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What a mile of elevation does to your equipment
Thinner air changes combustion input, cooling capacity and the airflow a duct system has to carry. These are the standard trade approximations, with your own elevation put into them.
The calculator
Approximations, not a design. Manufacturer's data, the appliance's own instructions and local code decide what is actually done to a piece of equipment — and orifice or pressure changes are the manufacturer's specification, never a rule of thumb.
At 5,300 feet
- 85%of sea-level air density — the same volume of air carries this much mass
- 96%of nameplate input, after the usual 4% per 1,000 ft above 2,000 ft
- 69,400BTU/h input after derate, at this elevation
- 30,700BTU/h of cooling from the nominal figure, on air this thin
- +17%more airflow needed to move the same heat as at sea level
What this is not: a specification. Derate rules vary by appliance and by code, some modern furnaces are certified without derate to a stated elevation, and the manufacturer's instructions always win. We use these figures to explain the shape of the problem, then work from the actual equipment in front of us.
In practice
What each of those numbers changes
- Furnace inputA furnace installed here delivers less heat than its nameplate suggests. Sizing from the label alone is how a house ends up cold on the two worst nights of the year.
- Cooling capacityLess dense air carries away less heat per cubic foot, so the sensible capacity of a given system falls. The fix is airflow and correct sizing, not a bigger badge.
- AirflowIf the ducts cannot carry the extra volume, the system runs high on static pressure, the furnace trips its limit in winter and the coil ices in summer.
- Combustion set-upGas pressure and orifice sizing are set to the manufacturer's instructions for the elevation. This is the part people skip, and it is the part that matters most for safety.
Worth asking
Three questions for whoever quotes your next system
- What heat load did you calculate?And was it done at this elevation, for this house, rather than from the size of the old unit?
- What does the derate make the real input?Anyone doing this work here should answer without hesitating.
- What static pressure did you measure?If the ducts were never measured, the airflow figure in the quote is a guess.
