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 it survives plan check.
A cooling-load calculation estimates how much heat an HVAC system must remove to hold indoor design conditions on a peak day. The method — Heat Balance, RTS, or CLTD — determines how solar gain, internal loads, thermal storage (lag), and ventilation are modeled, and therefore how big the equipment ends up. Getting the method (and its assumptions) right is upstream of every equipment decision that follows.
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 holds up. 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 methods estimate peak cooling load. The Heat Balance Method (HBM) is the rigorous, first-principles approach ASHRAE's software is built on. 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 CLTD/CLF is the older hand-calc method — still useful for checks and intuition, largely superseded by RTS in practice.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 frequently the largest single component — which is exactly the radiant, time-delayed kind, so the method's handling of it matters most.)
Heat Gain Isn't Cooling Load: Sensible, Latent, and Lag
The reason methods exist at all is that instantaneous heat gain is not the same as the cooling load. Two things separate them:
- 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 outdoor air has a low SHR and a real latent problem; miss it and the room is cold but clammy.
- Thermal lag. Radiant heat gain (sun through glass, warm surfaces) doesn't hit the air instantly — the mass of the building absorbs it and releases it later. That lag is why peak cooling load arrives after peak solar gain, and why a heavy building peaks differently than a light one.
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 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.
- 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 | 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 standard, 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, and it's overkill for a straightforward tenant improvement. 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 should 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 produces peak design loads that stand up to review. 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. Use it to check a number, not to defend one on a permitted set.
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, or an ASHRAE commercial method on a single-family home, 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 standard design percentile and the equipment is sized for reality, not for the worst hour of the decade.
- 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 true simultaneous peak for a whole system — is smaller than the sum of the individual room peaks. Sizing a central plant on the sum instead of the block is one of the most common ways 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 | Transfer-function / RTS-family |
| Trane TRACE 3D Plus | Load calc + energy | Heat-balance engine (EnergyPlus-based) |
| IESVE | Detailed energy modeling | Heat balance |
| EnergyPlus / OpenStudio | Research-grade energy | Heat balance |
| eQUEST | Whole-building energy | DOE-2 |
The practical point: choosing HAP vs. a heat-balance engine is choosing RTS-family vs. full HBM. 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 mid-size office floor (illustrative numbers only — every real load depends on climate, envelope, and use): build the load from its parts — envelope conduction, solar through the glazing by orientation, people (sensible + latent), lighting and plug loads on their schedules, and the ventilation air per ASHRAE 62.1. 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.
That block sensible-plus-latent total, at its SHR, is what selects the equipment: the air handler or rooftop unit (AHU/RTU) and cooling coil (sized for both sensible cooling and the latent/dehumidification the SHR demands), the chiller or condensing unit behind it, and the airflow that carries it — whether distributed through VAV boxes or a VRF system — through ductwork coordinated with the rest of the trades (see our MEP coordination best practices). 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
The most common way a load calc goes wrong isn't the method — it's the safety factor stacked on top of it. A modest allowance (often in the 10–20% range) for uncertainty and future load is reasonable. Padding every component "to be safe" is not, and it backfires: comfort suffers (oversized cooling equipment short-cycles, never runs long enough to dehumidify, leaves 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 a sensible allowance once, and stop. Remember too that ventilation drives a real share of the load: outdoor air per ASHRAE 62.1 is a required input, not a place to guess.
Documenting the Number So It Survives 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 ASHRAE 62.1, tied to occupancy.
- 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 doesn't have to question it. 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 a full energy balance at every surface — inside and outside — and in the room air, accounting for conduction, convection, and radiation without shortcuts. It's the ASHRAE reference standard (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 should be sized with RTS or, on high-stakes projects, the Heat Balance Method. Using Manual J on commercial work — or a commercial method on a single-family home — is a mismatch a sharp plan reviewer will notice.
A modest allowance, often in the 10–20% range, for uncertainty and future load is reasonable. Padding every component "to be safe" beyond that isn't — it causes 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 one sensible allowance, document where it was applied, and stop.
The amount of heat an HVAC system must remove to keep a space at its indoor design conditions on a peak day — the sum of envelope, solar, internal, 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 ASHRAE-grade load and energy software.
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), and because loads are split into sensible and latent — 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 — sizing to a standard design condition avoids inflating every load.
Tools like Carrier HAP, Trane TRACE 3D Plus, IESVE, EnergyPlus/OpenStudio, and eQUEST — with the load method (RTS-family vs. heat balance) built into each.
Senior electrical design engineer with 6+ years designing MEP systems for 900+ U.S. projects. Experienced third-party peer reviewer and city plan reviewer.
Connect on LinkedIn →Get a Defensible Number the First Time.
We size to the right method, document the trace, and reconcile the calc against the equipment schedule before it ships.
Schedule a Scope CallSend Us the Details