Heat loss and HVAC sizing
A room-by-room heat-loss and cooling-load estimate in the style of Manual J (not Manual J): U- or R-values and areas, air leakage, design temperatures for a few US cities with sources, window solar gain from the sun's position, and heat-pump sizing with the balance point — in imperial or metric.
Added
Works with
- HVAC ducts and airflow Building tradesimports these rooms and their heat loss to size ducts
- Gas pipe sizing Building tradesput the furnace or boiler this load calls for on the gas line and size the pipe
How to use it
A Manual J-style estimate, not ACCA Manual J. It shows the method and where the heat goes; for buying equipment, have a full Manual J (with S for equipment and D for ducts) done.
How to use it
Design conditions — choose a city to fill in the 99 % heating and 1 % cooling design temperatures (outdoors is colder than the first only 1 % of winter hours, hotter than the second 1 % of summer hours), or enter your own. Indoors is 70 °F heating and 75 °F cooling, as in Manual J. Set the summer daily range, the latitude (for the sun) and the design month. Infiltration is in air changes an hour at design conditions; from a blower-door test, divide ACH50 by about 20 (a rule of thumb). Add mechanical ventilation with any heat recovery, the ground and unheated-space temperatures, the summer moisture difference for latent load, and duct losses if ducts run outside the conditioned space.
Rooms — for each room, its floor area and ceiling height, and every surface between it and outdoors, an unheated space or the ground: walls (net of windows and doors), windows (U-factor, SHGC and interior shading), doors, roofs or attic ceilings, floors and slab edges (perimeter × F-factor). In imperial, walls, roofs and floors take the whole assembly’s R-value; windows and doors the U-factor from their label. A room can have its own air-change rate, people and appliance gains (cooling only). Export and import the house as JSON.
Loads — the design heating load and heat loss per degree (UA), the cooling load (total, sensible, latent, the sensible heat ratio, the peak hour, tons), heating per floor area, and room by room: heating, cooling at the house’s peak and each room’s own peak (west rooms peak late). Bars show where the heat goes; a chart follows the cooling load through the design day.
Heat pump — enter the unit’s heating capacity at 47, 17 and 5 °F from its spec sheet. The chart crosses the building’s load line with the capacity curve at the balance point; below it, backup heat makes up the difference, and the tool says how much at the design temperature. Use Chart full screen for a large view.
Method and R-values — an assembly R-value calculator (layers in series add: R = ΣR, U = 1/R), the formulas, what is left out, and the sources.
The method
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Conduction: Q = U × A × ΔT (slab edges F × perimeter × ΔT).
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Air: Q = 0.018 × ACH × volume × ΔT Btu/h (ft³, °F), standard air’s heat capacity; ventilation 1.08 × cfm × ΔT less heat recovery; latent 4840 × cfm × Δw.
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Cooling, hour by hour on the design day: outdoor temperature from ASHRAE’s daily-range fractions; walls and roofs at the sol-air temperature; windows A × SHGC × shading × the irradiance on that face from the sun’s position and the ASHRAE clear-sky model; people 230 Btu/h sensible and 200 latent each.
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Checked against a published example: A. Bhatia, “Heat Loss Calculations and Principles”, CED Engineering course M05-003, section 3 — zone 4 of an office in Montreal: walls 9,331 Btu/h, glass 24,192, roof 3,686, conduction 37,209, ventilation 15,924, zone total 53,133 Btu/h. The tool reproduces each figure (open it).
Limits
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No thermal mass or time lag, so heavy walls and roofs read high and early in cooling; no attic model (enter the attic ceiling’s R and treat the attic as outdoors, which errs high), no window overhangs or neighbours' shade, no basement walls by depth.
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Heating gets no credit for sun or people, as in Manual J; cooling sizing by ACCA Manual S has limits on oversizing that are not applied here.
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Design temperatures are for the county named; for anywhere else enter your local ASHRAE 99 % and 1 % values. A few figures are rules of thumb (ACH50 ÷ 20, 85–90 % charging, typical ranges in hints) and are labelled as such.
Sources: US EPA, ENERGY STAR Certified Homes County-Level Design Temperature Reference Guide (2015), from the ASHRAE 2013 Handbook—Fundamentals and ACCA Manual J 8th ed. design conditions; ASHRAE Handbook—Fundamentals ch. 14 Table 6 (daily-range fractions), 2001 ch. 30 Table 7 (clear-sky model), ch. 18 (sol-air temperature); ASHRAE 90.1 Appendix A Table A6.3 (slab F-factor); M. Sherman, LBL (1987), for the ACH50 rule; sun position by NOAA’s equations from the sun and moon tool.
Related: energy usage analyser (what a heat pump would do to your bill), Solar ROI, sun and moon, unit converter.
Linking to an example
The whole house travels in a share link. Add the tab after #: design, rooms, results,
heatpump or method.
| Parameter | Meaning | Example |
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A design-temperature city: |
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Indoor, 99 % heating and 1 % cooling outdoor temperatures, °C |
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Infiltration, air changes an hour |
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One of the presets below |
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example= |
Shows |
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The CED M05-003 worked example, zone 4: 53,133 Btu/h (open) |
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A small three-room house in Boston (open) |
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The same house with a 2-ton heat pump: balance point and backup heat (open) |
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