HVAC Load Calculator
One house, two loads. Cooling sizes the air conditioner, heating sizes the furnace or heat pump, and they are never the same number.
HVAC block load calculator
Both loads, component by component
| Component | Cooling | Heating | Basis |
|---|
What changes the heating number
| Scenario | Heating load | Furnace output | Change |
|---|
Two loads, two machines
A house loses heat in winter and gains it in summer, and those are separate calculations that happen to share a model. The cooling number sets the air conditioner's tonnage. The heating number sets the furnace's output, or a heat pump's capacity at the winter design temperature. On a typical mixed-climate house the two land in different places: roughly 30,000 BTU per hour of cooling and 45,000 of heating on the defaults here, which is why a 2.5 ton air conditioner is paired with a 60,000 BTU furnace rather than two machines of the same size.
The asymmetry is not an error. Summer design conditions are a 15 to 20 degree difference between indoors and outdoors; winter design conditions in zone 5 are a 50 to 60 degree difference. Conduction and infiltration both scale with that difference, so the winter number is usually the larger one — but only because the temperature gap is larger, not because the house is worse in winter.
Why the heating load takes no credit for sun or people
Manual J allows internal and solar gains to offset part of the heating load, and a careful room-by-room calculation does exactly that. This page does not, for one practical reason: on the coldest night of the year there is no sun, and the people in the house may be asleep under blankets rather than cooking dinner. Sizing heating to the sunny Sunday afternoon case is how houses end up with a furnace that cannot hold 68°F on the one night it matters.
The cooling side is the opposite. There, every internal and solar gain is counted, because the design case is a hot afternoon with the oven on and eight people in the kitchen, and the equipment has to remove all of it. Conservative on one side and inclusive on the other is the standard practice, not a shortcut.
Design temperature is the input that decides everything
The summer and winter design temperatures are the 1 percent and 99 percent values from ASHRAE's climate tables: the temperature the location exceeds for about 88 hours a year, and the one it drops below for about 88 hours. They are not records and they are not averages. Picking a design temperature ten degrees colder than your county's adds roughly 15 percent to the heating load and can push you into the next furnace size for no benefit.
The zone presets on this page are round numbers for the middle of each zone. If you know your county's numbers — from a permit set, a Manual J, or a load calculation your contractor already ran — type them in. That single change makes the result materially more useful than any zone-based estimate.
What the heating number means for a heat pump
A furnace is rated by output at steady state, so a 60,000 BTU furnace delivers close to 60,000 BTU per hour until it cycles off. A heat pump is rated at 47°F outdoor air, and its capacity falls as the outdoor temperature drops — a standard single-stage unit loses roughly 30 to 40 percent by 17°F, while a cold-climate unit holds much more of its rating. If your heating load is 43,000 BTU per hour at 18°F and the proposed heat pump delivers 30,000 at that temperature, the difference is made up by resistance strip heat, which is expensive to run.
That crossover is the balance point, and it is the number to ask a heat pump installer for: not the tonnage, not the HSPF2 rating, but the delivered capacity at your design temperature against the calculated load. A house in zone 6 with a 55,000 BTU load and a heat pump rated 36,000 at 5°F will spend a lot of January on strip heat unless the unit is a cold-climate model or the envelope is improved first.
Input versus output: why furnace numbers confuse people
Furnaces are sold by input, quoted by output, and the two differ by the efficiency rating. A 100,000 BTU input furnace at 80 percent AFUE delivers 80,000 BTU of heat into the ducts; the other 20,000 goes up the flue. A 90-plus condensing furnace delivers more than 90,000 from the same input. This page sizes to output because output is what the house needs, then converts back to a round input size at 80 percent, which is the commonest furnace in the US. If you are buying a condensing furnace, the same output comes from a smaller input — and the difference in first cost is real but so is the difference in annual gas use.
What this model does and does not count
It counts conduction through walls, ceiling, windows and floor, infiltration at a natural air-change rate, people, appliances, solar through glass, and duct gain or loss by location. It assumes a square footprint and an even distribution of glass, which is a simplification: an L-shaped house with a long north wall has more envelope than the model assumes. It does not count duct leakage, which on older systems can waste 20 to 30 percent of the delivered capacity and is often the cheapest problem to fix before buying equipment.
Its honest accuracy is roughly plus or minus 15 percent on a house whose construction you can describe accurately, and worse than that on a house you are guessing about. That is enough to catch a quote that is 50 percent oversized — the common case — and not enough to replace a room-by-room calculation for a permit.
Frequently asked questions
Why is my heating load bigger than my cooling load?
Because the temperature difference is bigger. Summer design might be 91°F outdoors against 75 indoors, a 16 degree gap. Winter design in the same climate is 18°F against 70 indoors, a 52 degree gap. Conduction and infiltration both scale with the gap, so heating usually comes out larger in mixed and cold climates.
Can I use this instead of a Manual J?
For planning, yes. For a permit or a final equipment selection, no. Permitting authorities generally want a room-by-room Manual J with a Manual S equipment match, and this is a single-zone block load.
What furnace size should I buy for a 43,000 BTU load?
The next standard output above the load, which is usually a 60,000 BTU output unit (roughly 75,000 BTU input at 80 percent AFUE). Two-stage or modulating furnaces tolerate being slightly oversized far better than single-stage, because they can run at 60 percent or less instead of cycling.
Should I size a heat pump the same as the furnace?
No. A furnace's rated output is available at any outdoor temperature, and a heat pump's is not. Size the heat pump on its delivered capacity at your winter design temperature, and accept that some hours will be covered by auxiliary heat unless you buy a cold-climate model.
Does duct location really change the load?
Yes, and it is the term most online calculators omit. Ducts in a vented attic lose heat to a 130°F attic in summer and to a 20°F attic in winter. This model adds 18 percent for attic ducts and nothing for ducts inside the envelope, which is the same order of magnitude as the difference between one equipment size and the next.
How accurate is a block load?
Within roughly 15 percent when the construction inputs are accurate, which is enough to catch a badly oversized quote and not enough to submit for a permit. The two inputs people get wrong most often are glass area and air-tightness, both of which are easy to overestimate in your own favour.
What if I am between two sizes?
Round up on the heating side and treat the cooling side more carefully. An undersized furnace cannot hold temperature on a design night, while a slightly oversized air conditioner costs comfort through poor dehumidification. Two-stage equipment narrows the penalty on both sides.
Disclaimer: This is a block load for planning and for checking quotes. It is not a room-by-room Manual J, does not model duct design, and should not be submitted for a permit. Design temperatures are representative of each climate zone rather than of your county.
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