BattWiseR-value targets by climate zone, and what it takes to hit them.

HVAC Sizing

Roughly how much heating and cooling a house needs — and, more usefully, how much smaller the equipment gets when the envelope is tightened.

The common mistake in HVAC sizing is not buying too little. It is buying too much.

An oversized air conditioner pulls the temperature down quickly and shuts off before it has run long enough to remove moisture. You get a house that is cold and clammy, a compressor that short-cycles itself to an early death, and higher bills than the correctly sized unit would have given.

It is also easy to sell. "A bit bigger, to be safe" is a comfortable sentence for everybody in the transaction except the person paying the electricity bill.

What a load actually depends on

Floor area is the crudest input. Real load depends on envelope quality, air leakage, window area and orientation, ceiling height, occupancy and internal gains — which is exactly what a Manual J calculation works through, wall by wall and window by window.

Many jurisdictions require one on a permit for equipment replacement. A contractor unwilling to produce one is telling you something useful.

Insulation buys 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.

A 1,800 ft² house in Zone 4 with a poor envelope needs about 4.5 tons. The same house sealed and insulated to a very good standard needs 2.5. Equipment is priced per ton. Sealing before replacing a system is one of very few home improvements that pays part of its own cost immediately.

Output, not input

A furnace rated 80,000 BTU input at 80% efficiency delivers 64,000. Size on delivered output, and check which number 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.

1 article

HVAC size for 1900 ft² in Zone 4 with a average envelope and 9 ft ceilings

Cooling
4tons
46,431 BTU/h for 1,900 ft² in Zone 4 — average envelope, 9 ft ceilings
Equipment size by envelope quality4 tonaverage envelopeas it is now2.5 tonvery good envelopesealed and insulated1.5 ton smaller — equipment is priced per ton
Cooling load46,431BTU/h
Nearest size soldair conditioners come in half-ton steps4 ton48,000 BTU/h
Heating loadfurnace output, not input — divide by AFUE for input rating80,750BTU/h
ClimateBaltimore, St Louis, SeattleZone 4 — 23 / 40 BTU per ft²
Envelopeinsulated walls and attic, double glazing, some air sealingAverage (× 1)
Ceiling adjustment9 ft against the 8 ft baseline× 1.06
If the envelope were very good1.5 ton smaller — that is money on install day2.5 ton

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.
  • A average envelope is costing you 1.5 ton of equipment. The same house sealed and insulated to a very good standard would need 2.5 ton instead of 4 — and equipment is priced per ton. Insulation does not only cut the fuel bill; it cuts the machine you have to buy, on the day you buy it.
  • 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 — 80,750 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.