Guides
Answers to the HVAC questions homeowners ask most
These guides cover the HVAC questions that come up every week for homeowners shopping for new equipment or trying to understand the one they have. Pricing is sourced from current installer quotes. Temperature and capacity numbers come from manufacturer extended-performance tables and the federal energy code.
Reviewed by licensed HVAC technicians whose day-to-day work matches the topic.
Updated May 2026
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Guide
Do heat pumps work in cold climates?
What temperature heat pumps stop working, when you need a cold-climate model, when you need backup heat, and what state rebates do to the install math.
Guide
Thermostat settings for summer and winter
What to set the thermostat to when you are home, sleeping, away, on vacation, or with a baby in the house. Schedules by season with the dollar math.
Guide
How long does an AC unit last?
Lifespan by equipment type (window, central, mini-split, heat pump, geothermal), what climate does to those numbers, and which components fail first.
The four HVAC numbers every homeowner should know
HVAC has its own vocabulary and most of the words exist to obscure pricing. Four numbers actually matter when you're talking to a contractor. Once you can read these on the spec sheet, the marketing fluff stops affecting your decision.
1. BTU per hour (capacity)
BTU per hour is how much heating or cooling the equipment can deliver at design conditions. Air conditioners are usually quoted in tons (1 ton = 12,000 BTU/hr). Furnaces are quoted in raw BTU/hr input. A 3-ton AC delivers 36,000 BTU/hr of cooling. A 60,000 BTU/hr furnace burns enough gas to deliver roughly that much heating at 95% efficiency. Wrong BTU is the single biggest install mistake; too big causes humidity and short cycling, too small fails on the hottest or coldest day. The BTU sizer and AC tonnage calculator return the number your home actually needs.
2. SEER2 (cooling efficiency)
SEER2 is the Department of Energy's 2023 rating standard for how efficiently an AC or heat pump cools across a typical season. Higher is better. The federal minimum is 14.3 SEER2 in the South, 13.4 SEER2 in the North. Premium variable-speed units hit 22 SEER2 and up. The savings between a 14.3 and 18 SEER2 AC in a hot climate is roughly 20 percent of the cooling bill, which the SEER2 savings calculator will put into dollars for your zip code.
3. AFUE or HSPF2 (heating efficiency)
For furnaces, AFUE (Annual Fuel Utilization Efficiency) tells you what percentage of the fuel burned becomes useful heat. The federal minimum is 80 percent. Condensing models reach 95 to 98 percent. For heat pumps, HSPF2 measures heating efficiency on the same DOE 2023 standard, with the federal minimum at 7.8 HSPF2 and premium cold-climate units hitting 11 HSPF2 and above. The AFUE payback calculator shows when the high-efficiency furnace upgrade pays back, and the HSPF to HSPF2 converter translates older heat pump ratings to the new standard.
4. COP (heat pump efficiency at a specific temperature)
COP (Coefficient of Performance) is heat output divided by electricity input at a specific outdoor temperature. A COP of 3.0 means 3 BTU of heat per BTU of electricity. The number drops as outdoor temperature drops, which is why cold-climate ratings exist. A typical air-source heat pump runs COP 3.5 at 47°F, COP 2.5 at 17°F, COP 1.8 at 5°F. Geothermal stays at COP 3.5 to 5.0 year-round because the soil temperature never changes. The COP calculator converts efficiency at your design temperature into operating cost at your electric rate.
From guide to your specific numbers
Every guide above ends pointing at a calculator that takes the explainer content and puts your specific zip code, square footage, or utility rate into it. The cold-climate heat pump guide hands off to the heat pump sizing calculator and the rebate finder for the state-by-state payment math. The thermostat guide hands off to the thermostat savings calculator with your specific utility rates. The lifespan guide hands off to the replace vs repair calculator when an aging system starts asking the harder question.
Read the guide first to know what the right answer looks like, then run the calculator to find out what it is for your specific home. The numbers always look different on the kitchen table than in the abstract.