# ARS HVAC — answers

Category: HVAC, Plumbing, and Electric
Hub: https://askdae.com/t/ars/answers

## Why is my AC running continuously but not cooling the house below 78 degrees?

An air conditioner running continuously at 78 degrees typically indicates a system hitting its outdoor design capacity limit during extreme heat, restricted airflow from a loaded filter, or low refrigerant causing the indoor evaporator coil to ice over.

Residential systems are engineered around regional outdoor design temperatures—typically 95°F ambient dry-bulb temperature. When outdoor temperatures climb above 100°F, or when airflow falls below 350 to 400 CFM (cubic feet per minute) per ton of cooling capacity, equipment cannot reach lower thermostat setpoints.

**If you only do one thing**: Measure the temperature split across the system using a probe thermometer at the supply and return air grilles to verify an active Delta-T (temperature difference) between 16°F and 21°F.

- **Design capacity limits**: Standard sizing protocols from ACCA (Air Conditioning Contractors of America) design systems to maintain 75°F indoors at local peak conditions; extreme ambient temperatures exceeding 98°F to 102°F will naturally limit a properly running unit to a sustained 78°F indoor baseline.
- **Airflow restrictions**: Air filters loaded with particulate matter restrict system airflow below the target 400 CFM per ton, which starves the evaporator coil (the indoor heat-absorbing component) of heat and prevents effective cooling delivery.
- **Evaporator coil icing**: Insufficient airflow or an undercharged refrigerant loop lowers suction pressure, causing coil surface temperatures to drop below 32°F; the resulting frost blanket insulates the tubing and stops thermal transfer entirely.
- **Fouled condenser coils**: Debris, grass clippings, or scale on the outdoor condenser coil (the exterior heat-dissipating component) prevent effective heat rejection, raising system operating pressures and decreasing total sensible cooling output by up to 30%.
- **Attic duct leakage**: Return duct separations in unconditioned attic spaces pulling in 130°F air can overwhelm equipment capacity, while supply leaks lose 20% to 30% of conditioned air directly into roof framing.

- **Watch out for**: Lowering the thermostat setting to 65°F during a heatwave, which does not speed up cooling but forces continuous compressor operation and causes complete coil freeze-ups.
- **Watch out for**: Leaving the thermostat fan switch set to "ON" instead of "AUTO," which blows warm, damp air back into living spaces by re-evaporating moisture off the shut-down indoor coil.
- **Watch out for**: "Topping off" refrigerant without fixing root leaks and checking superheat (vapor temperature rise) or subcooling (liquid line temperature drop below saturation), which risks compressor floodback.

Inspect the return air filter and replace it if loaded, then measure your supply and return register temperatures; if the temperature split remains under 15°F, schedule a licensed technician to measure static pressure and verify refrigerant circuit charge.

Sources:
- Frequently Asked Questions | ARS/Rescue Rooter: https://www.ars.com/faq
- About Us | Allgood Home Services: https://www.ars.com/allgood-plumbing-heating-cooling/about-us
- HVAC Company Near Me | Air Conditioning, Heating & Plumbing: https://www.ars.com/
- About Us | Brothers Air, Heat & Plumbing: https://www.ars.com/brothers-air-heat-plumbing/about-us
- AC Running But Not Cooling? 8 Causes and How to Fix Them: https://hvacchecker.com/resources/ac-running-but-not-cooling
- Why Your AC Runs Continuously But the House Stays at 78 Degrees: https://www.homefrontair.com/blog/why-ac-runs-continuously-house-stays-78-degrees

Permalink: https://askdae.com/t/ars/answers/why-is-my-ac-running-continuously-but-not-cooling-the-house-below-78-deg
Updated: 2026-09-22T16:34:29.790002+00:00

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## Cost to replace 100 feet of main sewer line using trenchless pipe bursting

Replacing 100 feet of main sewer line using trenchless pipe bursting typically costs between $6,000 and $16,000, averaging $80 to $160 per linear foot depending on line depth, pipe diameter, and insertion pit excavation requirements.

Aging clay, cast iron, or Orangeburg pipes frequently suffer from root intrusion and structural collapse, forcing property owners to balance pipe renewal against surface repair expenses. Traditional open-trench excavation across 100 feet routinely adds $4,000 to $10,000 in secondary concrete, driveway, and landscaping restoration.

**If you only do one thing**: Complete a full closed-circuit television (CCTV) sewer camera inspection to confirm the host pipe maintains a minimum 1/4-inch per foot drop before pulling the replacement line.

- **Access pit excavation**: Contractors dig an insertion pit and a receiving pit measuring 4x4 feet to 6x8 feet, costing $1,500 to $3,500 depending on whether depths exceed 6 feet or require breaking concrete.
- **Continuous HDPE pipe materials**: Crews pull continuous high-density polyethylene (HDPE) pipe rated DR-17 or DR-11 per ASTM F714 standards, which eliminates joint gaps and provides a 50-year to 100-year design life.
- **Pipe diameter sizing**: Technicians replace 4-inch sewer lines size-for-size or upsize to 6-inch lines using a 30- to 50-ton hydraulic expander head, adding a 15% to 25% material and equipment surcharge.
- **Couplings and cleanout installation**: Connecting the HDPE line to existing household plumbing and the city main using ASTM C1173 flexible mechanical couplings, alongside a two-way exterior cleanout, costs $800 to $1,800.
- **Permits and municipal inspections**: Municipal utility permits, street cut authorizations, and local plumbing inspections add $300 to $2,000 to the total 100-foot project cost depending on local jurisdiction codes.

- **Watch out for**: Host pipe sags or bellies exceeding 1 inch of standing water, because the bursting head tracks the existing grade and locks existing drainage dips into the new line.
- **Watch out for**: Parallel utility lines within 24 to 36 inches, where outward ground displacement from the expander head can rupture nearby gas, water, or electrical conduits.
- **Watch out for**: Concrete-encased repairs or ductile iron sleeves exceeding 0.5-inch wall thickness, which can halt bursting heads and trigger emergency open-trench excavations.

Schedule an on-site video pipe assessment to determine your lateral depth, host pipe material, and utility layout. Contact ARS/Rescue Rooter at 866-399-2885 to arrange a sewer camera evaluation and trenchless replacement estimate.

Sources:
- Frequently Asked Questions | ARS/Rescue Rooter: https://www.ars.com/faq
- HVAC Company Near Me | Air Conditioning, Heating & Plumbing: https://www.ars.com/
- Exceptional Service Guarantee | ARS/Rescue Rooter: https://www.ars.com/about-us/service-guarantee
- About Us | Brothers Air, Heat & Plumbing: https://www.ars.com/brothers-air-heat-plumbing/about-us

Permalink: https://askdae.com/t/ars/answers/cost-to-replace-100-feet-of-main-sewer-line-using-trenchless-pipe-bursti
Updated: 2026-09-22T16:34:29.790002+00:00

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## Should I repair or replace a 12-year-old R-410A air conditioner with a leaking coil?

Replace the 12-year-old R-410A air conditioner instead of repairing it, because a coil replacement costing $1,500 to $3,200 on equipment exceeding its ten-year warranty lifespan fails standard repair-versus-replace financial thresholds.

HVAC contractors frequently encounter leaking evaporator coils on aging systems due to formicary corrosion, which creates microscopic pitting in copper tubing from household airborne acids. Because central air conditioners carry an average service life of 12 to 15 years, major component failures past year 10 lead to compounding refrigerant recharge and labor costs under ongoing refrigerant production phase-downs.

**If you only do one thing**: Apply the industry-standard $5,000 rule by multiplying the unit age in years by the total repair estimate; any total exceeding 5,000 indicates that system replacement is the most cost-effective path.

- **The $5,000 calculation threshold**: Multiplying the 12-year unit age by an average coil repair estimate of $1,800 to $2,800 results in a score of 21,600 to 33,600, far exceeding the 5,000 benchmark used to justify replacement over repair.
- **Coil replacement economics**: A proper evaporator coil repair requires recovering existing refrigerant, brazing line connections, pulling a vacuum below 500 microns, and adding 5 to 10 pounds of R-410A, totaling $1,500 to $3,200 in parts and labor.
- **Warranty expiration**: Factory warranties on major components expire at 10 years, meaning a 12-year-old system carries 100% out-of-pocket risk while a replacement coil typically carries only a 1-year limited part warranty.
- **Efficiency improvements**: Replacing an aged 13 SEER (Seasonal Energy Efficiency Ratio) system with a modern 15.2 to 18 SEER2 platform lowers cooling electrical consumption by 20% to 35%.
- **Refrigerant regulatory phase-down**: Under the EPA AIM Act (American Innovation and Manufacturing Act), virgin R-410A production decreases significantly through 2029, escalating future repair costs for legacy cooling systems.

- **Watch out for**: Acid contamination from air and moisture drawn through the coil leak, which damages compressor motor windings and frequently causes secondary compressor failure within 3 to 12 months.
- **Watch out for**: Mismatched coil-to-condenser pairings where installing a new indoor coil with an aged outdoor condenser violates AHRI (Air-Conditioning, Heating, and Refrigeration Institute) standards and reduces system capacity by 15% to 30%.
- **Watch out for**: Liquid stop-leak sealant additives, which can clog thermostatic expansion valves (TXVs) and restrict 3/8-inch liquid line filter driers.

Schedule an on-site evaluation for a Manual J cooling load calculation to size a replacement cooling system accurately. Consult a licensed HVAC professional at ARS/Rescue Rooter by calling 866-399-2885 to explore system replacement options and local utility rebate programs.

Sources:
- Frequently Asked Questions | ARS/Rescue Rooter: https://www.ars.com/faq
- HVAC Company Near Me | Air Conditioning, Heating & Plumbing: https://www.ars.com/
- About Us | Florida Home Air Conditioning: https://www.ars.com/jacksonville/about-us
- About Us | Atlas Trillo Heating & Air Conditioning: https://www.ars.com/atlas-trillo/about-us

Permalink: https://askdae.com/t/ars/answers/should-i-repair-or-replace-a-12-year-old-r-410a-air-conditioner-with-a-l
Updated: 2026-09-22T16:34:29.790002+00:00

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## How do I safely replace a standard wall thermostat with a smart thermostat?

To safely replace a standard thermostat with a smart thermostat, isolate the 24-volt HVAC circuit at the breaker panel, verify zero voltage with a multimeter, and map existing conductors to standard terminal designations including a dedicated common wire.

Smart thermostats like Google Nest reduce space conditioning energy consumption by 8% to 12% based on EPA Energy Star data, but transitioning from mechanical switching to Wi-Fi microprocessors changes electrical requirements. Modern connected thermostats demand a constant circuit return path for continuous operation, making proper low-voltage power verification critical during replacement.

**If you only do one thing**: De-energize the entire HVAC system at the primary breaker panel and confirm 0 VAC across the power and common terminals with a digital multimeter before disconnecting any conductors.

1. **De-energize and verify power**: Shut off the dedicated circuit breaker supplying the furnace or air handler, then use a digital multimeter set to AC voltage to confirm 0 VAC between the 24-volt alternating current (VAC) power wire and ground.
2. **Document and label conductor terminals**: Remove the existing thermostat cover plate, photograph the terminal block, and attach physical wire labels to each 18 AWG (American Wire Gauge) conductor based on its terminal letter—primarily R (24V power), W (heating), Y (cooling), G (fan), and C (common).
3. **Verify or adapt the common wire**: Check for a connected C-wire providing a continuous 24VAC neutral return path; if only four wires exist without a C-wire, install an add-a-wire power adapter at the furnace control board or pull new 18/5 thermostat cable.
4. **Mount the sub-base and seal penetrations**: Secure the new backplate level against the drywall with wall anchors, guide the wires through the center opening, and seal the wall cavity hole with non-hardening electrical putty to block draft-induced temperature reading errors.
5. **Terminate wiring to terminals**: Strip 3/8-inch of insulation from each lead, insert the bare copper into the corresponding spring terminals (R/Rc, C, W, Y, G) on the sub-base, snap the smart thermostat display into place, and restore breaker power.
6. **Commission and test system cycles**: Complete the on-screen configuration and Wi-Fi pairing, then initiate manual 5-minute test calls for heat, cool, and continuous fan to verify contactor engagement and blower activation.

- **Watch out for**: High-voltage line-voltage systems (120V/240V baseboard heaters wired with 12 AWG or 14 AWG conductors), which will instantly short out standard 24VAC smart thermostats and create an electrical hazard.
- **Watch out for**: Running smart thermostats on "power stealing" without a dedicated C-wire, which causes compressor short cycling, Wi-Fi drops, and blown 3-amp or 5-amp low-voltage fuses on the control board.
- **Watch out for**: Unsealed drywall penetrations behind the wall plate, which allow unconditioned wall cavity air to flow over internal temperature sensors and skew room readings by 3°F to 5°F.

Remove the current thermostat faceplate to count your available conductors; if you lack a continuous C-wire or find 120V wiring, contact an ARS/Rescue Rooter technician at 866-399-2885 for professional smart control installation.

Sources:
- Frequently Asked Questions | ARS/Rescue Rooter: https://www.ars.com/faq
- HVAC Company Near Me | Air Conditioning, Heating & Plumbing: https://www.ars.com/
- Exceptional Service Guarantee | ARS/Rescue Rooter: https://www.ars.com/about-us/service-guarantee
- History | About Us | ARS/Rescue Rooter: https://www.ars.com/about-us/history

Permalink: https://askdae.com/t/ars/answers/how-do-i-safely-replace-a-standard-wall-thermostat-with-a-smart-thermost
Updated: 2026-09-22T16:34:29.790002+00:00

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## Can a cracked furnace heat exchanger be welded or must the unit be replaced?

A cracked furnace heat exchanger cannot be welded under ANSI Z21.47 safety standards and must be addressed by either installing an OEM replacement heat exchanger assembly or replacing the entire heating unit immediately.

A compromised heat exchanger breaches the physical barrier between combustion flue gases and supply airflow, introducing toxic combustion byproducts into living areas. According to the CDC (2024), unintentional non-fire carbon monoxide poisoning accounts for over 400 deaths and 100,000 emergency department visits annually in the United States.

**If you only do one thing**: Shut down the gas supply and tag out the furnace immediately upon discovering any crack, pinhole, or seam separation in the heat exchanger.

- **Code and listing restrictions**: Field welding alters the metallurgy of stamped aluminized or stainless steel (typically 18- to 20-gauge), voiding the equipment's UL or CSA listing and violating NFPA 54 (National Fuel Gas Code) mandates.
- **Thermal cycle failure**: Residential heat exchangers cycle repeatedly between 70°F (21°C) ambient and over 400°F (204°C) operating temperatures; welded heat-affected zones become brittle and consistently re-crack along the weld line.
- **Component replacement threshold**: Replacing only the heat exchanger core makes economic sense when the furnace is under 10 years old and covered by a 20-year or lifetime manufacturer parts warranty, with mechanical labor running between $800 and $1,800.
- **Full system replacement threshold**: If the heating system exceeds 12 to 15 years in age, full unit replacement is standard practice to restore heating efficiency up to 96%+ AFUE (Annual Fuel Utilization Efficiency, a measure of annual fuel-to-heat conversion).
- **Diagnostic verification**: Certified technicians verify suspected fractures using an infrared combustion analyzer to detect flue gas spill, a flexible digital borescope, and static pressure checks across the air plenum.

- **Watch out for**: Relying solely on cold visual checks, as hairline stress fractures often stay tightly closed until thermal expansion opens them during active burner firing.
- **Watch out for**: Liquid sealants, tapes, or high-temperature epoxies, which disintegrate above 300°F (149°C), release hazardous fumes into the duct stream, and fail to stop exhaust migration.
- **Watch out for**: Replacing the heat exchanger without measuring total external static pressure (TESP); high static pressure exceeding 0.50 inches water column restricts airflow and causes recurring thermal cracking.

Review the furnace manufacture date on the rating plate to verify warranty status, and contact ARS/Rescue Rooter at 866-399-2885 to schedule an emergency heat exchanger inspection and system replacement evaluation.

Sources:
- HVAC Company Near Me | Air Conditioning, Heating & Plumbing: https://www.ars.com/
- Community Outreach | ARS/Rescue Rooter: https://www.ars.com/about-us/community-outreach
- About Us | Hamilton Plumbing, Heating & A/C: https://www.ars.com/call-hamilton/about-us
- About Us | Atlas Trillo Heating & Air Conditioning: https://www.ars.com/atlas-trillo/about-us

Permalink: https://askdae.com/t/ars/answers/can-a-cracked-furnace-heat-exchanger-be-welded-or-must-the-unit-be-repla
Updated: 2026-09-22T16:34:29.790002+00:00

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## What's the difference between single-stage, two-stage, and variable-speed HVAC compressors?

Single-stage compressors run strictly at 100% capacity, two-stage units switch between high and low outputs around 65% to 70%, while variable-speed inverter compressors modulate continuously between 25% and 100% to match exact real-time thermal loads.

Compressor selection dictates cooling efficiency (measured by Seasonal Energy Efficiency Ratio 2, or SEER2), cycle frequency, electrical draw, and indoor humidity control. Department of Energy standards require residential cooling minimums between 13.4 and 14.3 SEER2 depending on region, making compressor design a central factor in system performance and electric utility costs.

**If you only do one thing**: Specify variable-speed inverter units for high-humidity climates or zoned systems, and use two-stage equipment as a balanced, cost-effective baseline for standard residential floor plans.

- **Single-Stage Operation (100% capacity)**: Operates strictly at full output, cycling on and off via standard 24-volt thermostat calls; delivers 13.4 to 14.5 SEER2 ratings, creates mechanical wear through frequent start-stop cycles, and draws full locked-rotor amperage on every startup.
- **Two-Stage Operation (65% to 100% capacity)**: Employs an internal unloading valve or dual-displacement scroll to run at low stage (typically 65% to 70% capacity) for roughly 80% of runtime; achieves 15 to 17.5 SEER2 ratings, reduces short-cycling, and enhances continuous air filtration.
- **Variable-Speed Modulation (25% to 100% capacity)**: Uses an inverter-driven motor modulating in 1% increments across an operating range of 25% to 100%; achieves 18 to 24+ SEER2, maintains room temperatures within 0.5 degrees Fahrenheit of the thermostat setpoint, and eliminates startup electrical spikes.
- **Latent Load and Dehumidification**: Variable-speed and two-stage units operate longer, low-airflow cycles that keep indoor evaporator coils cold, removing up to 30% more moisture from the air than single-stage systems without overcooling the living space.
- **Sound and Electrical Profiles**: Single-stage outdoor condensing units generate sound levels between 72 and 78 dBA, whereas variable-speed inverter outdoor units operate between 55 and 62 dBA at part-load while significantly lowering mechanical strain.

- **Watch out for**: Undersized existing ductwork with total external static pressure exceeding 0.5 inches water gauge, which starves modulating airflow and triggers high-pressure safety shutoffs on variable-speed blowers.
- **Watch out for**: Thermostat mismatch, where standard 24-volt single-stage controls are wired to variable-speed condensing units, locking the compressor into basic staged profiles and losing continuous modulation capabilities.
- **Watch out for**: System oversizing on single-stage equipment relative to ACCA Manual J load calculations, which causes short run times that lower temperature too quickly without removing indoor humidity.

Have an HVAC professional perform an ACCA Manual J load calculation and duct static pressure test to select the compressor design matched to your home's layout and climate requirements.

Sources:
- HVAC Company Near Me | Air Conditioning, Heating & Plumbing: https://www.ars.com/
- Frequently Asked Questions | ARS/Rescue Rooter: https://www.ars.com/faq
- About Us | ARS | HVAC Services: https://www.ars.com/blue-dot-services/about-us
- About Us | High-Quality HVAC Services in New Jersey: https://www.ars.com/air-experts/about-us

Permalink: https://askdae.com/t/ars/answers/what-s-the-difference-between-single-stage-two-stage-and-variable-speed
Updated: 2026-09-22T16:34:29.790002+00:00

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