Heat Balance, RTS, or CLTD: Choosing a Cooling-Load Method
Two engineers can look at the same building and produce two different cooling loads — both defensible. Here's how the main methods differ, and how to document the result so a plan reviewer can follow it.
- The heat entering a space at any moment is not the same as the cooling load at that moment.
- RTS is the method most commercial MEP work can default to.
- Size to a defensible calculation, apply any justified allowance once, and stop.
A cooling-load calculation estimates the heat an HVAC system must remove on a peak day. The method — Heat Balance, RTS, or CLTD — determines how solar gain, internal loads, and thermal storage are modeled, which shapes equipment size.
Two engineers can look at the same building and produce two different cooling loads — both defensible — depending on the method they used and the assumptions they fed it. That's not sloppiness; it's the nature of load calculation. But it means the method matters, because the number you land on has to survive two audiences: a plan reviewer who may ask how you got it, and a contractor who has to live with the equipment it selects.
Get the method and the assumptions right and the number is far easier to defend. Get them wrong — usually by oversizing "to be safe" — and you've bought a comfort problem, an energy-code problem, and a cost problem in one move. Here's how the main methods actually differ, when each earns its place, and how to document the result so nobody has to take it on faith.
Three main methods estimate peak cooling load. The Heat Balance Method (HBM) is the rigorous, first-principles reference method defined in the ASHRAE Handbook—Fundamentals (Ch. 18) and implemented as the engine inside the major heat-balance load and energy tools. The Radiant Time Series (RTS) is a simplified, transparent derivative of HBM good for most commercial work. CLTD/CLF is the older hand-calc method — still useful for checks and intuition, largely superseded by RTS in practice. The choice sets how thermal lag and solar gain are modeled.
First Principle: Heat Gain Is Not Cooling Load
The single idea that trips up load calculations is this: the heat entering a space at any moment is not the same as the cooling load at that moment. Radiant heat — sun through glass, warm surfaces, lighting — doesn't hit the air instantly. It's absorbed by floors, walls, and furniture, stored, and released over the following hours. So the peak cooling load lags and smooths out the peak heat gain, and the building's thermal mass sets how much.
Every credible method exists to model that lag. They just trade rigor for effort differently. (For commercial buildings, solar gain through glazing is often one of the largest components — which is exactly the radiant, time-delayed kind, so the method's handling of it matters most.)
Sensible, Latent, and Lag
One more idea matters here:
- Sensible vs. latent. Sensible heat changes air temperature (solar, lighting, equipment, warm outdoor air). Latent heat changes moisture (people, cooking, ventilation humidity). The coil has to handle both, and their ratio — the sensible heat ratio (SHR) — drives coil selection and dehumidification. A space with lots of people or humid outdoor air can have a low SHR and a real latent problem; miss it and the room is cold but clammy.
Every method is really just a different way of turning heat gain over the day into the cooling load the equipment must remove — which is why the method's treatment of lag matters so much.
Where the Load Actually Comes From
A cooling load is the sum of a few components — envelope load, solar load, internal loads, and ventilation and infiltration load — and knowing which one dominates tells you where the design attention goes:
- Envelope — conduction through walls, roof, and glass, driven by the outdoor-to-indoor temperature difference and, on walls and roofs, by sun on the outside surface.
- Solar — radiant gain through glazing, governed by the glass area, orientation, and SHGC (solar heat gain coefficient) — and delayed by thermal lag.
- Internal — people (sensible + latent), lighting, and equipment/plug loads. In offices and data-adjacent spaces, internal loads often dominate; in kitchens and medical spaces they spike.
- Ventilation — conditioning the outdoor air the space needs (which ties directly to the ASHRAE 62.1 rate, whether delivered by the main unit or a dedicated outdoor air system (DOAS)) — frequently the largest single latent load.
| Component | Driven by | Sensible / Latent |
|---|---|---|
| Envelope | ΔT, insulation | Sensible |
| Solar | glass area, orientation, SHGC, lag | Sensible |
| People | occupancy | Both |
| Lighting / equipment | connected load, schedule | Mostly sensible |
| Ventilation | outdoor-air rate, humidity | Both |
The mix shifts by building type — which is exactly why the same method gives very different-shaped loads for an office, a hospital, and a warehouse.
Heat Balance Method (HBM): The Rigorous Standard
The Heat Balance Method is the reference method, defined in the ASHRAE Handbook—Fundamentals (Chapter 18) and treated as the most accurate approach for non-residential loads. It solves an energy balance at every surface — inside and outside — and in the room air, accounting for conduction, convection, and radiation without the simplifying shortcuts the other methods take.
The trade-off is effort. HBM is iterative and computationally heavy; it's not a hand calculation, so in practice it runs inside software, including current versions of Carrier HAP and Trane TRACE 3D Plus. Where it earns its keep is the hard, high-stakes project — tight thermal tolerances, unusual geometry, heavy glazing, mission-critical spaces — where the accuracy is worth the compute and you may need to defend the number rigorously.
Radiant Time Series (RTS): The Practical Workhorse
RTS is the method most commercial MEP work can default to. ASHRAE derived it from the Heat Balance Method as a simplified, non-iterative approach: it uses radiant time factors to model how stored radiant heat is released over 24 hours, and conduction time series factors for heat conducted through the envelope.
In plain terms: RTS captures the same thermal-lag physics that makes HBM accurate, without the iteration. It's transparent, repeatable, well-documented in ASHRAE's Load Calculation Applications Manual, and it can produce traceable peak design loads when the inputs and assumptions are sound. For the large majority of commercial buildings, RTS is the right balance of rigor and practicality.
CLTD/CLF: The Legacy Hand Method
Before RTS, the manual go-to was CLTD/CLF — Cooling Load Temperature Difference, Cooling Load Factor, and Solar Cooling Load factors, pulled from published tables. It approximates the thermal lag with tabulated values instead of computing it, which made it doable by hand.
It still has a place: a fast sanity check, an early feasibility number, or a gut-check against software output. But it's a superseded, table-bound approximation — its assumptions are baked into the tables, and it's less accurate for anything outside the conditions those tables assumed. It can still support a documented design calculation within the building types and conditions its tables cover; outside those limits, use RTS or heat balance.
Where Manual J Fits (and Where It Doesn't)
If you work across project types, keep the line clear: ACCA's Manual J is a residential load-calculation method. It's the right tool for a house or a small residential unit. It is not the method for a commercial building — reach for RTS (or HBM) there. Using Manual J on commercial work is the kind of mismatch a sharp reviewer notices.
Design Conditions and Load Diversity
Two inputs quietly control the whole result:
- Design conditions. The calculation runs against ASHRAE climate design data — a summer design dry-bulb/wet-bulb chosen at a percentile (e.g., the 0.4% or 1% condition), not the all-time record. Pick conditions that are too conservative and every load inflates; pick the percentile that fits the project's requirements, and check the dehumidification design condition where latent load matters.
- Load diversity. Not every load peaks at the same moment — the sun on the east façade peaks in the morning, the west in the afternoon, and people/lighting follow the occupancy schedule. Block (coincident) load — the simultaneous peak of all the rooms a system serves — is usually smaller than the sum of the individual room peaks. Sizing a central plant on the sum instead of the block is a common way cooling equipment gets oversized.
The Software — and Why the Method Lives Inside It
At real scale these calculations run in load/energy software, and the method is baked into the tool:
| Tool | Typical use | Underlying approach |
|---|---|---|
| Carrier HAP | Load calc + energy | Heat balance (v6); Transfer Function Method (v5 and earlier) |
| Trane TRACE 3D Plus | Load calc + energy | Heat-balance engine (EnergyPlus-based) |
| IESVE | Detailed energy modeling | Heat balance |
| EnergyPlus / OpenStudio | Whole-building energy simulation | Heat balance |
| eQUEST | Whole-building energy | DOE-2 |
The practical point: choosing an RTS tool over a heat-balance engine is a real methodological choice — but check the version: HAP moved to the ASHRAE Heat Balance method at v6, so a current HAP license is a heat-balance tool, while v5 and earlier used the Transfer Function Method. For most commercial load sizing, an RTS-based tool is transparent and sufficient; for detailed energy modeling, ASHRAE 55 thermal-comfort analysis, or unusual mass/glazing, a heat-balance engine earns its extra rigor. The method isn't an academic label — it's the assumption set running behind the load number you hand to equipment selection.
From Load to Equipment: A Quick Worked Example
Take a 20,000 ft² office floor and build the room load from its parts — envelope conduction, solar through the glazing by orientation, people (sensible + latent), and lighting and plug loads on their schedules; the ventilation air per ASHRAE 62.1 comes in later, as a system load. Apply the method's thermal lag so the radiant gains land at the right hour, then find the block peak — the moment the coincident total is highest — rather than adding every room's individual peak. The room-only block load might come out near 60 tons at an SHR of about 0.82, while the sum of the individual room peaks reads closer to 70 tons — that roughly 14% gap is the load diversity the block calculation captures. Those numbers are illustrative: climate, envelope and use move all three. System loads such as ventilation and fan heat are then added hour by hour, since the coil peak can fall at a different hour than the room peak.
That coil (system) sensible-plus-latent total, at its SHR, is what selects the central cooling capacity: the cooling coil (sized for both sensible cooling and the latent/dehumidification the SHR demands), the air handler or rooftop unit (AHU/RTU), and the chiller or condensing unit behind it. The system airflow is sized from the zone sensible loads, supply-air conditions and ventilation needs, through ductwork coordinated with the rest of the trades (see our MEP coordination best practices). VAV boxes and VRF indoor units are sized to their own zone peaks. Change an assumption upstream — a heavier envelope, a lower design percentile, honest diversity — and the peak moves, and so does the equipment. That traceable chain from climate → components → block load → equipment is the whole reason to get the method and its inputs right.
The Oversizing Trap
A common way a load calc goes wrong isn't the method — it's the safety factor stacked on top of it. A single documented allowance, applied only where an input is genuinely uncertain or where the owner has directed future-capacity margin within the energy code's equipment-sizing limits, is defensible. Padding every component "to be safe" is not, and it backfires: comfort suffers (oversized cooling equipment tends to short-cycle, often doesn't run long enough to dehumidify, and can leave the space cold and clammy); energy compliance suffers (oversized equipment can work against your ASHRAE 90.1 / energy-code position); cost suffers twice (bigger equipment costs more up front and often more to run).
The discipline is to size to a defensible calculation, apply any justified allowance once, and stop. Remember too that ventilation drives a real share of the load: outdoor air per the IMC or ASHRAE 62.1 is a required input, not a place to guess.
Documenting the Number for Review
A load calc is only as good as its defensibility. The method itself is usually locked in early — see our guide to MEP design phases (SD, DD & CD) — but whatever method you use, make the result traceable:
- State the method and the source — e.g., RTS per the ASHRAE Load Calculation Applications Manual.
- Show the design conditions — the outdoor design temperatures and the assumptions behind them.
- Show the ventilation basis — OA per the IMC or ASHRAE 62.1, tied to occupancy and floor area.
- Note the safety factor and where it was applied — so it's clearly one allowance, not hidden padding.
- Make the calc agree with the schedules — the equipment you selected has to match the load you calculated.
A reviewer who can follow the number from assumptions to equipment can verify its basis more easily. That's the whole game.
We size to the method the project calls for, document the trace, and reconcile the load against the equipment schedule before it ships. See our HVAC engineering services, or schedule a scope call.
Related: ASHRAE 62.1 Ventilation · Duct Sizing (Equal Friction / Static Regain) · COMcheck vs. Title 24
Common Questions
Heat Balance (HBM) solves an energy balance at every surface — inside and outside — and in the room air, accounting for conduction, convection, and radiation. It's the ASHRAE reference method (Handbook—Fundamentals, Chapter 18) but iterative and computationally heavy. Radiant Time Series (RTS) is ASHRAE's simplified, non-iterative derivative of HBM: it uses radiant time factors and conduction time series factors to model the same thermal-lag physics without the iteration, which makes it the practical default for most commercial work.
No. ACCA's Manual J is a residential load-calculation method, intended for a house or a small residential unit. Commercial buildings are typically sized with a commercial method such as RTS or, on high-stakes projects, the Heat Balance Method. Using Manual J on commercial work is a mismatch a sharp plan reviewer is likely to notice.
A single documented allowance, applied only where an input is genuinely uncertain or where the owner has directed future-capacity margin within the energy code's equipment-sizing limits, is defensible. Padding every component "to be safe" beyond that isn't — it can cause oversized equipment to short-cycle and lose dehumidification, works against your energy-code compliance position, and costs more both to buy and to run. Apply any justified allowance once, document where it was applied, and stop.
The rate at which an HVAC system must remove heat to keep a space at its indoor design conditions on a peak day — the sum of envelope, solar, internal, infiltration and ventilation gains, adjusted for thermal lag.
The rigorous, first-principles method that solves the energy balance at each surface and the room air; it's the engine behind many detailed load and energy simulation tools.
A simplified, transparent derivative of the Heat Balance Method suitable for most commercial load calculations, using radiant time factors to model thermal lag.
Largely superseded by RTS for production work, but the CLTD/CLF method is still useful for hand-check estimates and building intuition about how loads behave.
Because radiant heat gain is absorbed by the building mass and released later (thermal lag) — so the load the coil sees is time-shifted from the instantaneous gain.
Heat that changes air temperature (solar, lighting, equipment, warm outdoor air), as opposed to latent heat, which changes moisture.
Heat associated with moisture — from people, cooking, and humid ventilation air — that the cooling coil must remove through dehumidification.
The ratio of sensible load to total load; it drives coil selection and how much dehumidification the system must provide.
ASHRAE climate design conditions at a chosen percentile (e.g., 0.4% or 1%), not record extremes, which avoids inflating every load.
Tools like Carrier HAP, Trane TRACE 3D Plus, IESVE, EnergyPlus/OpenStudio, and eQUEST — with the load method built into each. Carrier HAP moved to the ASHRAE Heat Balance method at v6, while v5 and earlier used the Transfer Function Method.
Senior electrical designer with 6+ years in U.S. MEP design, contributor to 900+ projects, and an experienced third-party peer and plan reviewer.
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