Room Heat Load Heater AC Sizer Calculator

Purpose and model boundary

Builds a room-level steady-state heating and cooling load from envelope UA, outdoor airflow, solar, sensible, and latent gains, then screens user-entered equipment candidates against the active capacity targets.

Do not treat this result as an energy-code determination, Manual J, HVAC or ventilation design, moisture-control guarantee, structural or concrete design, permit approval, equipment selection, cost quote, or endorsement by a cited source. Site climate, assemblies, room use, ventilation, windows, material properties, geometry, manufacturer instructions, locally adopted codes, and approval thresholds remain the user's responsibility.

Inputs and units

• calculation scope: a listed route or text value.
• infiltration basis: a listed route or text value.
• solar basis: a listed route or text value.
• candidate objective: a listed route or text value.
• room volume: entered in ft^3.
• heating indoor temperature: entered in deg F.
• heating outdoor temperature: entered in deg F.
• cooling indoor temperature: entered in deg F.
• cooling outdoor temperature: entered in deg F.
• air changes per hour: entered in 1/hour.
• entered outdoor airflow: entered in cfm.
• air density: entered in lb/ft^3.
• air specific heat: entered in Btu/(lb deg F).
• entered total solar gain: entered in Btu/hour.
• internal sensible gain: entered in Btu/hour.
• cooling latent gain: entered in Btu/hour.
• heating sizing margin: entered in fraction.
• cooling sizing margin: entered in fraction.

The planning grid contains 14 fixed data rows plus one header row with these columns: Include, Row label, Row kind, Area or heat capacity (ft^2 or Btu/hour), U-factor or cool capacity (Btu/(h ft^2 deg F) or Btu/hour), SHGC or installed cost (fraction or user currency), Incident solar (Btu/(h ft^2)), Shading factor (fraction), Source note. Numeric columns retain the units shown here; text, route, and include columns are intentionally unitless.

Governing relationships

For included envelope component j, component and total conductance are:

UA_j = A_j U_j

UA = sum(UA_j)

UA_j and UA are in Btu/(h-deg F), A_j is area (ft^2), and U_j is transmittance (Btu/(h-ft^2-deg F)). Outdoor airflow is entered directly or calculated as:

q_air = ACH*V_room/60

q_air is in cfm, ACH is 1/h, and V_room is ft^3. Air conductance is:

UA_air = 60 q_air rho_air c_p

UA_air is in Btu/(h-deg F), with air density rho_air in lb/ft^3 and specific heat c_p in Btu/(lb-deg F).

Heating design load is:

Q_heat = (UA+UA_air)(T_in,h-T_out,h)

Window solar is:

Q_solar = sum(A_j SHGC_j I_j f_shade,j)

Both are in Btu/h. Window solar may instead use the entered total; SHGC and shade factor are fractions, and I_j is incident solar (Btu/(h-ft^2)). Cooling sensible and total loads are:

Q_cool,sensible = (UA+UA_air)(T_out,c-T_in,c)+Q_solar+Q_internal

Q_cool,total = Q_cool,sensible+Q_latent

Heating and cooling targets multiply their respective loads by (1+m), where m is the entered sizing-margin fraction.

An equipment row passes when its entered heating capacity is at least the active heating target and its cooling capacity is at least the active cooling target, all in Btu/h. Passing rows are ranked by combined active capacity or entered installed cost according to the selected objective.

Calculation sequence

1. Validate every active selector, scalar bound, type, and every fixed-grid entry.
2. Apply the selected route and ignore values from inactive fields.
3. Normalize unit-dependent inputs into the workbook's calculation basis.
4. Evaluate every included row or candidate and aggregate the active schedule.
5. Calculate primary and supporting outputs, populate the authored chart series, and apply derived numeric checks.
6. Return the workbook status with the computed results; invalid input is never converted into a plausible engineering answer.

Outputs and interpretation

• total ua (Btu/(h deg F)): Sum of included component area times U-factor.
• heating design load (Btu/hour): Room envelope and outdoor-air heating load before planning margin; zero when heating is inactive.
• heating target capacity (Btu/hour): Heating design load after the entered planning margin.
• cooling sensible load (Btu/hour): Envelope, airflow, solar, and internal sensible cooling load; zero when cooling is inactive.
• cooling target capacity (Btu/hour): Total sensible plus latent cooling load after the entered planning margin.
• selected candidate: Adequate candidate selected by the entered objective, or a clear no-candidate result.

The chart is an explanatory view of the workbook's named chart ranges. It does not add a separate calculation or replace the numeric outputs.

Validation and status logic

The workbook evaluates Model_Status in the order shown. The first matching condition wins.

Assumptions and limitations

• All shipped rows, properties, and equipment capacities are synthetic and illustrative.

• Heat transfer and infiltration are steady-state at entered design temperatures.

• Latent and internal gains are entered rather than inferred.

• This is a room or zone screening estimate, not a whole-building load procedure, duct design, or equipment selection.

• Distribution losses, cycling, part-load performance, ventilation standards, thermal mass, and adjacent-zone heat flow are not modeled.

Restrictions and non-computing states

Each numeric control offers a working range chosen for usable entry, which is not always the limit this calculator enforces; a value outside that limit is rejected with a message rather than quietly accepted. The calculator additionally enforces allowed choices, active-field types, complete fixed-grid rows, and conditional or cross-field rules before computing.

Unsupported route values, incomplete rows, out-of-domain inputs, relational failures, nonfinite derived arithmetic, and workbook error tokens are non-computing states. Hidden inputs remain in the values sent to the calculator but must not affect results while those fields are inactive.

Errors and warnings

A rejected entry means the submitted state failed the published input rules. Workbook NOT VALID identifies an invalid active domain or unsupported derived arithmetic. Workbook CHECK retains a finite result while flagging a planning condition that needs review. OK means only that the delivered workbook logic closed within the entered assumptions; it is not code compliance, professional approval, or a safety guarantee.

This version is checked against 118 saved test cases, covering both calculated results and entries that should be refused, run against this exact workbook.

• U.S. Department of Energy heating and cooling sizing guidance - Primary public context for avoiding square-foot-only equipment sizing and obtaining qualified load evaluation. Official public U.S. government source; no copied sizing table, text, figure, or proprietary procedure is embedded.
• U.S. Department of Energy air-sealing guidance - Primary public context for infiltration and project-specific measurement. Official public U.S. government source; airflow arithmetic is independently implemented without copied text, tables, or figures.

Licensing conclusion: No licensed, proprietary, manufacturer, utility, product-price, code-table, climate table, material table, forecast, or external dataset is embedded. The calculator independently implements general heat-transfer, moisture, airflow, energy, geometry, and quantity arithmetic from cited public and official sources; all defaults, labels, property rows, equipment assumptions, diagrams, and test values are synthetic illustrations. No equipment catalog, climate table, load standard, or product performance dataset is reproduced.