Calculators
HVAC planning and comfort calculators
Zoning, lifespan and replacement timing, checking the size of an existing unit, humidity and dew point comfort, heat index, heat loss through walls and generator sizing for HVAC.
Around the equipment sit the decisions that decide whether it works: whether the house should be one zone or three, whether the fifteen-year-old furnace is worth another repair, whether the 4-ton unit the previous owner installed was ever the right size, and whether the generator you are considering can actually start the compressor. These calculators handle those questions with the same transparency as the sizing tools.
Two of them are about comfort rather than equipment. Dew point, not relative humidity, is what the body feels and what decides whether a cold pipe sweats, and heat index is the figure forecasters use for the danger of hot, humid air. Both come from published formulas, NOAA's for heat index and the Magnus approximation for dew point, and both are useful when deciding how hard to run a dehumidifier or an air conditioner.
7 calculators
HVAC Zoning Planner
Groups up to eight rooms into zones by floor, use and exposure, with the airflow each zone needs and the warnings that matter.
HVAC Lifespan & Replacement Planner
The replacement window for an AC, heat pump, furnace, boiler or water heater from its install date, conditions and repairs.
Is My AC/Furnace the Right Size?
Compares an installed air conditioner or furnace with the house's estimated load and the symptoms of over- and undersizing.
Dew Point & Humidity Comfort Calculator
Dew point and comfort band from temperature and humidity, and whether a window, pipe or wall will condense.
Heat Index Calculator
How hot it feels from temperature and humidity by the NOAA formula, with the National Weather Service caution bands.
Heat Loss Through Walls, Windows and Roofs
BTU/hr lost through up to five walls, windows, ceilings, doors and floors from their area and R-value, and each one's share.
Generator Size for HVAC Calculator
Running and starting watts for an air conditioner or heat pump with the blower and other loads, and the generator to buy.
Guides for this topic
Dehumidifier sizing: pints per day, really
A "50-pint" dehumidifier bought in 2018 and one bought today are not the same size. Here is what the rating measures, how to size for a damp space, and why the old numbers still circulate.
Furnace input, output and AFUE: reading the nameplate
A "100,000 BTU furnace" usually means 100,000 BTU of gas in, not heat out. Knowing which number is which is the difference between a right-sized furnace and a guess.
How to size an air conditioner (and why the rule of thumb is only a start)
The square-foot rule gets you to the right conversation. Here is what it leaves out, what Manual J adds, and how to tell whether the size on your quote is right.
Humidity, dew point and comfort at home
Relative humidity changes with the temperature; dew point does not. Once you read the air by its dew point, comfort, condensation and mould all make more sense.
Mini-splits: sizing each zone and the outdoor unit
A mini-split is sized twice, once for each room and once for the outdoor unit that serves them all. Most mistakes happen in the first step, with a 12k head in every bedroom.
Why an oversized AC or furnace costs you comfort and money
Bigger feels safer and works worse. Here is how an oversized system leaves a house clammy, cycles itself to an early end, and what to check before the next one is the same size.
How to read an HVAC quote (tonnage, SEER2, ductwork, warranty)
Three quotes for the same house can differ by thousands of dollars and still not be comparable. Here is what each line should say and what to ask when it does not.
Repair or replace: the age-times-cost rule and what it misses
Multiply the age by the repair cost. Over 5,000, lean towards replacing. It is a good start, and here is what it leaves out.
Questions people ask
- How do I know if my air conditioner is oversized?
- Compare the nameplate size with the house's load. The model number usually encodes the capacity in thousands of BTU: a "36" is 3 tons. If the calculated load for your house, climate and insulation is 2 tons and the unit is 3.5, it is oversized, and the symptoms confirm it: cooling cycles under 10 minutes, a house that is cool but humid, and large temperature swings. The size check calculator does the comparison and lists the symptoms to look for.
- When should a furnace be replaced instead of repaired?
- Gas furnaces last 15 to 20 years in the service-life data this site uses, and the common rule is to replace when the age in years multiplied by the repair cost in dollars exceeds 5,000: a $700 repair on a 12-year-old furnace is at the threshold. Replace sooner for a cracked heat exchanger, which is a safety fault, and later if the unit is well maintained and the repair is minor. The lifespan planner applies the rule with your dates.
- What indoor humidity is comfortable?
- Most people are comfortable between 30 and 50% relative humidity, which at 72 °F corresponds to a dew point between about 39 and 52 °F. Above a 60 °F dew point the air feels sticky whatever the thermostat says, and above 65 °F it is oppressive. In winter, humidity above 40% can condense on cold windows; the dew point calculator shows the surface temperature at which that starts.
- How much heat is lost through a wall?
- Heat loss is area × temperature difference ÷ R-value. A 200 sq ft wall insulated to R-13 loses 200 × 50 ÷ 13 = 770 BTU per hour when it is 20 °F outside and 70 inside. The same area of single-pane glass at R-1 loses 10,000 BTU per hour, which is why windows dominate the heat loss of most houses and why the wall heat loss calculator reports the share of each element.
- What size generator runs a 3-ton air conditioner?
- A 3-ton AC draws roughly 3,500 watts running but needs 3 to 5 times that for a fraction of a second to start the compressor, so a generator must supply about 12,000 to 15,000 starting watts unless a soft-start kit is fitted, which cuts the surge by 60 to 70%. With a 600 W furnace blower and 2,000 W of household basics, the peak when the compressor starts is about 16.6 kW, so a 20 kW standby generator with a 10% margin; with a soft start and the nameplate figures of a typical 3-ton unit, about 12 kW is enough. The generator calculator adds up running and starting watts from your nameplate.
More calculators on our other sites
Drywalls Calculator
Sheets, screws, joint compound, tape, corner bead, studs, soundproofing and what hanging and finishing drywall costs.
Floorings Calculator
How much tile, wood, vinyl, carpet, underlayment and trim a floor needs, and what it costs.
Paints Calculator
How much paint, primer, stain and wallpaper a job needs, and what it costs.