What 5,280 feet actually does to your HVAC equipment

Air at sea level weighs about 0.075 lb per cubic foot. At Denver's 5,280 ft it weighs about 0.0637 lb per cubic foot. That is roughly 15 to 17 percent less mass in the same volume, depending on how you handle temperature, and it is the reason a system selected from a catalogue printed in Ohio underperforms here.

None of what follows is controversial. It is in ACCA Manual J, in the ASHRAE Fundamentals Handbook, and in every manufacturer's high-altitude installation supplement. It is simply left out of a lot of proposals.

1. The sensible heat equation changes

Almost every airflow calculation in this trade starts from the same relationship. At standard sea-level conditions:

Q_sensible = 1.08 x cfm x delta T

That 1.08 is not a universal constant. It is the product of the specific heat of air, the density of air, and 60 minutes per hour. Change the density and the constant changes with it. Multiply by the Denver density ratio of about 0.83 and you get:

Q_sensible = 0.90 x cfm x delta T

Read that again as a practical statement: the same air handler moving the same cfm across the same temperature difference removes about 17 percent less heat in Denver than it does in Cleveland. Everything else in this article is a consequence of that one line.

What it does to airflow per ton

The familiar rule of 400 cfm per ton is a sea-level rule. To move the same mass of air at 5,280 ft you need more volume, so selections here typically land between 450 and 480 actual cfm per ton. If the duct system was sized for 400 cfm per ton and the equipment is now asking for 460, the velocity goes up, the friction loss goes up as roughly the square of velocity, and the static pressure problem you did not have becomes one you do.

2. Fans generate less pressure

Fan laws are density laws. A fan at constant speed moves the same volume flow rate regardless of density, but the static pressure it develops and the power it draws both scale directly with density. At 0.83 density ratio a fan rated to produce 1.0 in. w.c. at sea level produces about 0.83 in. w.c. here.

That cuts in two directions. The fan has less pressure available to push air through the duct, which is bad. It also draws about 17 percent less brake horsepower for the same volume, which matters when you are checking a motor selection. Both need to be in the calculation.

3. Gas equipment has to be derated, and usually re-orificed

Combustion needs oxygen, and there is less of it per cubic foot up here. Above 2,000 ft, gas appliances are installed to the manufacturer's high-altitude listing, which for most equipment means an orifice change and a reduced input rating. A commonly cited rule is around 4 percent less input per 1,000 ft above 2,000 ft, which at 5,280 ft works out near 13 percent, but the number that governs is the one in the specific listing for the specific appliance.

This is not a formality. A furnace running at full sea-level input at 5,280 ft is running rich, producing carbon monoxide it should not be producing, and sooting a heat exchanger that was not designed to be sooted. The way you know it has been done correctly is a combustion analysis at startup, with the printout in the handover file.

4. Refrigerant side, condenser side

On the refrigerant side, reduced air density means the condenser rejects heat less effectively for the same face velocity, which pushes head pressure up. Evaporator sensible capacity falls with airflow mass. Manufacturers publish altitude correction tables for exactly this. Gross capacity corrections for typical DX equipment at 5,280 ft usually land in the 3 to 6 percent range on cooling, on top of the airflow effect above.

The upside, and Denver genuinely has one: our design wet bulb is low. At a 0.4 percent cooling condition of about 91 °F dry bulb and 59 °F mean coincident wet bulb, the latent load in most buildings here is small, and evaporative strategies that would be useless in Houston are genuinely effective on the Front Range.

5. What a correct selection sheet looks like

Three numbers, on one line, for each piece of equipment:

1. The catalogue capacity at the rating condition, with the rating condition stated.
2. The altitude and density correction applied, shown as a factor.
3. The resulting capacity at the site design condition.

If a proposal gives you only the first number, it has not been through this process. That does not necessarily make it wrong, but it does mean nobody can check it, including the person who wrote it.

The short version

At 5,280 ft, plan for roughly 0.90 rather than 1.08 in the sensible heat equation, 450 to 480 actual cfm per ton rather than 400, about 83 percent of rated fan static pressure, and gas input derated to the manufacturer's high-altitude listing with the orifices to match. Then measure it at startup and write the readings down.

Denver is not a hard climate to design for. It is a specific one, and the specifics are all published.