Why the old size can mislead
Many Denver houses have been insulated, re-windowed or finished since the old AC went in. The old unit was sized for the house as it was then, and it was often too big even then.
A heat pump also has to heat. In Denver the heating load on a cold night is usually bigger than the cooling load in July, so sizing from the AC label can leave a dual fuel system leaning on the furnace more than it needs to.
How a proper sizing works
A Manual J load calculation goes room by room through your house, using floor area, windows and which way they face, insulation, air leaks and local design temperatures. Then the ducts are checked to see whether they can move the air that size needs.
Why variable speed helps
Variable-speed heat pumps such as the Bosch IDS Ultra slow down on mild days and speed up on cold ones. That forgives small sizing errors and keeps the house even. You still need the calculation.
Get a size
The sizing calculator on unicolorado.com gives a first estimate from a few facts about the house. For a final size, send photos or book an in-home estimate.
Common questions
Can I match my old AC size?
That's a guess. Old ACs were often oversized, and a heat pump also has to heat. Size it from a load calculation.