Case Study: Replacing a Leaking, Mismatched 3-Ton System With a 4-Ton Carrier Infinity Variable Speed Gas System With Two Zones in Austin
Installed April 15 and April 20, 2026 · Austin, Texas
In plain words. The homeowner's story. James lives in a two-story home in Austin with one 3-ton gas system that served two zones. The system was 11 to 15 years old, the indoor and outdoor units were mismatched, the coil was leaking refrigerant, and it needed too many repairs. Two days before the install, Atlas added 4 lb of R-410A to keep the house cool until the new system went in. It had kept the house cool in summer, but it ran all the time to keep up, and James thought the 3-ton system was undersized. No airflow problems were reported. He wanted to replace the complete AC and heating, keep two-zone control with new dampers, and add an LED REME HALO.
In plain words. Short answer. We replaced the leaking, mismatched 3-ton system in this Austin home with a 4-ton Carrier Infinity variable speed air conditioner, a matched cased coil and an Infinity 80 two-stage gas furnace, with two zones, part of our HVAC installation in Austin. The ACCA Manual J for the 2,073 sq ft house gives a total cooling load of 35,837 BTU/h, about 3.0 tons. The Manual S report lists 37,240 BTU/h of total cooling capacity for the selected 4-ton Carrier match, 104% of the cooling load and inside the Manual S range of 90 to 130% for variable speed equipment. We also removed the old bypass zone system, installed a new supply plenum, upsized the zone ducts and installed Infinity zoning. The installed price was $15,916.51.
Key facts
In plain words. Here are the home, equipment and project details.
| Item | Details |
|---|---|
| Location | Austin, Texas. Atlas AC Repair in Austin |
| Home | Two-story home, 2,073 sq ft conditioned per the Manual J, built in 2012. Furnace and coil in the attic. Ceilings 9 ft downstairs and 8 ft upstairs |
| Old system | 3-ton gas system, 11 to 15 years old, with mismatched indoor and outdoor units and a coil refrigerant leak. Existing bypass zone system with two 16 inch dampers and two thermostats |
| Load result | Whole house, 2,073 sq ft (ACCA Manual J 8th Edition): cooling 35,837 BTU/h (32,416 sensible, 3,421 latent), about 3.0 tons; heating 39,165 BTU/h. Design airflow: 1,632 CFM cooling and 1,186 CFM heating |
| Why 4 tons | The Manual S report lists 37,240 BTU/h of total cooling capacity for the selected 4-ton match. That is 104% of the 35,837 BTU/h cooling load, inside the Manual S variable speed range of 90 to 130% |
| New system | 4-ton Carrier Infinity 21: 26VNA148W003 variable speed air conditioner (R-454B), CVAMA4921XMA cased coil, 58TN0B090C21--20 Infinity 80 two-stage gas furnace (80% AFUE), Infinity System Control |
| Zoning | Two zones, downstairs and upstairs, with a 16 inch damper downstairs, a 14 inch damper upstairs and an Infinity smart sensor. The old bypass zone system was removed |
| Ductwork | New supply plenum, downstairs zone supply upsized from 14 to 16 inch, upstairs duct upsized from 12 to 14 inch, new 9 inch supply in the upstairs loft, foyer supply grill upsized from 6 to 9 inch |
| Rated efficiency | The Carrier Infinity 21 match is rated 19 SEER2 and 11 EER2 (AHRI 215698378, as listed in the Manual S report); furnace 80% AFUE |
| Permit and inspection | City of Austin permit pulled by Atlas AC Repair; inspection passed May 4, 2026 |
| Installed price | $15,916.51. See what is included in the Cost section |
| Install time | Two days, April 15 and April 20, 2026. Installed by Israel (lead) and Adonis |
The problem: a leaking, mismatched system
In plain words. The old 3-ton gas system was leaking refrigerant at the coil, the indoor and outdoor units did not match, and it needed too many repairs. It was 11 to 15 years old. Two days before the install, on April 13, 2026, Atlas added 4 lb of R-410A to the old system to keep the house cool until the new system went in. It still kept the house cool in summer, but it ran all the time to keep up. James thought the 3-ton system was undersized. No airflow problems were reported. James wanted to replace the whole system and keep two-zone control.
James called Atlas AC Repair on April 10, 2026, and Mateo did the site survey the same day. What the records show:
- A refrigerant leak. The coil was leaking refrigerant. Two days before the install, on April 13, 2026, Atlas added 4 lb of R-410A to the old system to keep the house cool until the new system went in.
- Mismatched equipment. The indoor and outdoor units were not a matched set.
- Too many repairs. Repairs were the main pain point.
- It ran all the time. The old system kept the house cool in summer, but it ran all the time to keep up. James thought the 3-ton system was undersized.
- An older bypass zone system. The house had a bypass zone system with two 16 inch dampers and two thermostats.
- A weak supply plenum. During the install, the crew found the existing supply plenum loose and weak, with the insulation fibers falling apart.
What the homeowner wanted, and the result
In plain words. James wanted a new complete AC and heating system, two-zone control with new dampers, and an LED REME HALO. We installed a matched 4-ton Carrier Infinity gas system, rebuilt the two-zone setup with Infinity zoning and installed the LED REME HALO. The table shows each request, what we did and how it turned out.
| What the homeowner wanted | What we did | Result |
|---|---|---|
| Replace the complete AC and heating | Installed a matched 4-ton Carrier Infinity variable speed air conditioner, cased coil and Infinity 80 two-stage gas furnace | Manual S compliant match, AHRI 215698378. “Atlas bid was a superior product with detailed work and service at the most affordable price.” (homeowner's review) |
| Keep two-zone control and replace the dampers | Removed the old bypass zone system and installed Infinity two-zone control with a 16 inch damper downstairs and a 14 inch damper upstairs | Zoning completed April 20, 2026 |
| An LED REME HALO | Installed an LED REME HALO | Installed April 20, 2026 |
How the size was chosen
In plain words. James thought the old 3-ton system was undersized because it ran all the time to keep the house cool in summer. The Manual J gives the whole 2,073 sq ft home a cooling load of 35,837 BTU/h, about 3.0 tons. A leaking, mismatched system can lose capacity and run longer. The Manual S report lists 37,240 BTU/h of total cooling capacity for the selected 4-ton Carrier match at the home's design conditions, or 104% of that load. That is inside the Manual S range of 90 to 130% for variable speed equipment, so the 4-ton match passes the sizing check.
The ACCA Manual J 8th Edition report uses Austin, Bergstrom IAP weather data. It separates sensible cooling, the heat removed to lower the temperature, from latent cooling, the heat removed with moisture. These are calculated design loads, not measured system output. The Manual S report follows ANSI/ACCA Manual S 2023 (3rd Edition).
| Manual J and Manual S item | Result |
|---|---|
| Conditioned area | 2,073 sq ft (1,233 sq ft downstairs, 840 sq ft upstairs) |
| Cooling load | 35,837 BTU/h total, about 3.0 tons |
| Sensible cooling load | 32,416 BTU/h |
| Latent cooling load | 3,421 BTU/h |
| Load sensible heat ratio | 0.905, as reported in Manual J |
| Heating load | 39,165 BTU/h |
| Summer design temperatures | 99°F outdoors / 72°F indoors, 50% indoor relative humidity, 37 design grains, medium daily range |
| Winter design temperatures | 21°F outdoors / 71°F indoors |
| Weather station | Austin, Bergstrom IAP, elevation 495 ft |
| Selected Carrier match (Manual S report) | 26VNA148W003 air conditioner and CVAMA4921XMA coil, AHRI 215698378, variable speed: 37,240 BTU/h total cooling at 99°F outdoors and 72°F db / 63°F wb mixed air |
| Total capacity ÷ cooling load | 37,240 ÷ 35,837 = 104% (rounded). Within the Manual S variable speed range of 90 to 130% |
| Latent capacity ÷ latent load | 5,586 ÷ 3,421 = 163% (rounded). Meets the Manual S minimum of 100% |
| Sensible capacity and sensible load | 31,654 BTU/h sensible capacity against a 32,416 BTU/h sensible load, about 98% |
| Furnace output ÷ heating load | 72,000 ÷ 39,165 = 184% (rounded). Within the report's Manual S furnace range of 100 to 200% |
| Calculated design airflow | 1,632 CFM cooling; 1,186 CFM heating |
Why this 4-ton match. The capacity check uses the total cooling capacity listed in the Manual S report at design conditions, not the nominal 48,000 BTU/h size. The 37,240 BTU/h result lands inside the Manual S total cooling range, and the latent capacity meets the Manual S latent check. The report lists sensible capacity at 31,654 BTU/h, slightly under the 32,416 BTU/h sensible load. Carrier lists the 26VNA1 compressor with a capacity range of 25 to 100% (26VNA1 product data), so it can run below full capacity on mild days. See another 4-ton Carrier Infinity gas system in The Woodlands.
What the Manual J was based on
In plain words. The Manual J uses construction inputs based on the home's age, a 2012 two-story home, with no radiant barrier. These are the report's model inputs, not a measured insulation survey of the house.
- Walls. 2x4 wood frame with R-9 board insulation and no cavity insulation, U-value 0.082.
- Windows. 16 windows, 128 sq ft in total, double pane low-e glass in aluminum frames, U-value 0.64 and SHGC 0.61.
- Doors. Two metal doors with a fiberglass core, 40 sq ft, U-value 0.6.
- Ceilings. Upstairs, R-28 under a vented attic with no attic fan, no radiant barrier and dark asphalt shingles. Downstairs, R-30 below the roof joists. Both have a U-value of 0.034.
- Ducts and air leakage. R-8 ducts in the attic, and average air leakage.
- Ceiling heights. 9 ft downstairs and 8 ft upstairs.
Zoning and ductwork
In plain words. The house already had two zones, but on an older bypass zone system. We removed the bypass and its two 16 inch dampers and installed Carrier Infinity zoning with two new dampers, a 16 inch damper for downstairs and a 14 inch damper for upstairs. For another zoned install, see our zoned Carrier Infinity replacement in League City. We replaced the loose supply plenum and upsized the ducts that feed each zone. The result is a two-zone Infinity system with a new supply plenum and larger zone ducts.
The Manual J also breaks the load into the two zones. These are calculated design loads and design airflow from the report, not measured airflow.
| Zone | Area | Heating load | Sensible cooling | Latent cooling | Design airflow (heating / cooling) |
|---|---|---|---|---|---|
| 1 | Downstairs, 1,233 sq ft | 26,910 BTU/h | 21,377 BTU/h | 2,057 BTU/h | 823 / 1,098 CFM |
| 2 | Upstairs, 840 sq ft | 12,256 BTU/h | 10,467 BTU/h | 1,364 BTU/h | 363 / 533 CFM |
Zone sensible cooling values are before the report's 572 BTU/h AED excursion adjustment. The two zones add up to 31,844 BTU/h, and with the adjustment the whole house is 32,416 BTU/h. Heating, latent and airflow add up to the whole-house totals within rounding.
Zone dampers and zone control
In plain words. Each floor is its own zone with its own temperature setting. A Carrier Infinity System Control and an Infinity smart sensor run the zones. In the attic, a 16 inch damper serves the downstairs zone and a 14 inch damper serves the upstairs zone, and they open and close so air goes to the zone that is calling.
Ductwork changes
In plain words. We changed the ductwork at the equipment and the zone ducts that feed each floor. Each change is listed below. For typical ranges, see our ductwork replacement cost guide.
- Old zone system removed. The bypass and the two old 16 inch dampers came out, and the plenums were patched.
- New supply plenum. The existing supply plenum was loose and weak, and its insulation fibers were falling apart. With the homeowner's approval, we replaced it.
- Downstairs zone supply. Upsized from 14 inch to 16 inch.
- Upstairs zone duct. Upsized from 12 inch to 14 inch.
- Upstairs loft. A new 9 inch supply was added in the upstairs loft area.
- Foyer. The foyer supply grill was upsized from 6 inch to 9 inch.
- Fittings and registers. The crew added a 3-way register, a 9 inch start collar and a 14 inch 90 degree elbow.
What was installed
In plain words. The old 3-ton system, its mismatched indoor and outdoor units and the old bypass zone system came out. In their place went a 4-ton Carrier Infinity variable speed air conditioner, a matched cased coil, an Infinity 80 two-stage gas furnace, an Infinity System Control, an Infinity smart sensor, two zone dampers and an LED REME HALO. The new matched system, a Carrier Infinity 21, is rated 19 SEER2 and 11 EER2. This is an example of our AC installation in Austin.
| Part | Old | New |
|---|---|---|
| Outdoor unit | 3-ton air conditioner, mismatched with the indoor unit | Carrier Infinity 26VNA148W003 variable speed air conditioner, 4 tons, R-454B (product page, product data) |
| Indoor coil | Leaking coil | Carrier CVAMA4921XMA cased coil (product data, spec sheet) |
| Furnace | Gas furnace | Carrier Infinity 80 58TN0B090C21--20 two-stage gas furnace, 80% AFUE (product page, product data) |
| Control | Two thermostats on the old zone system | Carrier Infinity System Control SYSTXCCITC01-C (product page) and Infinity smart sensor SYSTXZNSMS01 (product page) |
| Zone dampers | Bypass and two 16 inch dampers | Infinity two-zone setup: 16 inch damper downstairs and 14 inch damper upstairs; bypass removed |
| Supply plenum | Loose, with insulation falling apart | New supply plenum |
| In duct air treatment | None | LED REME HALO (RGF HALO LED) |
| Electrical | Existing breaker | New 40 amp breaker for the condenser |
The matched system is listed in the Manual S report under AHRI 215698378 as the 26VNA148W003 air conditioner and CVAMA4921XMA coil with a gas furnace: a Carrier Infinity 21 rated 19 SEER2 and 11 EER2 (AHRI Directory).
How the install went
In plain words. The new system went in on April 15, 2026. The attic opening was smaller than the new equipment, so the crew took the coil and furnace apart to get them into the attic. The zoning needed additional parts, so the crew came back on Monday, April 20, 2026 to finish the ductwork and zoning and test the heating. An owner reviews every job before installation. Every Atlas install day includes an acid flush, a filter drier, locking caps and a pressure test.
Israel (lead) and Adonis, both Carrier Infinity trained, installed the system.
April 15. The crew removed the old system. The attic access is 20 1/2 inches and the new equipment is 21 inches, so they took apart the coil and furnace to get them in. They found the existing supply plenum loose and weak, with the insulation falling apart, and got the homeowner's OK to replace it. They installed the new supply plenum, upsized the downstairs zone supply from 14 to 16 inch, reconnected the ductwork, brazed the system and ran the pressure test. The 4-ton Carrier Infinity gas system was installed that day.
April 20. The crew installed the two-zone Infinity zoning with a 14 inch damper upstairs and a 16 inch damper downstairs, upsized the upstairs duct from 12 to 14 inch, added the new 9 inch supply in the upstairs loft, installed the LED REME HALO and a new 40 amp breaker for the condenser, finished the wiring and tested the heater.
Permit and inspection
In plain words. Atlas AC Repair pulled the City of Austin permit for the job, and the city inspection passed on May 4, 2026.
Cost
In plain words. The installed price was $15,916.51. For typical ranges, see our Carrier Infinity cost guide. The price covers the 4-ton Carrier Infinity variable speed air conditioner, the cased coil and the Infinity 80 two-stage gas furnace, the Infinity two-zone upgrade with two new zone dampers, the Infinity System Control and smart sensor, removal of the old bypass zone system, the new supply plenum and the ductwork changes, the LED REME HALO, a new 40 amp breaker for the condenser and the City of Austin permit. Atlas AC Repair is an Austin Energy participating contractor.
Follow-up after the install
In plain words. On August 17, 2026, James called because both thermostats were blank. On August 18, 2026, Mateo found a clogged condensate drain and cleared it. The thermostats came back on, and the system was performing to factory specifications.
What the homeowner said
In plain words. Here is James's review, word for word.
“Atlas AC is an outstanding company. I received three estimates to replace my complete AC and heating. Atlas bid was a superior product with detailed work and service at the most affordable price. I love the fact that they did not throw a sales pitch like the other guys! I can not speak highly enough about their work. Israel is knowledgeable, professional, and timely. This company goes beyond the call of duty. If you are looking for a honest AC company, look no further!”
James, Austin, Texas · 5-star Google review, April 20, 2026.
Common questions
In plain words. These answers explain the equipment, sizing and project details.
What did Atlas install in this Austin home?
In plain words. A 4-ton Carrier Infinity gas system with two zones: a Carrier Infinity 26VNA148W003 variable speed air conditioner (R-454B), a CVAMA4921XMA cased coil, a Carrier Infinity 80 58TN0B090C21--20 two-stage gas furnace, an Infinity System Control and smart sensor, two new zone dampers and an LED REME HALO. It replaced a leaking, mismatched 3-ton gas system and an older bypass zone system. The installed price was $15,916.51, and the work took two days, April 15 and April 20, 2026.
Why replace the old system instead of repairing it?
In plain words. Because the coil was leaking refrigerant, the indoor and outdoor units were mismatched, the system was 11 to 15 years old and it needed too many repairs. It kept the house cool in summer, but it ran all the time to keep up, and James thought the 3-ton system was undersized. Two days before the install, on April 13, 2026, Atlas added 4 lb of R-410A to the old system to keep the house cool until the new system went in. James chose to replace the complete AC and heating.
Why replace a 3-ton system with a 4-ton system?
In plain words. Because at Austin's 99°F design temperature, the selected 4-ton Carrier Infinity match (AHRI 215698378) delivers 37,240 BTU/h, about 3.1 tons. That is 104% of the home's 35,837 BTU/h Manual J cooling load and inside the Manual S range of 90 to 130% for variable speed equipment. Equipment is checked by its capacity at the home's design conditions, not by its nominal size. James had also noticed the old 3-ton system ran all the time, and a leaking, mismatched system can lose capacity and run longer.
Is a 4-ton system too big for this house?
In plain words. No. The sizing check uses the 37,240 BTU/h the Manual S report lists for this match at design conditions, not the nominal 48,000 BTU/h of a 4-ton unit. That is 104% of the 35,837 BTU/h cooling load, inside the Manual S range of 90 to 130% for variable speed equipment. Carrier lists the 26VNA1 compressor with a capacity range of 25 to 100%, so it can run below full capacity on mild days.
Does the new system cover the sensible and latent load?
In plain words. Mostly. Total cooling capacity is 104% of the total load, and latent capacity is 5,586 BTU/h, 163% of the 3,421 BTU/h latent load, which meets the Manual S minimum of 100%. Sensible capacity is 31,654 BTU/h against a 32,416 BTU/h sensible load, about 98%, so it is slightly under the sensible load at design conditions.
How does the two-zone system work in this house?
In plain words. Each floor is its own zone with its own temperature setting. A Carrier Infinity System Control and an Infinity smart sensor run the zones, and two zone dampers in the attic, a 14 inch damper for upstairs and a 16 inch damper for downstairs, open and close so air goes to the zone that is calling. The Infinity zoning replaced the old bypass zone system.
What ductwork was changed?
In plain words. The old bypass and its two 16 inch dampers were removed and the plenums were patched. The existing supply plenum was loose and its insulation was falling apart, so with the homeowner's approval it was replaced with a new supply plenum. The downstairs zone supply was upsized from 14 inch to 16 inch, the upstairs duct was upsized from 12 inch to 14 inch, and a new 9 inch supply was added in the upstairs loft. The foyer supply grill was upsized from 6 inch to 9 inch, and the crew added a 3-way register, a 9 inch start collar and a 14 inch 90 degree elbow.
What size AC does a 2,000 sq ft home in Austin need?
In plain words. For this two-story 2,073 sq ft Austin home, the Manual J total cooling load is 35,837 BTU/h, about 3.0 tons (32,416 sensible and 3,421 latent), and the heating load is 39,165 BTU/h. The selected 4-ton variable speed match delivers 37,240 BTU/h at design conditions, 104% of that load. Another house of the same size can have a different load because of its windows, insulation, ducts and orientation, so the load has to be calculated for each house.
How efficient is the new system?
In plain words. The matched system, AHRI 215698378, is a Carrier Infinity 21 rated 19 SEER2 and 11 EER2, as listed in the Manual S report. The Carrier Infinity 80 furnace is an 80% AFUE two-stage gas furnace.
How much does a 4-ton Carrier Infinity zoned gas system cost?
In plain words. This job was $15,916.51 installed. That covers the 4-ton Carrier Infinity variable speed air conditioner, cased coil and Infinity 80 two-stage gas furnace, the Infinity two-zone upgrade with two new zone dampers, the ductwork changes, the LED REME HALO, a new 40 amp breaker for the condenser and the City of Austin permit. Prices for other homes depend on the equipment, zoning and ductwork needed. For typical ranges, see our 4-ton HVAC system cost.
How long did the install take?
In plain words. Two days. The crew installed the new system on April 15, 2026. The zoning needed additional parts, so the crew returned on Monday, April 20, 2026 to finish the ductwork and zoning and test the heating.
Did the install pass inspection?
In plain words. Yes. Atlas AC Repair pulled the City of Austin permit, and the inspection passed on May 4, 2026.
What indoor air quality equipment was added?
In plain words. An LED REME HALO in-duct air purifier. The homeowner asked for it at the survey, and the crew installed it on April 20, 2026.
Was there any follow-up service after the install?
In plain words. Yes. On August 17, 2026, James called because both thermostats were blank. On August 18, 2026, Mateo found a clogged condensate drain and cleared it. The thermostats came back on, and the system was performing to factory specifications.
Proof and sources
In plain words. These records support the project details and calculations.
- ACCA Manual J 8th Edition load calculation and ANSI/ACCA Manual S 2023 (3rd Edition) equipment sizing report; survey and install notes; follow-up service notes; install photos and equipment data tags; invoice; City of Austin permit and inspection record.
- Carrier product data: 26VNA1 product page and product data; Infinity 80 furnace product page and 58TN0B product data; CVAMA product data and spec sheet; SYSTXCCITC01-C; SYSTXZNSMS01; RGF HALO LED.
- AHRI 215698378, AHRI Directory listing.
- TDLR license search for TACLB00153401E, the license Atlas AC Repair operates under.
