Calculators
Air equipment calculators: inputs you already have, one formula, one answer, with the formula printed so the number can be checked.
Each calculator takes numbers a homeowner already has, applies one formula, and returns one answer. The formula is printed on the page so the result can be checked by hand.
Space heaters 2 pages
Size
Space heater size calculator: watts per sq ft, BTU
Watts = length x width x 10 W per sq ft at an 8 ft ceiling (Dimplex, Stelpro), 12.5 at 10 ft. A 10 x 14 ft room needs 1,400 W: one 1,500 W heater.
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Space heater running cost calculator: per hour, month
kWh = watts / 1,000 x hours x duty cycle; cost = kWh x rate. A 1,500 W heater on 8 hours a day at EIA's 18.31 cents is 12 kWh, $2.20 a day, $65.92 a month.
Air purifiers 2 pages
Size
Air purifier running cost calculator: filters + kWh
Yearly cost = filter price x 12 / interval months + watts x hours x 365 / 1,000 x rate. A 56 W unit at 24 h is 490.56 kWh, $89.82 at EIA's 18.31 cents (July 2026).
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Air purifier CADR calculator: room size and ACH
AHAM's rule: smoke CADR at least two-thirds of the room's square feet at an 8 ft ceiling (room size = 1.55 x CADR); ACH = CADR x 60 / volume. 180 sq ft: 120 cfm.
Humidifiers 1 pages
Size
Indoor humidity by outdoor temperature calculator
CEE's winter table: indoor RH 35% at 20F outside, 30% at 10F, 25% at 0F, 20% at -10F, 15% at -20F. Calculator interpolates the table and checks dew point.
Thermostats and heat pumps 1 pages
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Thermostat setback savings calculator: DOE 10% rule
DOE: as much as 10% a year by setting back 7-10 F for 8 hours a day; ENERGY STAR smart thermostat average 8%. On $1,600 of heating and cooling: $160 or $128.