Temperature Humidity Load Calculator

Calculator Inputs

Example Data Table

Formula Used

This calculator estimates the load required to condition incoming air (ventilation or infiltration) from an outdoor state to an indoor target state, then optionally adds internal sensible and latent gains.

• Saturation vapor pressure (kPa): Psat = 0.61078 · e^{(17.2694·T)/(T+237.3)} where T is in °C.
• Humidity ratio: W = 0.621945 · Pv / (P − Pv), with Pv = RH · Psat.
• Moist air enthalpy (kJ/kg dry air): h = 1.006·T + W·(2501 + 1.86·T).
• Mass flow (SI): ṁ ≈ ρ · V̇, using ρ = P / (R·TK·(1+1.6078·W)).
• Total load (SI): Q̇total = ṁ · (hout − hin) in kW.
• Sensible load (SI): Q̇sens = ṁ · 1.006 · (Tout − Tin). Latent is the remainder.
• Quick Imperial check: Q̇total ≈ 4.5·CFM·(hout − hin), and Q̇sens ≈ 1.08·CFM·ΔT.

Sign convention: positive values mean cooling and/or dehumidification demand; negative values indicate heating or humidification tendency.

Practical Notes for Temperature and Humidity Load Planning

Temporary and semi-permanent construction spaces rarely behave like finished buildings. Doors open frequently, partitions leak, and work activities generate moisture. A temperature and humidity load check helps you estimate how much cooling and dehumidification capacity is needed to keep a space workable, protect materials, and support curing, finishes, and equipment reliability.

The first driver is outside air that enters as ventilation or infiltration. When hot and humid air is brought to a cooler indoor setpoint, the load splits into sensible (temperature change) and latent (moisture removal). Latent load is often the larger portion in coastal or monsoon climates, which is why a unit that “meets kW” can still fail if its moisture removal is low.

Next, add internal gains. People add both heat and moisture; equipment and lighting are mostly sensible; process and solar gains vary by project. For planning, it is better to be slightly conservative, then validate with site observations. If the space is tightly sealed and occupied lightly, the air-change portion may be small. If the space is leaky, the air-change portion can dominate.

Worked example (matches the table above)

• Space: 150 m² floor area, 3 m height, estimated 2 ACH (≈ 900 m³/h).
• Outdoor: 35°C at 60% RH. Indoor target: 24°C at 50% RH.
• Internal gains: 6 people, 1200 W equipment, 12 W/m² lighting.
• Result: ventilation total ≈ 12.06 kW, moisture transfer ≈ 12.42 kg/h, grand total ≈ 15.84 kW.

Use the moisture transfer number to judge dehumidification needs. If moisture transfer is positive, the system must remove that water vapor rate to maintain the target RH. If the project is sensitive (wood flooring, coatings, stored gypsum, or electronics), prioritize stable humidity over only meeting temperature.

When translating results into equipment, separate “temperature control” from “humidity control.” A cooling-only unit sized to the grand total may cycle off quickly and leave RH high. Favor systems with continuous runtime and low coil temperatures, or pair cooling with a dedicated dehumidifier. Maintain drainage, avoid long uninsulated ducts, and keep supply air paths short to prevent condensate and slip hazards. If you use negative-pressure dust control, account for extra outside air makeup. For material protection, keep interior surface temperatures above the indoor dew point to reduce condensation on steel, glazing, and temporary enclosures.

Finally, remember that this is an engineering estimate, not a substitute for commissioning. Confirm airflow assumptions (or ACH) with fan data and site conditions, and adjust internal loads as the work phase changes. Re-run the calculator whenever occupancy, openings, or weather conditions shift.

FAQs

1) What does “latent load” mean here?

Latent load is the energy required to remove moisture from incoming air so indoor relative humidity stays near the target. High latent load usually indicates strong dehumidification demand.

2) How do I choose an ACH value for construction spaces?

Start with 1–3 ACH for semi-enclosed areas and 3–8 ACH for very leaky spaces. Use site observations, door activity, and temporary enclosure quality to refine the estimate.

3) Why can total kW look acceptable but humidity still rises?

Some cooling units deliver sensible capacity but limited moisture removal, especially at higher airflow or warmer coils. Check the moisture transfer and ensure the selected equipment has adequate dehumidification performance.

4) What pressure should I enter?

Leave it blank unless you have a measured site value or high elevation. Standard pressure works for most planning. Pressure mainly affects psychrometric properties slightly.

5) Are people heat defaults suitable for all tasks?

They represent light activity. For heavy labor, increase per-person sensible and latent inputs. For seated office-type work, the defaults are usually appropriate.

6) Can I use this for heating season checks?

Yes. If outdoor conditions are cooler or drier than indoor targets, loads can become negative, indicating heating or humidification tendency rather than cooling or dehumidification demand.

7) How should I size equipment from the results?

Use the grand total as a starting point, then apply project contingency and verify latent capability against the moisture transfer. Consider staging or modular units for changing phases.