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What size, and how would you know?
The tonnage on your quote decides how the house will feel for the next fifteen years, how much it costs to run and how long the equipment lasts. It is also the number nobody explains, and the one almost nobody calculates.
Start here. Sizing is a calculation with a name — a load calculation, usually done to the ACCA Manual J method. It takes the floor area, the ceiling heights, the orientation, the window area and what kind of glass it is, the insulation levels, the air tightness, the duct location and leakage, how many people live there and what the design temperature is for this part of Texas. It produces a heat gain figure in BTU per hour, and the equipment is chosen to meet that.
What usually happens instead is that somebody reads the model number off the old unit and quotes the same again, or applies a square-footage rule of thumb. Neither is a calculation. The first carries forward whatever mistake was made twenty years ago; the second is a guess with a confident voice.
Pick what your quote says and what has changed about the house. Everything below is written out in full, so if you would rather just read it, nothing is hidden.
This is the commonest thing we see and it is worth saying before anything else. If the size is not stated, you cannot compare two quotes, you cannot check the work, and you have no way of knowing whether a calculation was ever done. Ask for three things in writing: the tonnage or BTU rating of the equipment, the calculated design load for the house, and the method used to get it. A contractor who has done the sums will hand them over in a minute. One who has not will tell you it is not necessary, and that is your answer.
The same size as what you have now
This is the default, and it is not automatically wrong — it is just unexamined. The question is whether the original size was right, and whether anything has changed since.
Two things make it suspect. The first is that the house you have now is very rarely the house that was sized. The second is that if the original was oversized — and a great many systems in Abilene are, because oversizing has been the safe commercial choice for decades — then matching it simply buys the same mistake again with a new warranty on it.
If the house has genuinely never been touched and the system has kept up without struggling, matching is a reasonable answer. Ask for the calculation anyway. It costs the contractor an hour and it is the only thing that turns a match into a decision.
Bigger than what you have now
Ask why, and expect a specific answer. There are legitimate reasons: the house has been extended, the old unit genuinely could not hold temperature on the worst days, or the original really was undersized. Those are all findings, and they should be written down.
The illegitimate reason is the common one — bigger feels safer, and if the customer is ever too warm the bigger unit is the one nobody complains about. The trouble is that oversizing has costs that arrive quietly. The system satisfies the thermostat quickly and shuts off, so it never runs long enough to remove moisture, and you get a house that is cold and clammy. It short cycles, which is hard on the compressor and hard on the contactor. It costs more to buy, and often needs larger ductwork that is not in the quote.
Going up a size is not a cautious choice. It is a different set of problems.
Smaller than what you have now
Counter-intuitively, this is often the sign of a contractor who has actually done the calculation. A house that has been re-glazed, re-roofed, insulated and air-sealed since 1978 genuinely needs less cooling than it did, and the calculation will say so.
What you want to see is the arithmetic. A smaller system arrived at from a load calculation is a good outcome: lower purchase price, longer run times, better dehumidification, less wear. A smaller system arrived at because it is the cheapest thing in the van is a different matter entirely, and the two look identical on a quote.
So ask the same question you would ask about a bigger one: what is the calculated design load, and may I see it? If the answer is a sheet with the window areas and orientations on it, that is a contractor doing the job.
It does not state a size at all
A quote without a size is a price attached to nothing. You cannot compare it, you cannot check it, and you have no way of knowing what will actually turn up on the day.
In our experience the number came off the old rating plate, and the reason it is not written down is that nobody wants to be held to it. Occasionally it is genuinely an oversight by somebody who does good work — which is why the right move is to ask rather than to assume.
Ask for it in writing, along with the design load and the method. If the reply is that every contractor does it this way, that is true and it is still not a reason.
Two contractors gave you different sizes
This is the most useful situation you can be in, because you now have a question that will separate them. Ring both and ask the same thing: what design load did you calculate, and how?
A difference of half a ton between two real calculations is normal — different assumptions about air tightness or duct leakage will do that, and either could be defensible. A difference of a ton and a half is not an assumption, it is that one of them did not calculate anything.
The one who can hand you the sheet is the one to talk to, even if their number is the larger one. What you are buying is not the tonnage, it is whether anybody looked.
Nothing has changed since it was built
Then the original sizing is the whole question, and the honest answer is that you cannot know whether it was right without redoing it. What you can do is use the house as evidence.
If on the hottest week of the year the system ran more or less continuously in the afternoon and held the temperature, it was about right. If it ran continuously and lost ground, it was small. If it cooled the house quickly and then sat idle for long stretches while the house still felt damp, it was too big — and that is far and away the commonest finding in houses of this vintage.
Windows, insulation or the roof have been upgraded
Then the load has genuinely gone down, often by a surprising amount, and matching the old size is very likely to oversize you.
Windows are the biggest single lever in a West Texas house, because west-facing glass in July is an enormous heat gain and modern coated glass cuts it substantially. Attic insulation is the next. Air sealing is the quiet one — less measurable to a homeowner, and frequently the largest reduction of the three.
This is the case where a smaller system on the quote is a good sign rather than a worrying one. Make sure whoever calculated it knows what was done and when, because none of it is visible from the kerb.
An addition, or a converted garage or attic room
This is the case where the answer is least likely to be a bigger system, even though that is what everybody assumes.
Added space needs cooling, but it also needs duct, and that is where these conversions usually fail. A converted garage tied into the existing system with one small run and no return will be uncomfortable no matter what size the equipment is, because air cannot get in and it cannot get out. Meanwhile the rest of the house now shares its air with a room that has an uninsulated slab and a roll-up door in one wall.
A load calculation has to treat that room as its own thing. Sometimes the right answer is a bigger system and new duct. Quite often it is the existing system left alone and a separate mini-split for the converted space, which is cheaper, works better and does not compromise the rest of the house. Anybody who reaches for the bigger box without discussing this has skipped the interesting part.
How you use the house has changed
This matters less than people expect, and in one specific way it matters a great deal.
More people in the house adds load, but it is a small number next to the building fabric — a few hundred BTU per person. Working from home does not change the design load much either, although it changes when you need the house to be comfortable, which is a scheduling question rather than a sizing one.
The exception is equipment. A home office with several machines, a workshop, a grow setup, a server, a large aquarium, or a kitchen that is now used for real cooking every day are genuine additions to the load, and they are concentrated in one room. Tell whoever does the calculation. That is exactly the sort of thing that never appears on a sheet unless somebody mentions it, and it is the sort of thing that makes one room impossible while the rest of the house is fine.
What to ask
“What design load did you calculate for the house, and may I see the sheet?”
Matching the old size is only a decision if somebody checked. If the answer is that the old one was fine so this one will be too, you have learned something useful about the quote.
What to ask
“What changed that makes it need more, and does the existing ductwork carry the extra air?”
Both halves matter. A larger system on the existing duct will be noisy, will not deliver its capacity and will run at a static pressure that shortens its life. If the answer to the second half is not in the quote, it is not in the price either.
What to ask
“Show me the calculation, and tell me what you assumed about the ductwork.”
A smaller system from a real calculation is good news. A smaller system from a shrug is how you end up unable to hold temperature in August. The sheet tells you which one you have.
What to ask
“Please put the equipment size, the calculated design load and the method in writing.”
All three, in one email. It is a reasonable request, it takes a competent contractor a few minutes, and the reply — whatever it says — will tell you more than the rest of the quote put together.
What to ask
Ask both of them: “What design load did you calculate, and how?”
Then go with whoever can answer, even if theirs is the more expensive or the larger system. You are not choosing between two tonnages. You are choosing between somebody who measured and somebody who did not.
We will do this for you, and you keep the sheet. If you ask us to quote a replacement, the load calculation is part of it and the result is yours regardless of who you buy from. We would rather be the contractor who gave you the means to check everybody else than the one who hoped you would not ask.
What this page will not tell you. It will not tell you what size your house needs, because that requires somebody to be in it with a tape measure and a note of which way the glass faces. Any page on the internet that gives you a tonnage from a square footage is doing the exact thing this page is about.
If you want the calculation done
It takes a visit and an hour or so of work afterwards. You get the sheet, the design load and a note on what the ductwork can actually carry — which is the other half of the question and the half that gets left out.