How big should the air conditioner be? Sizing HVAC for a Houston house

The right size is the one a room-by-room Manual J load calculation says, run on your house as drawn, not on its square footage. Manual J is the Air Conditioning Contractors of America method, and it calculates loads at an indoor condition of 75°F and 50% relative humidity. The equipment is then picked under its companion standard, Manual S, so that its cooling capacity lands between 95% and 115% of the calculated load.

In Houston almost every sizing mistake runs the same direction. Too big. And an air conditioner that is too big for a house in climate zone 2A cools the air quickly and leaves the water in it.

That is the whole answer. The rest of this piece is how to tell whether you actually got one.

Why is a bigger air conditioner worse in Houston?

Because an air conditioner does 2 jobs, and the bigger unit is only good at 1 of them.

The first job is dropping the temperature. That is the sensible load, the part you feel on the thermostat. The second is pulling moisture out of the air, the latent load. A unit removes moisture only when the indoor coil is cold enough to condense it and stays cold long enough for the water to drip off into the pan and leave the house. That takes minutes of continuous running.

An oversized unit hits the thermostat setpoint before it gets there. It runs 5 or 6 minutes, shuts off, and the water still sitting on the coil evaporates back into the airstream when the blower keeps going. The thermostat says 74. The room feels like a gym locker.

In Denver nobody would notice. Houston is hot and humid for most of the year, and for a good share of it the latent load is the bigger problem. A house that never runs long enough to dry itself ends up above the 60% indoor humidity ceiling the EPA recommends, and that is where wood floors cup, closets smell and mold gets a start behind furniture on exterior walls. The piece on vapor barriers covers what that damp indoor air does once it reaches a cold wall.

Oversizing also costs money twice. You pay more for the larger unit, then you pay for the short cycles, which are the least efficient minutes a compressor runs.

How do contractors usually size the unit instead?

By rule of thumb. A ton for so many square feet, or whatever size was on the slab when they pulled the old one out.

Neither one knows anything about your house. A ton-per-square-foot rule cannot tell a 1985 slab house with single-pane aluminum windows and ducts in a hot vented attic from a new house with a conditioned attic and good glass. Replacing like for like just carries forward whatever error the last installer made, and in a lot of Houston houses that error has been carried forward 2 or 3 times.

The incentive points the same way. A contractor who sizes a unit 1 ton too big rarely hears about it. The house gets cold, the owner is happy in May, and the damp complaints in September get blamed on the weather. A contractor who sizes it right and gets a hot-afternoon callback in August hears about it immediately. So the safety margin creeps up, and nobody is being dishonest. The allowances piece shows how this turns up in a bid as an "HVAC allowance" with a tonnage and a brand and no calculation behind it.

What goes into a Manual J load calculation?

The house itself, wall by wall and window by window. A real Manual J needs:

• The plans and the orientation. A west-facing wall of glass in Houston is a very different load from the same glass facing north.
• Every window's size, plus its solar heat gain coefficient and U-factor from the spec, and any overhang that shades it.
• Wall, roof and floor assemblies with their insulation levels.
• Where the ducts run. Ducts in a vented attic gain heat and leak to the outside; ducts inside the conditioned space do neither.
• How tight the house is. A blower door number if one exists, an honest estimate if it does not.
• How many people live there, and appliance loads.
• The outdoor design conditions for Houston, which come from published weather data, not from the worst day anyone remembers.

The output that matters is room by room, not one number for the house. A room-by-room calculation tells the duct designer how much air each room needs. A single "block load" can size the equipment, but it cannot size the ducts, and a house with the right total and the wrong airflow per room still has a hot upstairs bedroom.

On a new house the inputs come from the drawings, which is one reason a complete drawing set matters past the permit counter. On an existing house someone has to measure. Either way, a Manual J run in 10 minutes from the square footage alone is a rule of thumb with a printout.

How can I tell if a load calculation was padded?

Ask for the report itself, not a summary line, and check 5 things.

Indoor design temperature. It should say 75°F for cooling. Setting it at 70 or 72 inflates the load, and it is the most common way a calculation gets quietly fattened.

Outdoor design temperature. It should match the published design value for Houston. If it reads like a record high, someone has picked the hottest day they could find.

Windows. The glass areas and ratings should match your drawings or your actual windows. Default "clear single pane" on a new house with low-e glass is a padded load.

Infiltration. A tight new house entered as "loose" adds load that is not there.

Safety factors. Manual J is already designed to the design-day condition. A line adding 10% or 20% on top is margin on margin.

Then check the equipment against the load. Under Manual S the cooling capacity should land between 95% and 115% of the calculated total, at your design conditions, from the manufacturer's expanded performance data. A 3-ton calculated load with a 4-ton unit is 133%. That is not a rounding choice.

The ratio you rarely see in a bid is how much of the unit's capacity goes to moisture. In Houston it is worth asking the contractor to show you that the selected unit's latent capacity covers the calculated latent load. Most will not have looked.

What size unit does a typical Houston house need?

There is no honest answer to this in tons per square foot, and anyone who gives one has just shown you how they size.

What can be said: the answer has been falling. A new house with a conditioned attic, ducts inside the envelope, low-e glass and a tight air barrier needs much less cooling per square foot than the house it replaced, sometimes dramatically less. Owners who rebuild on an inner-loop lot and carry over the tonnage from the old house are buying for a building that no longer exists.

The same applies when an existing house changes. Foaming the roof deck, replacing windows or tightening the envelope all drop the load. The spray foam piece covers the case where the attic is sealed and the old unit stays, which is how a lot of Houston houses become cool and damp in the same week. Going the other way, adding a second story adds a load the original unit was never sized for.

Is a single unit enough for a 2-story or 3-story house?

Usually not, or not without zoning.

Heat rises, and in a Houston summer the top floor carries a roof load the ground floor never sees. One thermostat on the ground floor of a 2-story house is satisfied while the bedrooms upstairs are several degrees warmer. Turning it down to fix the bedrooms overcools the living room.

The fixes are 2 separate systems, one per floor, or 1 properly designed zoned system with dampers and a bypass strategy that does not wreck the coil. The 3-story narrow-lot townhouses in Rice Military, Montrose and EaDo push this to the extreme, with a third floor that sits right under a roof and often a single system trying to cover all 3 levels. The townhouse buying guide has a hot-afternoon test you can run before you make an offer.

What about variable-speed units and dehumidifiers?

Both help, and neither excuses a bad calculation.

A variable-speed or inverter unit can slow down instead of shutting off. At part load, which is most hours of a Houston year, it runs long and slow, keeps the coil cold and keeps pulling water. That makes it more forgiving of a calculation that came out a little high. It does not make a 2-ton oversize acceptable. At its lowest speed it still has a floor, and a unit sized far above the load spends its life at that floor, cycling anyway.

A standalone whole-house dehumidifier, ducted and running on its own humidistat, handles moisture separately from temperature. In a tight new Houston house it is often the right call, oversized unit or not, because a well-insulated house has a small sensible load and still has people, showers and cooking adding moisture every day. It is also the usual repair for a house that already has an oversized unit that is too new to replace.

Tight houses also need fresh air on purpose, through a ventilation system rather than leaks. In this climate that outside air arrives wet, so it belongs in the moisture conversation as well as on the code checklist.

Is a right-sized unit ever the wrong answer?

Sometimes it does not feel right, and this is the honest concession.

A unit sized exactly to the design-day load is sized for the hottest hours of August. In April, October and the muggy nights in between, the temperature load is small and the moisture load is not. A right-sized single-stage unit barely runs on those days, and the house can feel clammy at 72°F. Correct sizing fixes the summer. It does not on its own fix the shoulder seasons. That is the case for variable-speed equipment or a dehumidifier even when the calculation is clean.

There is also a real argument from the other side. Contractors who size slightly high point to the August callbacks, a family that keeps the thermostat at 70, or a west-facing great room with more glass than the calculation credited. Some of that is fair. The answer is to put the real thermostat setting and the real glass into the calculation, not to add a ton at the end. And a calculation is only as good as its inputs: a careful Manual J on a guessed blower door number can still be off.

Who should run the calculation?

On a new house or a major renovation, whoever designs the mechanical system, checked by someone who does not sell the equipment.

On a lot of Houston residential jobs the HVAC is design-build: the subcontractor sizes and installs it, and the architect's drawings show a unit location and not much more. That works when the subcontractor runs real calculations. It goes wrong when the sizing and the sale come from the same person and nobody reviews it. Many permit offices expect a load calculation with the mechanical submittal, and what they check varies. Confirm with your jurisdiction what it requires rather than assuming someone looked.

The cheapest protection is in the contract: require a room-by-room Manual J, a Manual S equipment selection and a Manual D duct design before installation, and have your architect or an independent energy rater review them. It is one of the questions worth asking an architect before signing, because the architect who treats the mechanical design as someone else's problem is the one whose clients end up with damp closets.

Modern Acre Studio has written about how Houston humidity shapes the envelope and the air side together, which is the design-stage version of the same argument. For budgeting the mechanical system inside a whole project, the cost calculator keeps design and construction separate, and the firms on this register are a starting list for finding an architect who will ask for the calculation instead of the tonnage.