Size Replacement Heating and Cooling from the House, Not the Old Unit

When a furnace or air conditioner fails, the quickest quote is a like-for-like replacement: same size as the old one. It is also usually wrong. Houses get new windows, attic insulation and air sealing over the decades, and many original systems were oversized to begin with. This guide sizes the replacement for one real-style house from its envelope, checks the airflow the ducts must carry, and decides whether a premium high-efficiency air conditioner is worth the money.

The house and the old equipment

A single-storey 1,600 ft² house has 1,250 ft² of net wall insulated to R-13, 220 ft² of double-glazed windows and doors at U-0.32, a 1,600 ft² ceiling with R-30 in the attic and 12,800 ft³ of heated volume. A blower door test measured 0.4 natural air changes per hour. The local 99% winter design temperature is 5 °F and the thermostat is set to 70 °F, a difference of 65 °F. The old furnace is rated at 80,000 BTU/h input, typical of houses of its age.

Step 1: the design heat loss

The heat loss calculator works through each part. Walls: 1,250 × 65 ÷ 13 = 6,250 BTU/h. Windows and doors: 220 × 0.32 × 65 = 4,576. Ceiling: 1,600 × 65 ÷ 30 = 3,467. Air leakage: 0.018 × 12,800 × 0.4 × 65 = 5,990. Total: 20,283 BTU/h, or 5.9 kW, on the coldest design day. Adding 10 to 20% for ducts in an unheated crawlspace would bring it to roughly 22,000 to 24,000 BTU/h. Either way, the old furnace, which delivered about 64,000 BTU/h at 80% efficiency, was more than three times larger than the house needs.

Step 2: choose equipment close to the load

An oversized furnace short-cycles: it heats the house in a few minutes, shuts off, and repeats, with more temperature swings, more wear and lower real efficiency. The smallest common condensing furnaces are around 40,000 BTU/h input, about 38,400 BTU/h output at 96% efficiency, still well above the load but far better than 80,000. A two-stage or modulating model would run on low fire most of the winter. For a heat pump, the key figure is capacity at the 5 °F design temperature, not the nominal rating; a cold-climate model whose capacity at 5 °F is about 20,000 to 24,000 BTU/h would cover the load without resistance heat on most days. Before deciding, upgrading the attic from R-30 to R-49 would lower the load to 18,939 BTU/h, which may allow a smaller unit.

Step 3: check the airflow

A furnace delivers its heat through air, and the duct system must carry it. With a temperature rise of 50 °F through the furnace, the airflow calculator gives 38,400 ÷ (1.08 × 50) = 711 CFM. The same ducts must also carry cooling airflow: a 2.5-ton air conditioner at 400 CFM per ton needs 1,000 CFM, more than for heating. If the existing ducts were sized for a 2-ton system, a larger air conditioner would be noisy and starved of air. Size the equipment, then confirm the ducts can carry the higher of the two airflows.

Step 4: is the high-SEER2 upgrade worth it?

The installer offers a 2.5-ton, 30,000 BTU/h air conditioner at the regional minimum of 14.3 SEER2, or an 18 SEER2 variable-speed unit for $1,800 more. The house sees about 1,400 full-load cooling hours a year, and electricity costs 16 cents per kWh. The SEER2 savings calculator gives 2,937 kWh a year for the minimum unit and 2,333 kWh for the premium one: a saving of 604 kWh, or $96.60 a year. Simple payback is 18.6 years, longer than many systems last. Unless rebates cut the price difference or comfort and humidity control matter more than cost, the premium unit does not pay for itself here.

Common mistakes

• Copying the old unit's size. Original equipment is often oversized and the house has usually changed since.
• Using the record low temperature. Design temperatures are statistical; using the extreme oversizes everything.
• Ignoring air leakage. Here it was 30% of the load, more than the windows.
• Sizing heat pumps on nominal capacity. Capacity falls in cold weather; check the figure at the design temperature.
• Forgetting the ducts. Equipment that the ducts cannot supply with air will never deliver its rating.

Replacement checklist

1. Collect wall, window and ceiling areas, R-values and U-factors, and a blower door result if available.
2. Look up the 99% winter design temperature for your location.
3. Calculate the design heat loss and add an allowance for ducts outside the heated space.
4. Choose equipment close to the load, preferably two-stage or variable speed.
5. Check heating and cooling airflow against what the ducts can carry.
6. Run the SEER2 payback with your own cooling hours and electricity price before paying for a premium unit.