HVAC in Denver has to heat a mile-high house and survive hail on the condenser. Furnaces and boilers are the winter machines. Thin air changes combustion, so a swap copied from the old sticker or from a sea-level habit can be the wrong fire. Boilers and radiators in bungalows are not forced air and should not be diagnosed as if they were. Cooling matters in the hot stretch, and the outdoor unit sits where hail can dent the fins and bend the cabinet. A storm can leave you with wounded cooling and a furnace you still need the next time the night drops. Filters load when winter inversions hold the basin air down and the house stays shut.
Say heat or cooling first. No heat: does it try, is there a smell, is the flue intact. Ask whether the heat exchanger and combustion air are in the look. Ask whether a replacement is sized for this house and this air, and whether ducts or radiators are part of the scope. Old brick houses leak air and leak ducts. A new box on a ruined distribution system will disappoint you in January. If hail hit the condenser, say whether it is dented, leaking, or just ugly. Ask what is ruled out before a compressor is blamed. A dirty filter can counterfeit a big failure.
Condensate lines freeze and clog. Mention water near the air handler so it is not chased as a roof leak. Maintenance before the heating season and a look after hail season are both reasonable, and neither replaces a call when there is no heat tonight.
Inversion weeks change the complaint. The house stays closed, the filter loads, and a furnace that is merely dirty feels broken. Say how long the basin air has been sitting so that gets checked before a larger replacement is assumed. Hail damage on the condenser is a separate sentence.
Name the equipment in the basement or the closet, and say if the outdoor unit took the last storm. Call the number on this page.