The short answer
A heat pump should be sized from the house, not from the old unit. The method is a load calculation, ACCA Manual J, done room by room for local design conditions.
Bluffline Memphis Heat Pumps runs one before quoting any system, and the result goes on the written estimate next to the size of the old unit.
The three steps
| Step | ACCA manual | What it decides |
|---|---|---|
| Load calculation | Manual J | How much heat the house gains in summer and loses in winter |
| Equipment selection | Manual S | Which heat pump and air handler meet those loads |
| Duct design or check | Manual D | Whether the ducts can carry the chosen system’s air |
What the load calculation measures
A Manual J calculation adds up every way heat gets into the house in summer and out of it in winter.
Inputs from the visit
- Room sizes and ceiling heights.
- Windows: size, direction, glass type and shading.
- Insulation in the attic, walls and floor.
- Air leakage, estimated from the house’s age and condition.
- Duct location: attic, crawlspace or inside the conditioned space.
- Occupants and appliances, which add heat in summer.
What it produces
A cooling load in BTU per hour, split into sensible heat and latent heat (water in the air), and a heating load at the winter design temperature. Each room gets its own number, which also sets how much air each room’s duct should carry.
Memphis design conditions
The calculation uses design temperatures, not record extremes. For Shelby County they are 95 degrees for cooling and 20 degrees for heating, the conditions exceeded only about 1 percent of hours in summer and 1 percent in winter.
Why not size for the record?
Sizing for a once-a-decade heat wave or freeze would leave the system oversized for every other day of the year. Short cycles, poor dehumidification and higher wear would follow.
Cooling sets the size here
In Memphis the cooling load is usually the larger and more important one. Summer brings long, humid stretches near 95 degrees, and removing water from the air is half the job.
Why not size for heating?
A heat pump sized to carry the whole heating load at 20 degrees alone would be too big for summer. It would short-cycle and leave the house damp. Instead, it is sized for cooling, and backup heat covers the gap on the coldest mornings:
- Electric strips in the air handler on an all-electric system, sized to the gap at 20 degrees.
- A gas furnace on a dual fuel system.
The aux and emergency heat guide explains how backup heat is staged.
Why the old nameplate misleads
Many Memphis houses were fitted with generous AC sizes when they were built, often by a rule of thumb. Since then, plenty have gained:
- Replacement windows, which cut solar gain sharply.
- More attic insulation, blown in over the years.
- A new roof, sometimes with better ventilation.
- Sealed ducts, which stop losing cooling to the attic or crawlspace.
Each of those lowers the load. A house that had a 3-ton unit may need 2.5 tons now. Copying the old size carries the old mistake forward. The load calculation is the only way to know how much those changes added up to in your house.
What oversizing does
- Short cycles that cool the air fast and stop before removing much water.
- A clammy house at a normal setpoint.
- Uneven rooms, since the system never runs long enough to even out.
- More wear from frequent starts.
The humidity guide explains the moisture side in detail.
An East Memphis example
A 1978 ranch near Poplar and Mendenhall, about 1,850 square feet, loses its 3-ton heat pump in August. The owners ask for the same size. Here is what the load calculation finds against the house as it was built:
| What changed since 1978 | Effect on the cooling load |
|---|---|
| Double-pane replacement windows, installed in 2015 | Much less solar heat through the glass |
| Attic insulation blown in over the years | Less heat through the ceiling |
| Crawlspace ducts, partly sealed | Less cooling lost under the floor |
| Two occupants instead of a family of five | Less internal heat |
The result
The calculation comes to 2.5 tons. A 3-ton unit would satisfy the thermostat in short bursts and leave the house damp. The 2.5-ton unit runs longer cycles and pulls more water out, and the 8 kW strip kit covers the gap at 20 degrees. Our heat pump replacement page tells the rest of the job.
What the owners saw
Both sizes went on one estimate, with the calculation’s result printed beside them, so the owners could see why the smaller one was recommended rather than take it on trust.
Undersizing is a problem too
A system too small for the house runs constantly on the hottest afternoons and may not hold the setpoint.
In winter it leans harder on backup heat. The goal is a heat pump close to the calculated cooling load, typically within about 15 percent above it, not one deliberately shaved down.
Choosing the equipment
Manual S matches real equipment to the load. Heat pumps come in half-ton steps, and their actual capacity varies with indoor and outdoor conditions.
What gets checked
- Sensible and latent capacity at Memphis conditions, so the unit removes enough water.
- Heating capacity at 20 degrees, from the maker’s data, which sets the backup heat size.
- The matched air handler or coil, rated as a pair.
- Two-stage or variable speed options, which handle part-load days better.
Checking the ducts
A heat pump moves about 400 cubic feet of air per minute for each ton. The ducts have to carry that.
On the first visit we take a static pressure reading. Most residential blowers are built for about 0.5 inches of water column. If the reading is high, the ducts or the return need work before the new system will perform. The crawl space ductwork guide and heat pump ductwork cover the fixes.
A smaller system can relieve tight ducts
A system that is too big for the house is also too big for its ducts. Getting the size right often brings a borderline static reading into range.
Two-story houses and zoning
In a two-story house, upstairs and downstairs loads can differ sharply.
Where each floor has its own system, each gets its own calculation. Where one system serves both, the room-by-room numbers show how much air each floor’s ducts should carry, and whether a second return or zoning is needed.
What to look for on an estimate
A sized estimate shows its working:
- The calculated load in tons or BTU per hour.
- The proposed equipment size next to it, and next to the old unit’s size.
- Model numbers for both halves, so you can check the matched rating.
- Backup heat size in kW, or the furnace being kept.
- The static pressure reading and any duct work it calls for.
If a quote has none of these, ask how the size was chosen. The Memphis buying guide covers comparing quotes, and the cost page explains how size moves the price.