What a Line Set One Size Too Small Actually Costs You

Carrier answers this as a percentage, not a yes or no: its product data prints the cooling capacity you lose on each acceptable vapor line.

Somebody has measured your copper, compared it with the new unit’s spec, and told you it is a size small. The next question is always the same one, and it is usually asked as a yes or no: is that a problem?

Carrier does not answer it as a yes or no. It prints a percentage.

The manufacturer publishes the price

Every Carrier residential product data sheet carries a table called Vapor Line Sizing and Cooling Capacity Loss. For each nominal capacity it lists the vapor line diameters the manufacturer will accept, and beside each one the share of cooling capacity the system gives up — column by column, as the run gets longer.

Take a three ton 26SCA5, which is rated on a seven-eighths inch vapor line. At eighty to a hundred feet of total equivalent length, its own sheet says:

That is the whole answer to is a size small a problem on that unit, in Carrier’s own numbers: about four points of cooling between the copper you have and the copper the unit was rated with.

Note that the rated line is not free either. The published rating is taken at twenty-five feet, so a longer run costs something on any diameter — which is why the table’s first column starts at twenty-six.

A diameter that is not on the list is not a slower option

The chart’s purpose is not only to price the loss. It is also the list of what is acceptable at all, and the reason a diameter drops off the bottom is not that it is inefficient — it is that the refrigerant stops carrying oil back to the compressor reliably. That failure is not a percentage, so it is not printed as one.

A five ton 26SCA5 lists three-quarters, seven-eighths and one and one-eighth inch. Five-eighths is absent. For that unit five-eighths is not a slow choice; it is off the list, and no number on the page describes what it does.

So the first question about your line set is not how much does it cost me. It is is my diameter on the list for my capacity, and only then the percentage.

Twenty sheets, eight tables

This page used to say that the table belongs to the family. That was written from four sheets and it is wrong, so here is what all twenty Carrier sheets in this archive actually say.

They print eight different tables between them. Every Carrier single-stage air conditioner running R-410A — the 24SCA4, 24SCA5, 24SPA6, 24ABB3 and 24AHA4, five families across three tiers and a decade — prints one identical table, cell for cell. Every single-stage air conditioner running R-454B prints a second one. The variable-speed families print five more between them, and the single-stage heat pumps print another.

So a table can cover six families, and the badge on the box does not tell you which one you are looking at. What sorts them is the kind of machine and the refrigerant in it, and that is not something a homeowner can read off a quote.

On air conditioners, the newer refrigerant does cost more

The two single-stage air conditioner tables describe the same seven capacities and the same three diameters at each, so they line up exactly — 189 cells against 189 cells.

Not one cell goes the other way. The gap is usually a point or two and at its widest six: a 1.5 ton unit on a half-inch vapor line at 201–225 equivalent feet gives up 9% on R-410A and 15% on R-454B.

At the size most people are buying, three tons at eighty to a hundred feet:

There is one row where the two refrigerants agree, and it is worth a second look: the five ton on a one-and-one-eighth inch line, the largest pipe anywhere in either table. From twenty-six feet to a hundred and seventy-five the two sheets print the same number. Six of the nine cells where R-410A and R-454B break even are on that one row. Where the pipe is generous, the refrigerant stops mattering; everywhere else the newer system pays.

This is a comparison of two published tables, not a rule Carrier states. The company nowhere writes that R-454B loses more through a given pipe, and it prints no reason. And the pattern is not a property of the refrigerant either: its R-454B single-stage heat pump, the 27SCA5, publishes the older air conditioner’s numbers rather than the newer ones — same brand, same year, same refrigerant as the 26SCA5, and a different table.

That is the second half of this story, and it has its own page.

The good news on one common pair

If your old unit is a 24SCA5 and the quote is a 26SCA5, the copper is very likely already right. The two families are rated on the same vapor diameter at every capacity but one — and at that one, one and a half tons, the new unit is rated on the smaller line: five-eighths, where the old unit wanted three-quarters.

A line set that was correct for a 24SCA5 is therefore the rated size or larger for the 26SCA5 that replaces it, at every capacity the two families share. That is not permission to reuse it — the coil, the cleanliness of the copper and your local code all still have to be answered, and the installer signs for all three. It does mean the diameter is not what will stop you.

What it does not mean is that the job comes free. The same copper on the newer unit sits on the higher of the two tables, so a long run that cost 4% before costs 6% now.

Length is the other half of the question

Diameter never travels alone in these tables. The same five-eighths line on the same three ton 26SCA5 costs 2% at twenty-six to fifty feet and 14% at the far end of the chart.

Two things make that harder to eyeball than it looks:

• The number is total equivalent length, not the distance you can measure. Every elbow and fitting adds to it, which is a calculation the installer does and a homeowner cannot do from the driveway.
• On the 26SCA5, anything past eighty equivalent feet — or more than thirty-five feet of vertical separation between the two units — is a long-line application, and long line requires accessories rather than just accepting a bigger number.

What this does not tell you

• It is Carrier’s answer for Carrier’s families, and it does not travel. American Standard’s R454B Application Guide has since been read, and it does not price an undersized line as a percentage at all — it publishes rated and alternate diameters, and treats anything off that list as a question about oil return rather than about capacity. Two brands, two different shapes of answer to one question.
• These are cooling numbers. The tables are printed as cooling capacity loss, including on the heat pump sheets. What an undersized vapor line does to published heating capacity is not on these pages.
• It says nothing about whether your existing copper is reusable. Diameter is one question. Whether the line can stay at all is a separate one, answered on the brand’s rules page and by the person who opens the system.

What to ask your installer

• What total equivalent length are you calculating for my run, and does that put the job into long line?
• What vapor diameter is my capacity rated on, and is the copper I have on the acceptable list for it?
• If it is on the list but under the rated size, what capacity loss are you expecting — and is the unit still sized for the house after it?

Questions people ask

Is a line set one size too small a deal breaker?

Not by itself, and Carrier does not treat it as one. Its product data lists several acceptable vapor diameters for each capacity and prints the cooling capacity you give up on each. On a three ton 26SCA5 at eighty to a hundred feet, the rated seven-eighths line costs 2% and a five-eighths line costs 6%. What matters is whether your diameter is on the list at all, and what the installer says about the rest of the system.

Why does the chart stop at some diameters instead of getting worse?

Because below a certain size the problem stops being lost capacity and becomes oil return to the compressor, and that is not a trade you can price. A five ton 26SCA5 lists three-quarters, seven-eighths and one and one-eighth. Five-eighths is not a slow option for that unit — it is not an option, and no percentage is printed for it.

Does R-454B lose more through the same pipe than R-410A did?

On Carrier single-stage air conditioners, yes, and it is not a close call. The R-410A families and the R-454B families print two different tables covering the same seven capacities and the same three diameters each — 189 cells. The R-454B figure is higher in 180 of them, equal in 9 and lower in none. But it is a statement about those two tables, not about the refrigerant: Carrier publishes no reason, and its R-454B heat pump prints the older air conditioner numbers rather than the newer ones.

Can I read my model number off a table for a different Carrier family?

Sometimes, and you cannot tell which times without checking. Twenty Carrier sheets in this archive print only eight different tables between them — six families share one of them. But the sets do not follow the badge on the box: they follow the kind of machine and its refrigerant, and two families from the same tier and the same year can sit in different sets. The safe move is the one this site is built on, which is to read the sheet for your model.

Sources

• Carrier — 26SCA5 Comfort Series Single-Stage Air Conditioner with Puron Advance Refrigerant — Product Data . Read 2026-09-02.
• Carrier — 24SCA5 Comfort Series Single-Stage Air Conditioner with Puron Refrigerant — Product Data . Read 2026-09-02.
• Carrier — 27SCA5 Comfort Series Single-Stage Heat Pump with Puron Advance Refrigerant — Product Data . Read 2026-09-02.
• Carrier — 26SCA4 Comfort Series Single-Stage Air Conditioner with Puron Advance Refrigerant — Product Data . Read 2026-09-02.
• Carrier — 24SCA4 Comfort Series Single-Stage Air Conditioner with Puron Refrigerant — Product Data . Read 2026-09-02.
• Carrier — 24SPA6 Performance Series Single-Stage Air Conditioner with Puron Refrigerant — Product Data . Read 2026-09-02.
• Carrier — 24ABB3 Comfort 13 Air Conditioner with Puron Refrigerant — Product Data . Read 2026-09-02.
• Carrier — 24AHA4 Performance Series Air Conditioner with Puron Refrigerant — Product Data . Read 2026-09-02.

This is a starting point, not an approval. Everything here is read from published manufacturer documents and dated, but only the licensed contractor who signs your installation can approve a specific system for your home and your local code. Take the citations on this page to them and ask.