Determining Supplemental Air Conditioning Needs is a Breeze

IT rooms, conference rooms and other spaces can generate considerably more heat than their square footage might suggest.

Sometimes the issue is equipment. Sometimes it’s people. Sometimes the room is fine most of the day but gets too warm when fully occupied. And sometimes the space needs cooling after the building’s HVAC system shuts down.

Understanding the basic heat loads can help explain why.

Start With the Room

The first factor is the size of the space.

Room Area = Length × Width

A room measuring 20 feet × 10 feet is:
20 × 10 = 200 square feet

But 200 square feet doesn’t tell you the cooling requirement by itself.

Consider three 200-square-foot rooms.

A private office might have one person, a computer and typical lighting.

A conference room might have ten people, lighting, a large display and videoconferencing equipment.

An IT room might have almost no occupants but contain racks of continuously operating electronic equipment.

Same square footage. Very different heat loads.

Add the People

People generate heat.

Using a simplified planning assumption of approximately 400 BTU/hour per person, a ten-person conference room could have an occupant load of:

10 people × 400 BTU/hour = 4,000 BTU/hour

That helps explain why a conference room can be comfortable when empty and too warm when the meeting starts.

Add the Equipment

Electrical equipment also produces heat.

The basic relationships are:

Watts = Amps × Volts

1 watt = approximately 3.412 BTU/hour

Suppose an IT room contains equipment with a combined load of 10,000 watts:

10,000 watts × 3.412 = 34,120 BTU/hour

That’s a substantial heat load in a relatively small room.

Add the Lighting

Lighting contributes heat as well.

If the room has 500 watts of lighting:

500 watts × 3.412 = 1,706 BTU/hour

Now the individual loads begin to tell the story.

Put the Heat Loads Together

Suppose our 200-square-foot equipment room has the following preliminary loads:

Equipment: 34,120 BTU/hour
People: 1,600 BTU/hour
Lighting: 1,706 BTU/hour
Room and other loads: 6,000 BTU/hour

The total preliminary heat load is:
34,120 + 1,600 + 1,706 + 6,000 = 43,426 BTU/hour

Now we can translate that into cooling tonnage.

1 ton of cooling = 12,000 BTU/hour

So:
43,426 ÷ 12,000 = approximately 3.62 tons

That doesn’t mean you order a 3.6-ton air-conditioning unit. It means you now understand the approximate magnitude of the heat that needs to be removed.

The mechanical engineer determines the actual cooling load and appropriate system.

Capacity or Operating Hours?

There are two common reasons supplemental cooling becomes necessary.

The first is capacity.

A conference room may be comfortable when empty but too warm when ten people are using it. The building HVAC is operating; the room simply doesn’t have enough cooling for the load when fully occupied.

The second is operating hours.

An IT room may generate heat 24 hours a day while the building HVAC shuts down at night or on weekends. That room may need an independent supplemental system.

Plan for It Early

Supplemental cooling can require electrical capacity, equipment locations, piping, drainage and controls.

Finding out that a room needs it after construction is much more difficult than identifying the requirement during design.

That’s why the project team should understand three things early:

How big is the room?

What heat loads will be inside it?

When does it need to be cooled?

Leadership Takeaway: Square footage is the starting point, not the answer. Understand the people, equipment and other heat loads—and when the room needs cooling—before assuming the building’s standard HVAC system is enough.