Multi-zone room/building heat gain by ASHRAE CLTD — envelope, solar, people, lighting, equipment, ventilation & infiltration. Sensible + latent per zone, whole-building rollup with diversity, tonnage (TR), kW, SHR and supply airflow. Updated live.
About the Cooling & Heat Load Calculator
This multi-zone tool estimates the peak cooling and heating load of each room and of the whole building using the ASHRAE simplified CLTD/SCL/CLF method. For every zone it adds up the heat entering through the envelope, glass, people, lights, equipment and outdoor air, then rolls the zones up into a building total with a diversity allowance.
It suits HVAC and MEP engineers sizing zone equipment and central plant, contractors checking a design, and students learning where a building's load comes from. Every component is computed transparently, with the formulas listed below.
Zones and the building total
Add a zone for each thermally distinct room or space. Each zone carries its own geometry, envelope, glazing, occupancy, ventilation and system factors, while the design conditions are shared across the building. The building total is the sum of the zone loads multiplied by the diversity factor, which reflects that zones rarely peak at the same instant — east and west glass, for example, peak hours apart — so the central plant can be smaller than the simple sum of zone peaks.
Methodology & formulas
Envelope: Q = U × A × CLTD (heating: ΔT = indoor − outdoor)
Glass solar: Q = A × SC × SCL
People: Qₛ = N × SHGₛ × CLF | Qₗ = N × SHGₗ
Lighting: Q = W × factor × 3.412
Ventilation/Infiltration: Qₛ = 1.08 × CFM × ΔT | Qₗ = 0.68 × CFM × Δgrains
Building total = Σ(zone loads) × Diversity
Room sensible is grossed up for duct and fan heat; a safety factor is applied to sensible and latent. Tonnage = Total ÷ 12,000; kW = Total ÷ 3,412; supply airflow = Sensible ÷ (1.08 × Supply ΔT).
Input parameters explained
| Design conditions | Shared outdoor/indoor dry-bulb, indoor RH and outdoor wet-bulb. Use local 0.4% summer / 99% winter values. |
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| Zone geometry | Floor area and ceiling height per zone — used for ventilation-by-area and infiltration volume. |
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| Envelope surfaces | Each wall, roof, floor or partition: area, U-value and CLTD (corrected), or actual ΔT for surfaces to unconditioned space. |
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| Fenestration | Glass area, shading coefficient (clear ≈0.95, tinted ≈0.6, reflective ≈0.35) and the SCL for its orientation. |
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| Internal & air | Occupancy and activity, lighting and equipment watts, ASHRAE 62.1 ventilation, infiltration (ACH) and duct/fan/safety allowances. |
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| Diversity | Building-level factor (50–100%) applied to the sum of zone peaks for the plant total. |
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Worked example (one zone)
1500 ft² office · 104/78 °F outdoor · 75 °F / 50% indoor · seeded roof, walls & glass · 15 people · 1650 W lighting · 2200 W equipment:
Roof gain = 0.06 × 1500 × 554,950 BTU/hr
West glass solar = 45 × 0.6 × 2256,075 BTU/hr
Supply CFM = Sensible ÷ (1.08 × 20)≈ 2,450 CFM
Zone cooling load≈ 5.3 TR
Assumptions & limitations
- CLTD/SCL/CLF is a simplified single-peak method; the rigorous approach is the hourly Radiant Time Series (RTS).
- Default CLTD and SCL values are typical mid-latitude peaks — correct them for your latitude, month, hour and surface colour.
- Internal and solar gains are credited in cooling and ignored in heating (a conservative winter assumption).
- Building total uses a single diversity factor; a detailed block-load study may give a different plant peak.
- This is a preliminary estimate; for permit, LEED or stamped designs verify against ASHRAE Fundamentals or validated software.
Frequently asked questions
What is the difference between sensible and latent cooling load?
Sensible load is the heat that changes air temperature — from walls, roof, glass, solar, lights, equipment and warm outdoor air. Latent load is the heat tied to moisture that must be removed to control humidity — mainly from people, ventilation/infiltration air and wet processes. Total load = sensible + latent, and their ratio is the Sensible Heat Ratio (SHR), which drives equipment and airflow selection.
How does the multi-zone building total work?
Each zone is calculated independently from its own envelope, glazing, occupancy and ventilation, then the building total is the sum of all zone loads multiplied by the diversity factor. Diversity accounts for the fact that not every zone peaks at the same moment (for example east and west glass peak at different times of day), so a central plant can be smaller than the simple sum of zone peaks.
How many BTU or tons do I need per square foot?
It depends on climate, construction, glazing and internal loads, so there is no universal figure. As a rough check, comfort air-conditioning typically lands around 20–40 BTU/hr per ft² (about 300–600 ft² per ton). The calculator computes the actual value per zone and flags it if it falls outside the normal band.
What is the CLTD method?
CLTD (Cooling Load Temperature Difference) is an ASHRAE simplified method that captures conduction plus time-delayed solar heat through walls and roofs with a single corrected temperature difference: Q = U × A × CLTD. Glass solar gain uses Q = A × SC × SCL. It gives quick, transparent peak-load estimates; the rigorous alternative is the hourly Radiant Time Series (RTS) method.
Why is ventilation air such a large part of the load?
Outdoor air must be cooled and dehumidified from ambient down to indoor design conditions. In hot or humid climates this is a major sensible and latent load — often 20–40% of the total — which is why the ASHRAE 62.1 outdoor-air rate and energy recovery matter so much.
How do I size supply airflow (CFM) from the load?
CFM = Sensible BTU/hr ÷ (1.08 × Supply ΔT). A typical 20 °F supply ΔT gives roughly 400 CFM per ton. The calculator returns this per zone automatically.
Does this tool do heating load as well?
Yes. Switch to Heating mode and each zone computes transmission losses (Q = U × A × ΔT) plus the sensible load to heat infiltration and ventilation air, ignoring solar and internal gains for a conservative design. Results show in MBH (1000 BTU/hr) and kW.
Is this accurate enough for a permit or stamped design?
It is a transparent peak-load estimate suited to early sizing, checks, small projects and teaching. For permit, LEED or stamped submissions, verify against a full ASHRAE Fundamentals calculation or validated software (e.g. HAP, TRACE) using project-specific weather data.
Last updated: 2026-01-03 · Preliminary design only — verify against project specifications and local code.