HVAC size for 800 ft² in Zone 6 with a good envelope and 8 ft ceilings
| Cooling load | 11,520BTU/h |
|---|---|
| Nearest size soldair conditioners come in half-ton steps | 1 ton12,000 BTU/h |
| Heating loadfurnace output, not input — divide by AFUE for input rating | 35,200BTU/h |
| ClimateMinneapolis, Burlington, Helena | Zone 6 — 18 / 55 BTU per ft² |
| Envelopewell insulated and air sealed, modern windows | Good (× 0.8) |
| Ceiling adjustment8 ft against the 8 ft baseline | × 1 |
| If the envelope were very goodalready there | 1 ton |
- Conditioned area
- 800 ft²
- Climate zone
- Zone 6
- Envelope
- good
- Ceiling height
- 8 ft
The common mistake is buying too much
Not too little. An oversized air conditioner cools the air quickly and shuts off before it has run long enough to pull moisture out, leaving a house that is cold and clammy. It short-cycles, which wears the compressor, and it costs more to buy and more to run.
800 ft² in Zone 6 with a good envelope and 8 ft ceilings works out to about 11,520 BTU/h of cooling — 1 ton, since units are sold in half-ton steps — and 35,200 BTU/h of heating output.
If a contractor quotes noticeably more than this, ask which load calculation it came from.
Insulation buys you a smaller machine
This is the part that never appears in insulation payback figures, and it is cash on install day rather than spread over a decade.
The same house with a very good envelope would need 1 ton instead of 1. Equipment is priced per ton. Sealing and insulating before replacing a system is one of the few home improvements that pays part of its own cost immediately.
This is not a Manual J, and it does not pretend to be
A real load calculation works from every wall, window and door individually, their orientation, occupancy, internal gains and measured air leakage. Many jurisdictions require one on a permit for equipment replacement, and a contractor unwilling to produce one is telling you something useful.
Use this as a sanity check on a quote, not as a specification.
Output, not input
A furnace rated 80,000 BTU input at 80% efficiency delivers 64,000. The 35,200 figure above is delivered output — check which one a quote is using before comparing two of them.
Ducts are the other half
A correctly sized unit on undersized or leaky ductwork performs like an undersized one, and adding tonnage does not fix it. If the existing ducts are being reused, they deserve the same scrutiny as the box.
Notes
- **The common mistake is oversizing, not undersizing.** An air conditioner that is too large pulls the temperature down fast and shuts off before it has run long enough to remove moisture. The result is a house that is cold and clammy, a compressor that short-cycles itself to an early death, and higher bills than the right-sized unit. If a contractor quotes noticeably more tonnage than this, ask what load calculation it came from.
- At this envelope quality the load is already close to the floor for this floor area and climate. Further insulation still saves fuel, but it will not drop you another equipment size.
- **This is not a Manual J.** A real load calculation works from every wall, window and door, its orientation, the number of occupants, internal gains and measured air leakage. Many jurisdictions require one on a permit for equipment replacement, and any contractor unwilling to produce one is telling you something. Treat this figure as a sanity check on the quote you are given, not as a specification.
- Heating output is not furnace input. A furnace rated 80,000 BTU input at 80% AFUE delivers 64,000. Size on delivered output — 35,200 BTU/h here — and check which number the quote is using.
- Ductwork is the other half, and it is where most systems actually fail. A correctly sized unit on undersized or leaky ducts performs like an undersized one, and no amount of extra tonnage fixes it.
Nearby sizes
- HVAC size for 700 ft² in Zone 6 with a good envelope and 8 ft ceilings
- HVAC size for 900 ft² in Zone 6 with a good envelope and 8 ft ceilings
- HVAC size for 600 ft² in Zone 6 with a good envelope and 8 ft ceilings
- HVAC size for 1000 ft² in Zone 6 with a good envelope and 8 ft ceilings
- HVAC size for 800 ft² in Zone 5 with a good envelope and 8 ft ceilings
- HVAC size for 800 ft² in Zone 4 with a good envelope and 8 ft ceilings
- HVAC size for 800 ft² in Zone 6 with a average envelope and 8 ft ceilings
- HVAC size for 800 ft² in Zone 6 with a very good envelope and 8 ft ceilings
- HVAC size for 800 ft² in Zone 6 with a poor envelope and 8 ft ceilings
- HVAC size for 800 ft² in Zone 6 with a good envelope and 9 ft ceilings
- HVAC size for 800 ft² in Zone 6 with a good envelope and 10 ft ceilings
Read more
- Insulating a Garage: What to Insulate Depends on What You Want From ItMost garage insulation advice fails because it does not ask the first question: is the garage inside the thermal envelope or outside it?
- Blown-In or Batts? The Answer Depends on the Cavity, Not the ProductThe two products perform almost identically when both are installed perfectly. They are not installed perfectly, and that is the whole comparison.
- Finding Your IECC Climate Zone, and Why the Map Is Not the AnswerClimate zones are assigned by county, not by zip code. Where the two disagree, the county wins — and they disagree more often than you would expect.
- Zone 5 Attic Insulation: The Target, the Depth, and the Top-Up MathsZone 5 covers a large share of the country and sits right on the step where the code moved from R-49 to R-60. Which one applies to you is a local question.