Sizing a Heat Pump for Denver — The Part Nobody Talks About
Equipment brand is the thing homeowners research and the thing that matters least. Sizing is the thing nobody asks about and the thing that determines whether you like your system. Here is how it should be done in Colorado.
“Same Size as the Old One” Is Not a Calculation
The most common sizing method in residential HVAC is to read the nameplate on the equipment being removed and match it. This is fast, it is free, and it propagates whatever error was made the last time — often decades ago, before the house had new windows, added insulation, or a finished basement.
The second most common method is square footage times a rule of thumb. That number came from somewhere near sea level and it does not know whether your house faces south, whether the attic is insulated, or how leaky it is.
Neither approach is a load calculation. A Manual J is: a room-by-room accounting of heat loss and heat gain based on your actual construction, orientation, glazing, insulation, infiltration, and the design temperature for your location.
Oversized system
- Short-cycles — on and off repeatedly instead of running steadily
- Poor dehumidification in cooling season
- Temperature swings and uneven rooms
- More wear on the compressor, shorter life
- You paid more for equipment that performs worse
Oversizing is the more common error, because it feels safe.
Undersized system
- Cannot hold setpoint at design temperature
- Backup heat runs constantly — expensive
- Homeowner concludes heat pumps do not work in Colorado
- May fall below the 80% heating load threshold required for Colorado’s $1,000 credit
Why Sea-Level Numbers Are Wrong Here
Air density at 5,280 ft
Denver air is roughly 17% less dense than at sea level. Less mass per cubic foot means less heat carried per cubic foot of airflow. Equipment capacity tables and duct sizing both need correction, and a lot of quick calculations skip it.
Very low humidity
Colorado’s dry air changes the latent load significantly compared to most of the country. Cooling loads here are far more sensible-heavy, which affects equipment selection and how you evaluate a system’s dehumidification behaviour.
Huge diurnal swings
A 40–70°F swing between overnight low and afternoon high is routine in spring and fall. Thermal mass and solar gain matter more here than in steadier climates, and a system sized only for the extremes handles the middle badly.
Intense solar gain
Thin atmosphere and 300 sunny days. South and west glazing drives cooling load harder than the same window would at lower elevation. Orientation is a real input, not a rounding error.
What We Actually Do
- Measure the house
Room dimensions, ceiling heights, window sizes and orientations, door count. Not a walkthrough estimate.
- Assess the envelope
Wall and attic insulation where we can inspect it, window type and age, obvious infiltration paths, whether the basement is conditioned.
- Read your utility history
Twelve months of gas and electric use is a reality check against the calculation. If the model and the bills disagree, we find out why before we specify equipment.
- Run Manual J at your design temperature
Room by room, corrected for altitude, using the design temperature for your specific location — which is not the same in Denver as it is in Evergreen.
- Measure duct static pressure
Determines whether your existing ducts can carry the airflow a heat pump needs, and whether a return-air modification belongs in the proposal.
- Select equipment against the load
Matched to the calculated load using the manufacturer’s published capacity at Denver temperatures — not the nominal tonnage on the box.
- Give you the document
You keep the load calculation. It is yours, whether you hire us or use it to evaluate someone else’s proposal.
Sizing Questions
How long does a load calculation take?
Do you charge for it?
My contractor says a load calculation isn’t necessary. Is that ever true?
Does sizing affect my rebates?
Get a Real Load Calculation
Free, room by room, altitude corrected, and yours to keep — even if you use it to check someone else’s quote.