Optimizing Internal Loads: Key Factors for HVAC

Internal loads are one of the two dominant factors on the cooling side of any load calculation. They represent heat generated inside the home rather than heat transferring through walls, windows, or the roof. Get internal loads wrong, and your system sizing will be off regardless of how carefully you measured everything else.

The most common mistake? Overestimating occupancy.

The Thanksgiving Problem

It's tempting to think about a home at full capacity. Thanksgiving dinner with extended family. A Fourth of July party spilling into every room. Kids home from college with friends visiting.

That's not how you size an HVAC system.

Load calculations are based on normal occupancy, not peak events. The standard approach is straightforward: number of bedrooms plus one. A three-bedroom home gets calculated with four occupants. The plus one accounts for two people in the master bedroom.

If you sized systems for holiday gatherings, every home would have oversized equipment that short-cycles through 95% of the year. The homeowner would get poor humidity control, higher energy bills, and equipment that wears out faster than it should.

Where to Put the People

For block load calculations, total occupancy is what matters. But room-by-room calculations require you to place those occupants somewhere specific.

The key question: where will people be at 4 or 5 PM on a hot summer day?

That's your peak cooling demand period. That's when the load calculation needs to reflect reality.

Most people won't be in bedrooms during late afternoon. They'll be in living areas, kitchens, or home offices. Place your occupant loads accordingly.

A common error is distributing people across every room in the house. But people can only be in one room at a time. If you put an occupant in the living room, don't also put one in the bedroom they'll sleep in later. You're calculating for a specific moment in time, not a 24-hour average.

The Exception: Full-Time Guests

The standard bedroom-plus-one formula assumes typical residential use. But homes don't always follow typical patterns.

If a homeowner has converted a room into permanent guest quarters—an in-law suite, a long-term visitor situation, a roommate arrangement—you should account for those additional occupants. A "bonus room" that's actually functioning as a fourth bedroom with someone living in it needs to be treated as such.

Ask the homeowner how the space is actually used. The room's label on a floor plan matters less than what happens in reality.

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Heat Output Per Person

Each occupant adds roughly 230 BTU/hr of sensible heat to a space, plus additional latent load from moisture. That's not a huge number compared to solar gain through a large west-facing window, but it adds up.

In a four-bedroom home with five occupants clustered in the living area during peak cooling hours, you're adding over 1,000 BTU/hr to that space. Ignore it, and the room runs warm. Overcount by placing imaginary people throughout the house, and you've inflated your total load unnecessarily.

Other Internal Loads

People aren't the only source of internal heat gain. Appliances, lighting, and electronics all contribute.

Kitchens are heat factories. Ovens, stovetops, refrigerators, and dishwashers all release heat into the space. Standard load calculations include assumptions for typical kitchen appliance use, but a home with serious cooking habits may warrant additional consideration.

Lighting varies dramatically based on fixture type. Incandescent bulbs convert most of their energy to heat rather than light. LED fixtures run much cooler. A home that's been updated to LED throughout has a meaningfully different internal load than one still running older lighting.

Electronics and appliances throughout the home add up. Computers, televisions, gaming systems—anything plugged in and running generates heat. Most load calculation methods include standard allowances for this, but a home office packed with equipment or a media room with multiple large displays may exceed those assumptions.

Getting Internal Loads Right

The practical approach to internal loads comes down to a few principles:

Use the standard occupancy formula. Bedrooms plus one. Don't inflate based on occasional gatherings or hypothetical scenarios.

Place occupants realistically. For room-by-room calculations, put people where they'll actually be during peak cooling hours. Living spaces in the afternoon, bedrooms at night.

Account for actual use. If the home has unusual occupancy patterns or has been converted in ways that affect how many people live there, adjust accordingly.

Don't overthink appliances. Standard allowances work for most homes. Only adjust if something genuinely unusual is happening—a commercial-grade kitchen, a server room in the basement, that kind of thing.

Internal loads are part of the total picture, not the whole picture. They matter, but they shouldn't dominate your attention at the expense of envelope characteristics and solar gain. Get all the pieces right, and the load calculation reflects what the home actually needs.

Conduit Tech's platform handles internal load calculations automatically based on the room types and occupancy you specify. Book a demo to see how the entire load calculation process works.