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HVACBase

What size AC do I need? Why won't my furnace start? Is a heat pump worth it?

Free HVAC calculators, sizing guides, and troubleshooting help — with the reasoning shown, not just a recommendation.

What size AC?
BTU Calculator
AC not cooling?
12 Causes & Fixes
Heat pump worth it?
Heat Pump Guide
Install costs?
Central AC Install
EPA & DOE Data SourcesAHRI Certified EquipmentACCA Manual J BasedNo Affiliate Links

HVAC Calculators & Sizing Tools

Size your HVAC equipment correctly with our ACCA Manual J based calculators. Avoid oversizing, reduce energy costs, and ensure optimal comfort with ACCA Manual J based sizing tools.

Most Popular

AC BTU Calculator

Calculate exact cooling capacity for any room size. Factors in climate zone, insulation, windows, and heat sources.

ACCA Manual J Based
Professional

AC Tonnage Calculator

Convert BTUs to tons and size central AC systems. Includes ductwork considerations and SEER ratings.

1-5 Ton Systems
Save Money

SEER2 Savings Calculator

Compare energy costs between different SEER2 ratings. Calculate ROI and payback period for upgrades.

2026 Standards
Dual Mode

Heat Pump Size Calculator

Size heat pumps for both heating and cooling. Includes cold climate considerations and backup heat.

Heating & Cooling
Gas & Electric

Furnace Sizing Calculator

Calculate furnace BTU requirements. Factors in home size, insulation, climate zone, and heat loss.

40K-120K BTU
Multi-Zone

Mini Split Sizing

Size ductless mini splits for single or multi-zone applications. Includes line set calculations.

1-5 Zones

All calculators updated for 2026 efficiency standards and include regional climate adjustments

Water Heater Calculator →Generator Sizing →Energy Cost Calculator →

How much AC do you actually need?

Room size drives the BTU/hr your AC needs, at roughly 20 BTU per square foot as a starting point. Oversizing feels safe but causes short-cycling — the unit cools the air fast, shuts off, and never runs long enough to strip humidity, leaving a cold clammy room.

Four room-size bands mapped to typical AC capacity in BTU per hour: 100-300 sq ft needs about 5,000 to 8,000 BTU/hr, 300-550 sq ft needs 9,000 to 12,000 BTU/hr, 550-1,000 sq ft needs 14,000 to 18,000 BTU/hr, and 1,000-2,000 sq ft needs 24,000 to 36,000 BTU/hr (equivalent to 2 to 3 tons of central AC). These are starting-point figures based on the roughly 20 BTU per square foot rule; final sizing should use a Manual J load calculation that accounts for insulation, windows, and climate.Room size → AC capacity (starting-point BTU/hr)100–300 sq ft5,000–8,000 BTU/hr
Bedroom, small office, den
300–550 sq ft9,000–12,000 BTU/hr
Living room, primary bedroom
550–1,000 sq ft14,000–18,000 BTU/hr
Open floor plan, studio, small home
1,000–2,000 sq ft24,000–36,000 BTU/hr
2–3 tons central AC, multi-room whole floor
~20 BTU per sq ft baseline · Manual J refines by climate, insulation, windows, and heat sources
Rule-of-thumb sizing bands. Use the BTU Calculator above for a Manual J-based figure that reflects your climate zone and home.
Run the BTU Calculator with your actual numbers →

Complete HVAC System Guides & Resources

Whether you're replacing equipment, troubleshooting problems, or comparing options, our comprehensive guides provide the technical details and real-world insights you need.

Air Conditioning Systems

View All AC Guides →

Central AC

AC Not Cooling: 12 FixesComplete Troubleshooting GuideInstallation Costs 2026Maintenance Checklist

Mini Splits

Complete Mini Split GuideMini Split Sizing CalculatorInstallation PricingMini Split vs Central

Window Units

Window AC BTU SizingQuietest Window ACsLow Profile OptionsCompact 5000 BTU Units

Portable AC

Portable AC SizingQuiet Portable ModelsEnergy Usage GuidePortable vs Window AC

Heating Systems

View All Heating Guides →

Furnaces

Complete Furnace GuideBlowing Cold Air FixReplacement CostsAnnual Maintenance Guide

Heat Pumps

Heat Pump ExplainedHeat Pumps in Cold WeatherHeat Pump vs Furnace2026 Tax Credits

Alternative Heat

Best Space HeatersBaseboard Heating CostRadiant Floor Heating GuideElectric vs Gas Heating

Comparisons

Gas vs Electric CostBoiler vs Furnace GuideMini Split Systems GuideHeating Cost Calculator

How a heat pump moves heat

A heat pump doesn't generate heat — it moves it. The same closed refrigerant loop pulls heat from outdoor air (even cold air still has usable heat) and releases it indoors. Run the loop backwards in summer, and it moves indoor heat out. That's why a heat pump can deliver 2–4 kWh of heat for every 1 kWh of electricity it consumes.

A closed loop showing the four stages of vapor-compression refrigeration in the physical order the refrigerant flows: (1) the evaporator coil (indoor, bottom-left) absorbs heat from indoor air, evaporating the low-pressure refrigerant into a cool gas. (2) The compressor (outdoor, bottom-right) then pressurizes that gas into a high-pressure, high-temperature gas. (3) The condenser coil (outdoor, top-right) releases the absorbed heat to outdoor air, condensing the refrigerant into a high-pressure warm liquid. (4) The expansion valve (indoor, top-left) drops the pressure sharply, cooling the refrigerant back into a low-pressure cold liquid/vapor mixture, which returns to the evaporator to repeat the cycle. Common residential refrigerants following this cycle: R-410A (phase-down 2023+, GWP 2088), R-32 (single-component, GWP 675), R-454B (R-410A successor for 2025+ equipment, GWP 466).Vapor-Compression Refrigeration CycleINDOOROUTDOOR
4. Expansion Valve
Sudden pressure drop cools the refrigerant
output: low-pressure cold liquid/vapor mix
3. Condenser Coil
Releases absorbed heat to outdoor air
output: high-pressure warm liquid
1. Evaporator Coil
Absorbs heat from indoor air
output: low-pressure cool gas
2. Compressor
Pressurizes vapor, raising pressure and temperature
output: high-pressure hot gas
low-pressure cool gashigh-pressurehot gashigh-pressurewarm liquidlow-pressurecold mix
Common residential refrigerants following this cycle:
R-410A (phase-down 2023+, GWP 2088) · R-32 (single-component, GWP 675) · R-454B (R-410A successor, GWP 466)
The same cycle drives central AC, mini splits, and heat pumps — reversing the flow direction switches between cooling and heating.
Read the full heat pump guide →

Energy Efficiency & Ratings

View All Efficiency Guides →

Cooling Ratings

SEER2 Rating GuideEER2 ExplainedCEER for Window ACsCompare SEER2 Ratings

Heating Ratings

AFUE Rating GuideHSPF2 ExplainedCOP Coefficient

Tax Credits

Federal Tax CreditsHeat Pump CreditsState Rebates Map

Saving Tips

25 Energy Saving TipsWinter Thermostat SetpointsInsulation R-Value GuideDuct Sealing DIY

What efficiency actually saves

Higher SEER2 lowers annual cooling cost — but with diminishing returns. Going from the 13.4 SEER2 minimum to 16 saves more per point than going from 18 to 22. The chart uses a 3-ton AC with about 1,500 cooling hours a year at $0.17/kWh; run the SEER2 calculator to plug in your rates.

Line plot showing that annual cooling cost falls sharply as SEER2 increases from 13.4 to 16, then flattens as SEER2 rises further to 22. At SEER2 13.4 (2026 US minimum), the modeled annual cost is about $685; at 15 it's $612; at 16 it's $574; at 18 it's $510; at 20 it's $459; at 22 it's $417. The largest per-point savings come at the low end — the 13.4 to 16 jump saves about $111/year, while the 18 to 22 jump saves about $93/year despite covering twice as many SEER2 points. Model assumptions: 3-ton (36,000 BTU/hr) central AC, 1,500 equivalent full-load cooling hours per year (mid-Atlantic climate), electricity at $0.17 per kWh.$1,000$800$600$400$200$013.41516182022$685$612$574$510$459$417SEER2 RatingAnnual Cooling CostSEER2 rating vs annual cooling cost
Diminishing returns are real: the 13.4→16 jump saves ~$111/yr, but 18→22 only saves ~$93/yr despite covering twice as many SEER2 points.
Model your own rates and hours in the SEER2 savings calculator →

Indoor Air Quality

View All Air Quality Guides →

Air Purification

Air Purifier SizingHEPA Filter GuideUV Light HVAC SystemsHow Air Purifiers Work

Humidity Control

Dehumidifier GuideDehumidifier SizingIdeal Indoor Humidity

Air Filters

MERV Rating ChartFilter Direction GuideChange Frequency

Testing & VOCs

Indoor Air Quality Testing (IAQ, VOCs, Radon)Mold Prevention

Quick HVAC Answers & Common Questions

Sizing Questions

AC for 1500 sq ft?2.5-3 tonsAC for 2000 sq ft?3-3.5 tonsAC for 2500 sq ft?4-5 tonsHeat pump sizing?600 sq ft/ton

Cost Questions

AC install cost?$3,500-$7,500Furnace replacement?$2,500-$6,500Mini split install?$2,000-$5,000Heat pump cost?$4,000-$8,000

Efficiency Questions

Good SEER2 rating?16-18 SEER2Good AFUE rating?90-95% AFUEGood HSPF2?8.5+ HSPF2Best MERV rating?MERV 11-13

Troubleshooting

AC not cooling?12 fixes →Furnace cold air?8 causes →Heat pump issues?Diagnose →AC freezing up?5 causes →

Energy Usage

Mini split watts?500-1500WFurnace watts?300-800WHeat pump kWh?2-5 kW/hr

Maintenance

HVAC checklist21 tasks →Filter frequency?1-3 monthsAC tune-up?Annual →Furnace service?Maintenance guide →

Smart HVAC Technology & Controls

Optimize comfort and efficiency with smart thermostats and zoning systems for precise climate control.

Smart Thermostats

What smart thermostats actually save, and when a $30 programmable does the same job — with the payback math shown.

Programmable vs Smart Thermostat →Do Smart Thermostats Actually Save?Winter Thermostat Setpoints

Zoning & Multi-Zone Systems

Whether zoning is worth the added cost for your home, how many zones make sense, and how to size a multi-zone mini split.

Multi-Zone Planning Guide →Multi-Zone Sizing CalculatorMini Split Systems ExplainedInstallation Costs

HVAC Resources & Tools

Troubleshooting

Master Troubleshooting

How We Source Our Content

We provide HVAC information based on manufacturer data, industry standards, and AHRI-certified efficiency ratings — no affiliate links or paid product placements.

Data-Driven Content

Guides reference AHRI certifications, DOE data, and ACCA Manual J methods where applicable

Industry Standards

Following ASHRAE, ACCA, EPA, and ENERGY STAR guidelines in all recommendations

Reviewed and Updated Regularly

Content is refreshed as standards, tax law, and equipment specifications change

Our Editorial Standards

Research Process

  • Verify all specifications against AHRI Directory
  • Cross-reference EPA ENERGY STAR databases
  • Use ACCA Manual J for all sizing calculations
  • Include regional climate zone variations

Independence Commitment

  • No affiliate links or referral fees
  • No manufacturer sponsorships
  • Equal coverage of all major brands
  • Transparent about limitations and assumptions

Start With Our Most Popular Calculator

Size your AC correctly and avoid the #1 HVAC mistake: oversizing. Our calculator uses ACCA Manual J methodology for accurate results.

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