Heat pump or furnace? How to tell which one heats your house, and what that changes

Across our five cities, homes built with natural gas service mostly have a split system: a gas furnace in the attic or a closet, with an air conditioner's indoor coil sitting on top of it and the compressor outside. Subdivisions without gas, parts of The Colony and newer phases in McKinney and Frisco among them, have heat pumps: the outdoor unit runs in reverse in winter to pull heat out of the outside air, with electric heat strips in the air handler for backup. A few homes have dual fuel, a heat pump paired with a gas furnace that takes over below a set outdoor temperature.

• Thermostat has Emergency Heat or Aux Heat: heat pump
• Outdoor unit runs on a cold morning: heat pump
• Gas line and a metal or PVC flue at the indoor unit: furnace
• Air at the register in heating is hot to the hand, over 120 degrees: furnace
• Air at the register is warm but not hot, 90 to 105 degrees: heat pump

Longer run times and air that feels lukewarm are the design, not a defect; the heat pump moves heat steadily rather than blasting it. Every so often on a cold, damp morning the outdoor unit will stop, steam, and the house will get a short puff of cool air; that is the defrost cycle clearing frost from the outdoor coil. Below roughly 25 to 30 degrees the heat pump alone cannot keep up and the electric strips help, which is when the bill jumps. The costly mistake is turning the thermostat up several degrees at once, which calls the strips immediately.

A sequence of sounds at each start: a small fan, a click, a glow or spark, a whoosh, then the main blower a minute later. Air that is hot. Cycles of ten to twenty minutes. A burning-dust smell on the first run of the season. What is not normal is cycling every few minutes (usually a dirty filter), repeated clicking without flame (ignitor or flame sensor), or any smell of gas, which means leave the house and call the gas utility first.

In North Texas winters, which are mild with a few hard freezes, a heat pump is usually cheaper to run than a furnace through most of the season and more expensive during the freezes when the strips run. Over a year the two are closer than people expect, and the choice at replacement time depends more on whether the house has gas and how the ducts are built than on the fuel itself.