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.

The table belongs to the family, not to the refrigerant

This is the part worth carrying to the installer, because it is the trap.

The 26SCA5 replaces the 24SCA5. Same brand, same Comfort tier, same single-stage air conditioner, one refrigerant apart. Every cell of the newer table is equal to or higher than the older one:

It would be easy to read that as a refrigerant rule — the new stuff is fussier about small pipe — and it is not one. The 27SCA5 is an R-454B heat pump from the same generation of the same brand, and wherever it lists the same capacity and the same diameter, it publishes the 24SCA5’s numbers rather than the 26SCA5’s. Three families, two refrigerants, and the numbers group by family instead of by refrigerant.

The two also disagree about what is acceptable. At one and a half tons the 26SCA5 accepts a half inch, five-eighths or three-quarters; the 27SCA5 accepts a half inch or five-eighths and stops there.

There is no shortcut here, and that is the point of this site: the answer is on your model’s sheet, not on the sheet of the unit next to it in the catalogue.

The good news on this particular 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.

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. Every brand publishes its own, and this site has not read one for American Standard, so nothing here transfers to that brand.
• 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 this pair of Carrier air conditioners, yes: every cell of the 26SCA5 table is equal to or higher than the 24SCA5 it replaces. But it is not a refrigerant rule. The 27SCA5 is an R-454B heat pump of the same generation and it publishes the older air conditioner numbers, not the newer ones. The table belongs to the family, so read the one on 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.

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.