HVACs Calculator guides
The calculators give you a number. These guides explain where the inputs come from, which assumptions matter, and how to tell a good estimate from a bad one.
Ducts & air
Bathroom and kitchen exhaust: the code minimums and the better numbers
The code sets a floor for exhaust fans. The HVI rules set a better target, and the duct decides whether the fan ever gets there.
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.
Duct sizing basics: friction rate, velocity and equivalent length
A duct is sized from three numbers, the airflow it carries, the pressure the blower can spare, and the speed the air can travel quietly. Here is how they fit together.
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.
MERV filters: filtration vs airflow, and the replacement schedule
A better filter catches more and restricts more. The answer is rarely the cheapest or the highest MERV; it is the right rating in a filter deep enough for the system.
Return air: why undersized returns wreck a good system
Every cubic foot of air pushed into the rooms has to come back to the blower. When the return path is too small, the whole system suffers, and it rarely gets blamed.
Whole-house ventilation: the ASHRAE 62.2 formula
Tight houses save energy and trap whatever is in the air. ASHRAE 62.2 says how much outdoor air a home needs, and the arithmetic fits on one line.
Sizing basics
BTU per square foot by climate zone
Twenty BTU per square foot is right in St. Louis and wrong in Miami and Minneapolis. Here are the figures by climate zone, what changes them, and when to stop using them.
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.
SEER2, HSPF2, AFUE and COP: the ratings explained
Every efficiency rating is output divided by input. Once you know what each one counts, comparing equipment and estimating what it costs to run is simple arithmetic.
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.
Energy & costs
Comparing gas, propane, oil, electric and heat pumps per million BTU
Therms, gallons, kWh and tons do not compare directly. Turn each into dollars per million BTU of heat delivered and the cheapest fuel is obvious.
Duct leakage: the 20% you pay for and never feel
Ducts in an attic or crawl space typically leak a fifth or more of the air the system makes. Sealing them is one of the best paybacks in a house.
Heating and cooling degree days explained
Degree days measure how cold or hot a season was in a single number. Multiply by the house's heat loss per degree and you have the season's heating energy.
Thermostat setbacks: how much they save, and when they don't
Turning the heat down for eight hours saves about one percent of the bill per degree. The rule is simple; the exceptions are where people lose the saving.
Heating
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.
Heat pumps in cold climates: balance point and backup heat
A heat pump sized for summer can heat most of a northern winter. The question is what happens below the balance point, and the answer is a few lines of arithmetic.
Radiators and baseboard heaters: output, length and ΔT
A radiator rated at one temperature gives a different output at another. Once you know the ΔT rule, sizing emitters for a room and a boiler for the house is straightforward.
Cooling
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.
Window and portable AC sizing: BTU, SACC and the room
A window unit is sized from a chart; a portable unit is sized from a chart and then corrected for the cooling it loses to its own hose. Here is how both work, with the numbers.
Buying & hiring
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.
What HVAC installations cost, by system
Installed prices for the same system vary by two to one. Here are the published ranges, with their sources and dates, and the factors that put a quote at the top or the bottom.
Water heating
Water heater sizing: first-hour rating, tankless GPM and temperature rise
Tanks are sized for the busiest hour, tankless units for the most taps open at once. Both start from how a household actually uses hot water.
What a water heater costs to run, by fuel and type
Water heating is often the second-largest energy bill in a house. Three numbers, gallons a day, the temperature rise and the UEF, give its cost.