heat gain first, then ventilate correctly, then size equipment last.
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01 // How heat moves through a building
Every HVAC calculation ultimately traces back to three physical mechanisms. A building envelope rarely uses just one — a wall loses heat by conduction through its layers, then convection and radiation carry that heat away from its outer surface.
Conduction
Heat moving directly through a solid material, from its hot face to its cold face. Governed by the material's thermal conductivity (k) — insulation works by using materials with very low k, often trapping still air or gas in small pockets.
Convection
Heat carried by a moving fluid — air or water — across a surface. Natural convection relies on buoyancy from temperature differences; forced convection uses a fan or pump, which is why moving air removes heat faster than still air.
Radiation
Heat transferred by electromagnetic waves, needing no medium to cross — this is how the sun heats a roof and how a cold window surface makes a room feel chilly even if the air temperature is fine. Surface color and emissivity matter here, not just insulation.
02 // U-values, R-values and envelope heat flow
Almost every heat-loss or heat-gain calculation through a wall, roof, window or floor reduces to one formula.
The U-value (or overall heat-transfer coefficient) tells you how many watts pass through one square metre of an assembly for every degree of temperature difference across it, in W/m²K. The R-value is its reciprocal — thermal resistance, in m²K/W — and is what most insulation is actually rated in. Layers in series simply add their resistances:
| Quantity | Relationship | What it tells you |
|---|---|---|
| Thermal resistance, one layer | R = L / k | Thickness (L) divided by conductivity (k) of that single material |
| Total resistance, multiple layers | R_total = R₁ + R₂ + … + film resistances | Resistances in series simply add, including inside/outside air-film resistance |
| Overall U-value | U = 1 / R_total | Lower U-value = better insulated assembly |
| Heat flow through the assembly | Q = U × A × ΔT | Watts lost or gained through that surface right now |
Two consequences fall straight out of that last formula. First, heat loss is proportional to area — a large single-glazed window can lose more total heat than a much bigger, well-insulated wall next to it, purely because its U-value is so much higher. Second, heat loss is proportional to ΔT — the indoor-outdoor temperature difference — which is exactly why heating and cooling loads are calculated at a specific design temperature, not an average one.
03 // Interactive: envelope heat loss
Enter an assembly's area, U-value and the temperature difference across it to see the instantaneous heat flow. Try swapping in a window's U-value versus a wall's to see how much the difference matters.
This is steady-state heat flow through one assembly only — it excludes infiltration, thermal bridging, solar gain and every other surface. Whole-building load calculation is covered in section 08.
04 // Humidity: why "how hot" isn't the whole story
Air is never just air — it's a mixture of dry air and water vapor, and how much vapor it holds changes how it feels, whether it will condense on a surface, and how much work a cooling coil has to do.
Relative humidity
The ratio of the water vapor actually present to the maximum the air could hold at that temperature, as a percentage. The same absolute moisture content gives a higher RH reading as air cools — which is the whole reason condensation happens on cold surfaces.
Humidity ratio
The mass of water vapor per unit mass of dry air (g/kg or lb/lb). Unlike RH, it doesn't change just because temperature changes — it only changes when moisture is actually added or removed. This is the quantity load calculations actually track.
Dew point
The temperature air would need to cool to, at constant pressure and moisture content, for it to become saturated (100% RH) and start condensing. Any surface colder than the air's dew point — a window, a cold pipe, a duct — will collect condensation.
05 // Dew point and condensation risk
Condensation is a surface-temperature problem, not an air problem — it happens wherever a surface drops below the dew point of the air touching it. That's why a cold drink sweats on a humid day, why single-glazed windows fog up in winter, and why poorly-insulated cold water pipes need lagging: the pipe surface sits below the surrounding air's dew point, so moisture condenses on it continuously.
In buildings this matters for more than comfort. Sustained condensation inside a wall cavity — often from warm, moist indoor air reaching a cold layer without a vapor barrier — is one of the most common causes of hidden mold growth and structural rot, which is why vapor-retarder placement is taken seriously in cold-climate wall design.
06 // The properties a psychrometric chart ties together
A psychrometric chart plots several air properties against each other so that knowing any two of them (at a given pressure) fixes all the rest.
| Property | What it is | Typical unit |
|---|---|---|
| Dry-bulb temperature | What an ordinary thermometer reads — the one everyone means by "temperature" | °C / °F |
| Wet-bulb temperature | Temperature read by a thermometer with a wet wick, cooled by evaporation; always ≤ dry-bulb, and equal to it only at 100% RH | °C / °F |
| Dew point temperature | Temperature at which the air's actual moisture content would cause saturation | °C / °F |
| Humidity ratio | Mass of water vapor per unit mass of dry air | g/kg or lb/lb (grains/lb) |
| Relative humidity | Actual vapor pressure ÷ saturation vapor pressure, at the current temperature | % |
| Specific enthalpy | Total heat content of the moist air mixture (sensible + latent), referenced to a baseline | kJ/kg dry air |
| Specific volume | Volume occupied per unit mass of dry air — needed to convert between mass and volume airflow | m³/kg dry air |
The practical payoff is that psychrometric relationships let you derive additional moist-air properties from a small set of measured quantities at a known pressure. The calculator below demonstrates two of them — dew point and humidity ratio — from dry-bulb temperature, relative humidity and barometric pressure; a full psychrometric property calculation can also determine enthalpy, wet-bulb temperature and specific volume.
07 // Interactive: dew point & humidity ratio
Enter dry-bulb temperature, relative humidity and local barometric pressure to estimate dew point and humidity ratio.
Uses a Magnus water-vapor approximation for dew point; barometric pressure affects humidity ratio. Treat results outside the normal HVAC range with caution, especially below freezing where dedicated ice/frost-point relations may be needed. For design-grade psychrometrics use ASHRAE property methods or validated software.
08 // Sensible heat, latent heat, and the sensible heat ratio
Every cooling or heating load splits into two very different kinds of work.
Sensible heat
changes temperatureHeat that changes dry-bulb temperature without changing moisture content. A common near-standard-air shortcut is Qsensible (Btu/h) ≈ 1.08 × cfm × ΔT, or in SI Q ≈ 1.2 × L/s × ΔT; the coefficient changes with air density and specific heat, especially with altitude or unusual conditions.
Latent heat
changes moistureHeat associated with adding or removing water vapor — evaporation and condensation — without necessarily changing dry-bulb temperature. People's breathing and perspiration, cooking, showers and outdoor air infiltration are major latent-load sources.
Sensible heat ratio (SHR)
Qsensible ÷ QtotalThe fraction of total cooling load that is sensible. A typical comfort-cooling SHR is roughly 0.7–0.8; a low SHR (humid climate, many occupants) needs a coil selected for more dehumidification, not just more total tons.
This split matters because total cooling capacity is not one number split however the equipment likes — a coil has a specific sensible/latent split determined by its temperature, airflow and geometry. Undersizing dehumidification (oversizing sensible capacity relative to latent) is a common cause of a room that reads the right temperature but still feels clammy.
09 // What actually goes into a cooling or heating load
A real load calculation sums contributions from every heat-gain (or heat-loss) path into the space, at a chosen design condition — not an average day.
| Load component | Type | Depends mainly on |
|---|---|---|
| Walls, roof, floor conduction | Sensible | U-value × area × ΔT (section 02) |
| Windows — conduction | Sensible | Window U-value × area × ΔT |
| Windows — solar (glazing) gain | Sensible | Glass area, orientation, shading, solar heat gain coefficient (SHGC) |
| Infiltration & ventilation air | Sensible + latent | Airflow rate and the indoor/outdoor temperature and humidity difference |
| Occupants | Sensible + latent | Number of people and their activity level |
| Lighting | Sensible | Installed wattage and usage schedule |
| Equipment / plug loads | Mostly sensible | Installed and diversified equipment wattage |
Two subtleties separate a rough estimate from a real design. First, loads use statistical climatic design conditions rather than averages or record extremes: current ASHRAE Fundamentals data presents heating conditions at 99% and 99.6%, and cooling conditions at 0.4%, 1% and 2%. Second, internal loads and solar gain require appropriate schedules, diversity and coincident conditions — exactly the kind of detail an area-only rule of thumb cannot capture.
10 // Rule-of-thumb sizing — and why it's only a first guess
These figures are useful for a sanity check or an early budget estimate. They are not a substitute for the load calculation in section 09.
| What an area-only rule ignores | Why it changes the load |
|---|---|
| Climate and design weather | The same building can have very different heating and cooling peaks in different locations. |
| Envelope and infiltration | Insulation, airtightness, thermal bridges and glazing U-values directly change conduction and outdoor-air loads. |
| Solar exposure and glazing | Orientation, shading and SHGC can make two equal-area rooms peak at very different times and loads. |
| People, equipment and ventilation | Occupancy, plug/process loads and required outdoor air add both sensible and latent load. |
A ton of cooling is exactly 12,000 Btu/h of capacity, but there is no fixed number of square feet that one ton can serve. Area-based Btu/h-per-ft² or ft²-per-ton rules are useful only as rough reasonableness checks after the real inputs above are understood; they should not be used to select equipment for a real project.
11 // Ventilation and air changes per hour
Ventilation adds a fresh-air requirement on top of the conditioning load — every cubic metre of outdoor air brought in has to be heated or cooled and often dehumidified, which is why over-ventilating wastes energy and under-ventilating hurts indoor air quality.
Air changes per hour (ACH) is the number of room volumes represented by an airflow each hour: ACH = volumetric airflow / room volume, with consistent time units. It is a useful descriptor, but not a universal ventilation-design method. ASHRAE 62.1 addresses nonresidential ventilation, ASHRAE 62.2 addresses residential buildings, and specialized occupancies can have separate ventilation, exhaust, pressure or containment requirements.
| Application | How ventilation is normally specified | Design reference |
|---|---|---|
| Homes / apartments | Whole-building outdoor air plus local exhaust provisions, not one generic room ACH | ASHRAE 62.2 and local residential/mechanical code |
| Offices / classrooms / many commercial spaces | Outdoor-air rates based on occupancy and floor area, with system-level adjustments | ASHRAE 62.1 and local code |
| Toilets / kitchens | Local exhaust is commonly specified directly as airflow, sometimes with makeup-air requirements | Applicable mechanical code / ventilation standard |
| Healthcare | May combine outdoor air, total ACH, pressure relationship and filtration requirements | ASHRAE 170 and health-care regulations |
| Laboratories / hazardous processes | Hazard-specific exhaust and containment based on the process; generic ACH alone is not a safety design | Applicable laboratory/process-safety standards and risk assessment |
note The calculator below converts a chosen ACH into airflow. It does not tell you what ACH or outdoor-air rate is required; that value must come from the applicable occupancy standard, code or hazard-control design.
12 // Interactive: required airflow for a target ACH
Enter a room's volume and a target air-changes-per-hour rate to see the airflow needed to achieve it.
For a full airflow, duct-velocity and fan-power workflow, see the Ventilation, Airflow & ACH Calculator.
13 // How cooling systems actually reject heat
Every cooling system does the same fundamental job — move heat from where it's unwanted to where it can be rejected — using one of a handful of architectures.
DX split / window units
A direct-expansion refrigerant coil in the space absorbs heat and rejects it outdoors through a compressor-condenser loop. Simple, common in homes and small commercial spaces, sized in tons or Btu/h per zone.
VRF / VRV systems
Multiple indoor units share one variable-refrigerant-flow outdoor unit, each modulating independently. Efficient for buildings with many zones that need different amounts of cooling (or even simultaneous heating and cooling) at once.
Chilled water systems
A central chiller cools water, which is pumped to air-handling units or fan-coils throughout a large building. Standard for large commercial and institutional buildings where a central plant is more efficient than many small units.
Cooling towers
Reject the chiller's condenser heat to the atmosphere by evaporating a small fraction of a recirculating water stream — see the HVAC Formula Sheet for approach and range definitions.
Evaporative / adiabatic cooling
Cools air by evaporating water directly into the airstream. Effective and low-energy in hot, dry climates; adds humidity, which limits its usefulness in already-humid climates.
Heat pumps
The same vapor-compression principle as a DX cooling system, but reversible for space heating. Depending on jurisdiction and test procedure, ratings include COP and seasonal heating metrics plus EER/SEER; current U.S. residential central systems use EER2/SEER2 and HSPF2. Capacity and efficiency both vary with operating conditions.
Whichever architecture is used, the equipment is selected to match the total load calculated in sections 08–10, split correctly between sensible and latent capacity, plus the ventilation airflow from section 11–12 — which is why load calculation, psychrometrics and ventilation all have to be worked out before equipment selection, not after.
14 // Background, FAQ, references and limitations
Expand for deeper context and the sources behind the figures used here.
Modern HVAC design formalized around the mid-20th century as mechanical refrigeration, forced-air distribution and psychrometric theory matured together. The field is built on a chain of physics: heat-transfer theory (conduction/convection/radiation) predicts how much heat crosses the envelope; psychrometrics predicts how that heat interacts with moisture in the air; load calculation sums both into a design requirement; and refrigeration-cycle and airflow engineering turn that requirement into selected, sized equipment.
ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers) publishes the reference data — design temperatures, material properties, ventilation rates — that most of the industry designs against, alongside national and local mechanical codes that set minimum legal requirements.
What is the difference between sensible heat and latent heat in HVAC? Sensible heat changes air temperature without changing its moisture content, and is what a thermometer registers. Latent heat changes moisture content without necessarily changing dry-bulb temperature, and is what a cooling coil removes when it dehumidifies. Total cooling capacity is the sum of both.
What is the difference between relative humidity and humidity ratio? Relative humidity is the ratio of actual to saturation vapor pressure at the current temperature, so it changes with temperature alone. Humidity ratio is the mass of water vapor per unit mass of dry air, and only changes when moisture is actually added or removed — which is why load calculations track humidity ratio, not RH.
Why does a cooling coil dehumidify air? Whenever the coil surface drops below the entering air's dew point, water vapor condenses on it and drains away. If the coil stays above the air's dew point, only sensible cooling happens and no moisture is removed.
How many air changes per hour does a room need? There is no universal value. ACH describes airflow relative to room volume; it is not by itself a compliance method. Use the applicable occupancy standard — for example ASHRAE 62.1 for many nonresidential spaces, ASHRAE 62.2 for residential ventilation, and specialized standards for healthcare, laboratories or hazardous processes.
What is a SEER or EER rating? They are cooling-efficiency metrics rather than capacity. EER represents performance at specified test conditions and SEER is seasonal; current U.S. residential central systems use the newer EER2/SEER2 nomenclature and HSPF2 for seasonal heating. Other jurisdictions and equipment categories may use different metrics.
Can I use rule-of-thumb BTU-per-square-foot sizing for a real design? Only as a rough reasonableness check. Area-only rules omit climatic design weather, envelope and leakage, glazing and solar exposure, occupancy, internal loads, ventilation and latent moisture loads. Real design needs an accepted project-specific load calculation such as ACCA Manual J for applicable residential work or an equivalent commercial procedure.
- ASHRAE Handbook — Fundamentals — heat transfer, psychrometrics, load calculation and climatic design data referenced throughout this guide.
- ASHRAE Standards 62.1 & 62.2 — ventilation and acceptable indoor-air-quality standards for nonresidential and residential applications respectively.
- ASHRAE Standard 90.1 — energy standard for buildings, including envelope U-value and equipment efficiency baselines.
- ACCA Manual J — the standard room-by-room residential load calculation procedure referenced in section 10.
- U.S. Department of Energy — Heating & Cooling — consumer-facing explanation of system types, efficiency ratings and sizing pitfalls.
- U.S. Department of Energy — Central Air Conditioners and Heat Pumps — current U.S. test procedures and SEER2/HSPF2 rating terminology.
Numeric values were reviewed in August 2026. They remain rounded educational comparisons and may differ from a specific climate, jurisdiction, product or edition of these standards.
This page is a simplified educational overview, not a design document. U-values are illustrative and must be replaced by project-specific assembly data. The heat-loss calculator is steady-state conduction through one assembly only. The dew-point calculator uses a Magnus approximation and the entered barometric pressure; it is not a full psychrometric property engine, particularly below freezing. The ACH calculator is only a volume/rate conversion and does not determine a required ventilation rate. HVAC sizing and ventilation compliance require the governing standards, climatic data and project-specific inputs.
Nothing on this page is validated for real HVAC design, permitting, energy-code compliance or safety-critical ventilation (such as laboratory fume control or infection control). Always rely on a qualified mechanical engineer, a proper load calculation, and your applicable code and standards for real projects.