How Heat Gets Into a Space You Are Trying to Condition

Why this matters

"The unit is too small" is the most common diagnosis in the trade and one of the least often verified. A room that is fine at noon and unbearable at four is not describing a capacity shortfall. It is describing a path that only opens for three hours a day, and no amount of extra tonnage fixes a path problem cheaply or well.

Heat reaches a conditioned space by a handful of distinct routes. Each has its own driver, its own daily timing, and its own telltale in the complaint. Add them up and you get a number that tells you what size to buy. Break them out and you get something more useful: a ranked list of what to change. This card is the worksheet for the second one.

Safety for the measurements this sheet requires

• Reading airflow at a ceiling diffuser. Ladder falls are the injury on this task and standing on the top step is how they happen. Use a ladder rated for the load and long enough that you never work above the manufacturer's marked highest standing level, keep three points of contact, and set it on a level surface clear of a door swing. The duty sits at 29 CFR 1910.23 in general industry and 29 CFR 1926.1053 on construction work.
• Entering a ceiling plenum, attic, or crawlspace to trace a duct. Step only on framing or planking, because ceiling material will not hold you. Loose fibrous insulation and duct wrap go airborne when disturbed and the route of exposure is inhalation, so wear respiratory protection selected under 29 CFR 1910.134 rather than relying on gloves and glasses, and treat older thermal system insulation as asbestos-containing until sampled under 29 CFR 1910.1001, or 29 CFR 1926.1101 for maintenance and renovation work. Summer plenum temperatures drive heat illness fast; work short intervals with water and a second person aware you are up there.
• Measuring lighting and equipment load at a panel. Clamping a conductor inside an energized panel is qualified-person work with an arc flash hazard, not a routine reading. Take it from a permanently installed meter or a receptacle-level meter where one exists. Where the panel must be opened, electrical work is governed by 29 CFR 1910.333(b)(2), and the live-dead-live proving sequence is NFPA 70E-2021, 120.5.

The paths, and what drives each one

Conduction through opaque envelope. Driven not by outdoor air temperature alone but by sol-air temperature: air temperature plus the effect of absorbed sunlight on that surface, minus its re-radiation to the sky. A dark flat roof in the afternoon behaves as though the air outside it were far hotter than it is. Take the effective difference from tabulated cooling-load data for the surface type and orientation rather than using air temperature difference on a sunlit surface.

Conduction through glazing. Driven by air temperature difference, and small compared to what comes through the same glass as light.

Solar transmitted through glazing. Driven by orientation, latitude, month, hour, and shading. Nearly independent of outdoor air temperature, which is why it confuses people: a cool bright day can produce a bigger gain than a hot overcast one.

Outdoor air, brought in deliberately or leaking in. Carries both sensible heat and moisture. Ventilation quantity is set by the applicable standard and the occupancy, infiltration by pressure differences across the envelope.

Occupants. Each person adds sensible heat and moisture in proportions that shift with activity level. Tabulated values for moderately active office work sit near 250 Btu/h sensible per person with latent of the same order; pull the row matching the actual activity rather than reusing an office figure for a gym.

Lighting and equipment. Essentially all electrical input to anything inside the space ends up as heat in it. Convert with 1 kW equals 3,412 Btu/h.

Distribution. Not a gain to the room at all, but a loss of delivered capacity: duct or pipe through an unconditioned space arrives warmer than it left, and that difference is capacity the equipment made and the room never got.

The column that names the fault

A complaint that is clock-locked and directional has told you it is a solar problem before you calculate anything, and a complaint about clamminess at setpoint has told you the sensible-to-latent split matters more than the total.

The sheet, filled in

West-facing conference room, 400 ft2, 80 ft2 of glazing, 12 occupants at peak, flat dark roof above, one branch duct through a hot ceiling plenum. Complaint: unusable from about 3 pm to 6 pm on clear days, fine the rest of the time. Conditions at the time of the survey: 92 F dry bulb and 75 F wet bulb outdoors, 74 F and about 50 percent relative humidity indoors.

Solar through glazing. Incident radiation on a vertical west surface on a clear mid-latitude afternoon commonly lands in the low hundreds of Btu/h per square foot; the tabulated value for this latitude, month and hour was 220. Glazing solar heat gain coefficient 0.45, interior blinds present but tied back and not in use. 80 ft2 x 220 x 0.45 = 7,920 Btu/h sensible

Envelope conduction. Wall 200 ft2 at U 0.08 gives UA 16; roof 400 ft2 at U 0.04 gives UA 16; glazing 80 ft2 at U 0.5 gives UA 40. Sunlit surfaces use tabulated sol-air effective differences (45 F for the dark low-mass roof, 30 F for the sunlit west wall); the glazing uses the 18 F air difference. (16 x 45) + (16 x 30) + (40 x 18) = 720 + 480 + 720 = 1,920 Btu/h sensible

Outdoor air. Ventilation for a conference room under the applicable standard works out to 5 cfm per person plus 0.06 cfm per ft2: (12 x 5) + (0.06 x 400) = 84 cfm. Sensible: 1.08 x 84 x 18 = 1,633 Btu/h Latent: humidity ratio outdoors about 103 grains per pound at 92 F dry bulb and 75 F wet bulb, indoors about 64 at 74 F and 50 percent, a difference of 39 grains. 0.68 x 84 x 39 = 2,228 Btu/h

Occupants. 12 x 250 = 3,000 Btu/h sensible; 12 x 200 = 2,400 Btu/h latent

Lighting and equipment. Measured 1.4 kW during a meeting. 1.4 x 3,412 = 4,780 Btu/h sensible

Totals. Sensible 7,920 + 1,920 + 1,633 + 3,000 + 4,780 = 19,253 Btu/h. Latent 2,228 + 2,400 = 4,628 Btu/h. Room total 23,881 Btu/h, of which the sensible share is 81 percent.

Distribution, kept separate on purpose. Supply air measured 3 F warmer at the diffuser than at the unit, at 400 cfm: 1.08 x 400 x 3 = 1,296 Btu/h of capacity made and not delivered. This does not belong in the gain column, because adding it there would double count it against equipment capacity that never reached the room.

Shares of the 23,881 Btu/h peak: solar through glass 33 percent, lighting and equipment 20 percent, occupant sensible 13 percent, occupant latent 10 percent, ventilation latent 9 percent, envelope conduction 8 percent, ventilation sensible 7 percent.

What the mix says to do, and in what order

Two results jump off that list, and neither is visible in the total.

A third of the peak arrives through 80 square feet of glass, for about three hours a day. That is the largest single item by a wide margin, it is present only during the complaint window, and it is addressable without touching the equipment. The blinds being tied back is not a footnote; it is a third of the problem sitting unmanaged.

The envelope is 8 percent. A proposal to add wall or roof insulation for this complaint would be attacking the sixth largest item on a seven-item list. That intervention is not wrong in general, it is simply irrelevant to a clock-locked afternoon complaint, and the sheet is what lets you say so with a number instead of an opinion.

Measured delivered capacity at the room, taken on the air side at these conditions, was 21,500 Btu/h total. Against a 23,881 Btu/h peak the room is short by 2,381 Btu/h, exactly 10 percent. That is the size of the actual gap, and it is worth noticing how small it is relative to how bad the room feels: 10 percent short for three hours is a room people walk out of.

Running the two corrections through the same sheet

Correction one, solar. An exterior shading device or a spectrally selective film taking the effective solar heat gain coefficient from 0.45 to 0.25 gives 80 x 220 x 0.25 = 4,400 Btu/h, down from 7,920, a reduction of 3,520 Btu/h. New peak: 23,881 - 3,520 = 20,361 Btu/h, about 15 percent below the original.

Against the unchanged 21,500 Btu/h of delivered capacity, the room now has 1,139 Btu/h of margin instead of a 2,381 Btu/h deficit. The complaint closes without a single change to the equipment.

Correction two, distribution. Sealing and insulating the branch through the plenum recovers the 1,296 Btu/h the duct was absorbing, raising delivered capacity to 22,796 Btu/h. Measured against the corrected 20,361 Btu/h load rather than the original figure, since the shading is already installed, that is 2,435 Btu/h of margin, about 12 percent of the corrected load.

The alternative that looks obvious and is not. Replacing the unit with the next size up adds capacity the room needs three hours out of twenty-four. For the other twenty-one it runs shorter cycles, and shorter cycles remove less moisture, because latent removal depends on the coil staying wet and cold long enough to condense. Latent is 19 percent of this room's peak, so the failure mode is a room that is cold and clammy at once, a new complaint the customer will not connect to the equipment they just bought.

What this sheet deliberately does not tell you

It is one hour, not a season. These numbers describe the peak the complaint is about. Equipment selection needs the same exercise at the design condition for the whole system, with diversity between spaces, because two rooms do not peak at the same hour and a system sized to the sum of individual peaks is oversized every hour of the year.

It counts energy, not comfort. A person sitting beside 80 ft2 of sunlit glass feels that surface radiating at them regardless of air temperature, and no amount of cold air fixes radiant asymmetry. That is a second reason exterior shading outperforms extra capacity here, and the sheet cannot show it because it has no column for where in the room the heat lands.

Mass moves the peak, and the sheet does not. Heat absorbed by a heavy floor or wall reappears hours later, so the load a system sees is a smoothed and delayed version of the instantaneous gains. On light construction the two are close; on heavy construction the difference is significant, and a proper cooling load calculation method accounts for it while a straight sum of gains does not.

It is gains, not equipment output. Delivered capacity at site conditions is not the nameplate rating, and that difference is a separate subject with its own card.

References

• ASHRAE Handbook, Fundamentals volume, chapters on nonresidential cooling and heating load calculations, on solar gain and sol-air temperature, and on tabulated occupant heat gain by activity
• ANSI/ASHRAE Standard 62.1 for the outdoor air rates applied per person and per unit floor area
• 29 CFR 1910.23 and 29 CFR 1926.1053 for ladders; 29 CFR 1910.134 for respiratory protection; 29 CFR 1910.1001 and 29 CFR 1926.1101 for asbestos-containing insulation; 29 CFR 1910.333(b)(2) and NFPA 70E-2021, 120.5 for electrical work
• See related: How to Think About Capacity Versus Output; What a BTU Actually Is