This is not a Manual J. Washington State Energy Code requires equipment to be sized from an ACCA Manual J load calculation, and this estimator is not one and cannot be used for a permit. It runs the simplified whole-house heat-loss method the state publishes for permitting purposes, which is a real published method and a much blunter instrument. It is here so you can tell whether a quote is in sensible territory.
Your house
Rough answers are fine. The output is a range for exactly that reason.
Estimated heat loss at design conditions
— to — BTU per hour
How to read that number
The range is the answer, not an apology for one. A single figure would look more confident and be less true.
What it is good for. Telling whether a quote is in the right territory. If your estimate lands around 40,000 BTU per hour and somebody has quoted equipment producing 90,000, that gap is worth asking about. Washington itself publishes oversizing allowances — up to forty percent over the calculated load for a furnace, twenty-five percent for a heat pump — so being somewhat above is normal and expected. Being double is a different conversation.
What it is not good for. Choosing equipment. This method cannot see which way your windows face, how leaky the house actually is, where the ducts run within the house, or how the load is distributed between rooms. Those are the three or four things that most often move the real answer, and they are exactly what a proper load calculation measures.
Why we are not just showing you BTU per square foot. Because the honest version of that number is a range of roughly ten to twenty-four in this climate, and which end you land on is decided almost entirely by the envelope rather than by the size of the house. A published analysis of forty homes that had all been through real load calculations found more than a fivefold spread in square feet per ton. Any single multiplier is wrong for most houses in both directions.
Why oversizing matters, stated honestly
There is a standard sales argument that an oversized system will wear out early, and we are not going to make it, because the research does not support it. The Department of Energy explicitly lists equipment durability as an open research question, and one federal study that pulled furnaces out of homes after fifteen to twenty-four years found performance had not meaningfully degraded. Another found short cycling was not the problem it is usually described as, at least with modern variable-capacity equipment. We would rather tell you that than repeat something convenient.
What is genuinely well supported is narrower and still worth knowing. Oversized equipment costs more to buy and can need larger ductwork and more electrical capacity. It measurably raises peak electrical demand. On a variable-capacity heat pump it reduces the range the system can modulate across, which the regional efficiency alliance identifies as causing shoulder-season cycling here specifically. And it is a code issue: Washington requires the smallest available unit that exceeds the calculated load. That is a real enough list without inventing anything.
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What this estimator assumes
Published openly, because a calculator that hides its assumptions is just an opinion with a text box.
It uses the outdoor design temperature Washington assigns your city, and an indoor temperature of seventy degrees. It derives wall, roof and floor areas from the floor area, storey count and ceiling height you enter, assuming a roughly rectangular footprint. It applies typical assembly insulation values for the vintage you select. It assumes the air-leakage figure the state method fixes for all houses, which is its single biggest limitation — a 1962 house and a 2018 house genuinely differ here and this cannot tell them apart. It adds ten percent if the ducts run outside the heated space. And it presents the result as a band of plus or minus fifteen percent, which is honest about the uncertainty rather than hiding it.
It calculates heating only. It does not account for window orientation, shading, actual air leakage, internal gains, or room-by-room distribution.
Quick answers
Is this a Manual J?
No, and we would rather say so plainly than let you assume otherwise. Washington State Energy Code requires equipment to be sized from an ACCA Manual J load calculation, and this is not one. A Manual J uses your actual window areas and which way they face, your actual insulation, your actual air leakage and your duct layout. This estimator uses the simplified whole-house heat-loss method that Washington publishes for permitting, which is a real published method but a much blunter instrument. Use it to tell whether a quote is in sensible territory, not to choose equipment.
Why does it give a range instead of a number?
Because a single number would be dishonest. Two houses of identical size in the same street can have genuinely different loads depending on insulation, leakage and glazing, and even the outdoor design temperature is uncertain here — Kent and Renton are about eight miles apart and Washington assigns them design temperatures three degrees apart, which is roughly six percent of the answer on its own. A range is what the method can actually support.
My quote is bigger than this range. Is the contractor wrong?
Not necessarily, and we would not want you using this to accuse anyone. There are legitimate reasons to go above a calculated load, and Washington itself publishes allowances of forty percent for a furnace and twenty-five percent for a heat pump. What the range is useful for is spotting the quote that is not merely above the load but in a different league entirely. If someone has quoted you roughly double, that is worth a conversation.
Does it cover air conditioning?
Only indirectly, and this is an honest limitation rather than laziness. The published method calculates heating only, and cooling here is a genuinely different problem. Our summer design temperature is around eighty-two degrees against a seventy-five degree indoor setting, so the temperature difference is about seven degrees — which means cooling load in this region is driven overwhelmingly by sunlight through glass and heat from people and appliances, not by conduction through walls. A square-footage calculation cannot see which way your windows face, so it cannot get cooling right. That one genuinely needs measuring.
What actually changes the answer most?
The envelope, by a long way. Across the vintages in this estimator the heat loss per square foot varies by well over twice, and that is before accounting for air leakage, which the simplified method cannot see at all. Floor area matters less than people expect, and the load per square foot actually falls as a house gets larger. Which is precisely why a flat rule of thumb per square foot is wrong in both directions.
This estimator is a screening tool, not a design, a quote, or a document for permitting. Washington State Energy Code section R403.7 requires equipment to be sized in accordance with ACCA Manual S from a load calculation performed in accordance with ACCA Manual J, and requires selection of the smallest available equipment size that exceeds the calculated load. Manual J and Manual S are standards of the Air Conditioning Contractors of America; we are not affiliated with ACCA and this tool is not ACCA-approved software. Actual requirements for your home may differ from the values shown.
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