Ask three contractors for a system size and you will often get three answers, because tonnage is where quotes get quietly fudged. A bigger unit is not a better offer, and a number arrived at by multiplying square footage by a rule of thumb is not the same as a number arrived at by measuring your house.
What size hvac system do you need? The short answer
Start with 600 square feet of conditioned space per ton of cooling, then check the exceptions below. For a normal single storey house in the Central Valley, that number gets you into the right neighbourhood. The number you sign for should come from a Manual J load calculation of your specific home.
If the two numbers disagree by a full ton or more, ask why. Sometimes the answer is a feature of the house, such as a sunroom full of glass. Sometimes the answer is that nobody did the calculation.
What a ton actually means
A ton of cooling is 12,000 British thermal units of cooling per hour. It is not a weight, despite the name. A 3 ton system can move 36,000 BTU per hour, and a 2 ton system can move 24,000. Residential systems here run from about 1.5 tons in a small home up to 5 tons in a large one.
What a Manual J load calculation actually is
A Manual J load calculation is a step by step measurement of how much heat a building gains in summer and loses in winter, and how much moisture it pulls in. It is the standard method for deciding how big a system should be, and it is called Manual J because it is a specific published procedure rather than a rough formula.
The calculation takes in the conditioned floor area, the orientation of each wall, the area and type of windows, the insulation in the walls and attic, the ceiling height, shading, internal heat from appliances and people, and the local climate design temperatures. What comes out is a number in tons, and that is the number that should be on the quote. We do a full Manual J load calculation before sizing a replacement system, and it comes free with the estimate.
The 600 square feet per ton rule of thumb
This is the shortcut almost everybody starts with. It works because, in an average house, the heat the system has to remove works out at roughly 12,000 BTU per hour for every 600 square feet of space.
| Conditioned square footage | Rough cooling tonnage |
|---|---|
| 600 to 1,000 | 1 to 1.5 tons |
| 1,000 to 1,200 | 1.5 to 2 tons |
| 1,200 to 1,500 | 2 to 2.5 tons |
| 1,500 to 1,800 | 2.5 to 3 tons |
| 1,800 to 2,400 | 3 to 4 tons |
| 2,400 to 3,000 | 4 to 5 tons |
Read that table as a starting point for a conversation, not as a specification. A contractor who quotes from a table like this without measuring the house is guessing.
When the 600 square feet per ton rule does not apply
There are six common situations where the rule leads you badly wrong. If any of them describe your house, the square footage number is close to meaningless.
- Two storey homes with a second floor over the garage. The ceiling in the garage area is much hotter than the rest of the house, so the load is not spread evenly across the floor plan.
- Tall ceilings. If your main rooms are nine feet or higher, the square footage figure understates how much heat the ceiling space collects.
- A lot of glass. A sunroom, a wall of west facing windows or a room full of glass doors adds load that has nothing to do with the size of the house.
- Poor insulation or an unfinished attic. A house with no attic insulation, or with old single pane windows, gains far more heat than the same floor plan in a well insulated house.
- Big internal heat sources. A room with several computers, a large aquarium or a workshop changes the numbers.
- Ducts that are the problem. If the existing ductwork is undersized, leaking or crushed, the system cannot be any bigger than the air path allows.
Add to that a heat pump, where the heating load matters as well as the cooling load. A house that needs a lot of heat in the morning and a lot of cooling by the afternoon can be a poor match for a single capacity system.
What happens when the size is wrong
A system that is too large is the mistake that causes the most trouble, and it is common because oversized equipment feels powerful and modern on the day it is installed.
It reaches temperature quickly, then shuts down, then starts again a few minutes later. That short cycling wastes electricity, wears the compressor and blower faster, and leaves the house less able to pull moisture out of the air, so the air can feel clammy even with the thermostat turned right down. It also blows cold air in a blast, which some people find uncomfortable.
A system that is too small is less dramatic and just as expensive. It runs constantly and never quite catches up, so the compressor works for long hours, the bill climbs, the house is uncomfortable on the hottest afternoons, and the equipment reaches the end of its life early. An undersized system also does a poor job of removing humidity, so a small unit in a humid summer is the wrong tool entirely.
How to work out your own number before you call anyone
- Get the conditioned area, not the total area. Exclude the garage, the carport and any room that is not air conditioned. A 2,000 square foot house with a 400 square foot garage is a 1,600 square foot cooling problem.
- Divide by 600. That gives you a rough tonnage. Treat it as a question to ask, not an answer.
- Check your ceilings. Any main room over eight feet, and a second storey over a garage, move the answer up.
- Look at the glass. Note the largest window or glass door wall in each room, and which way it faces. South and west facing glass is the expensive kind.
- Look at the ducts before the equipment. Open a register in a few rooms and compare how much air each one pushes. If they are wildly different, the ductwork is the first project.
- Ask for the load calculation in writing. Ask for the tonnage it produced and what it was based on. A contractor who will not show the working is telling you something.
When to stop and call a licensed technician
Working out a rough number is safe. Buying a system on a rough number is not. Call a licensed HVAC technician when any of these apply.
- You are replacing a system and the quote does not include a load calculation. In California the sizing has to be right, and the paperwork should say it was done.
- The proposed tonnage is more than a ton away from your own estimate and nobody has explained the gap.
- Your house is over 2,400 square feet, has two storeys, or has a sunroom or a wall of windows.
- Your ducts are leaking, crushed, disconnected or visibly undersized. That is a ductwork conversation before it is an equipment conversation.
- You are adding a room, a second storey or a major window wall. The load changes, and so does the system.
RB HVAC Inc. is a licensed HVAC contractor based in Stockton, licensed in California under number 1094161. We have served the Central Valley since 2010 across Stockton, Sacramento, Modesto and surrounding areas. We provide transparent diagnostics and upfront written pricing before any work starts, with free phone guidance for installation planning.
