Furnace Installation in Kirkwood and West St. Louis County

A furnace install in Climate Zone 4A starts with one calculation: how much heat does this specific home lose at 3°F outdoor with the desired interior setpoint? Manual J load calculation answers that question with the actual home’s window U-factors, insulation R-values, infiltration rate, and orientation — not with a “1 BTU per square foot per degree” rule of thumb that produces equipment oversized by 40–60% in a typical Kirkwood retrofit. Oversized furnaces short-cycle, deliver uncomfortable temperature swings, accelerate heat exchanger fatigue through repeated thermal expansion cycles, and waste fuel through start/stop inefficiency. Undersized furnaces run continuously and still can’t maintain setpoint during the January 2019, February 2021, or December 2022 Polar Vortex stretches. Sized correctly, the equipment cycles 4–6 times per hour at design conditions, runs longer cycles at higher efficiency at typical operating conditions, and meets the 3°F design load reliably. The rest of this page documents how we get equipment selection right.

Step 1: Manual J Heating Load Calculation

ACCA Manual J using Wrightsoft Right-J software with St. Louis Lambert climate data (ASHRAE 99% winter design 3°F, Climate Zone 4A). Inputs:

• Square footage by floor with ceiling heights documented
• Window U-factors and SHGC values, with orientation noted (north-facing windows lose more heat than south-facing at the same U-factor)
• Wall, ceiling, and floor insulation R-values (older Kirkwood homes in Meramec Highlands and Central Place historic districts often have R-7 to R-13 wall insulation; newer construction is R-19 to R-21)
• Air infiltration rate (measured via blower door if available, or estimated from construction era and tightness category)
• Indoor design temperature (typically 70°F for heating calculations)
• Indoor humidity setpoint if applicable
• Duct gains and losses if ductwork runs through unconditioned space

Manual J output gives heating load in BTU/hr at the 3°F design temperature. A typical 2,400-square-foot Kirkwood home built in the 1970s with average insulation typically calculates to 55,000–80,000 BTU/hr heating load, matching a furnace with 60,000–90,000 BTU/hr input (depending on AFUE).

Step 2: Manual S Equipment Selection

Manual S equipment selection matches AHRI-rated furnace capacity to the calculated heating load. Selection considers:

• Output capacity within 0–15% of Manual J calculated load. Equipment more than 15% oversized short-cycles.
• AFUE efficiency tier. 80% (non-condensing single-stage), 92–96% (condensing single-stage or two-stage), or 97%+ (modulating variable-speed condensing).
• Blower performance match to existing ductwork. Higher AFUE equipment typically uses ECM variable-speed blowers requiring static pressure under 0.8" WC; existing ductwork must accommodate.
• Venting category. Non-condensing equipment uses traditional masonry chimney or B-vent; condensing equipment requires sidewall PVC venting (typically Schedule 40 or polypropylene).
• Gas service capacity. Existing Spire Missouri gas service must support the new equipment’s input BTU/hr.
• Electrical service. 115V/15A typical for furnaces; modulating variable-speed equipment may require dedicated circuit.
• Manufacturer ecosystem. Communicating thermostats (Carrier Infinity, Trane ComfortLink, Lennox iComfort) match specific furnace series.

Recommended Equipment Tiers

80% AFUE Single-Stage Furnaces

Carrier Comfort 80, Trane S8X1, Lennox Merit 80AF, Bryant Legacy. Used in budget retrofits, rental properties, and homes where existing masonry chimney venting is required (no easy sidewall PVC routing). Lower upfront cost ($2,800–$4,500 installed), higher operating cost over equipment life.

92–96% AFUE Condensing Furnaces

Carrier Performance 96, Trane S9X2, Lennox Elite EL296V, Bryant Preferred. The mid-tier sweet spot for most Kirkwood residential replacements. Sidewall PVC venting, condensate drain to nearest drain. $4,800–$7,200 installed. Best balance of upfront cost, operating cost, and longevity for typical residential applications.

97%+ AFUE Modulating Variable-Speed Furnaces

Carrier Infinity 98, Trane S9V2, Lennox SLP99V. Modulating gas valves with variable-speed ECM blowers and communicating controls. $6,400–$10,800 installed. 20-year heat exchanger warranty on top units (Lennox SLP99V has lifetime heat exchanger warranty). Optimal comfort with continuous-low-fire operation through most of the heating season, ramping up only at design conditions.

Step 3: Manual D Duct Verification

If existing ductwork is staying (which is typical on residential furnace replacements), Manual D verification confirms the new equipment’s blower can move design CFM through the existing duct system. Measurements include trunk and branch dimensions, calculated equivalent length, verification against design friction rate (0.05–0.08 inches WC per 100 feet equivalent length). Static pressure measurement on the existing system establishes the baseline.

Common Manual D findings on Kirkwood retrofits:

• Trunk and branch sizing adequate for existing equipment but undersized for new equipment with higher CFM blower — resolution typically involves selecting different equipment or limited duct modifications
• Excessive elbows and length runs producing elevated equivalent length — harder to address without significant duct rework
• Undersized return air path producing elevated return-side static pressure — resolution typically involves adding return air drops or grilles
• Ductwork in unconditioned space (attic, crawlspace, garage) with inadequate insulation, sealing, or both — resolution involves duct sealing and additional insulation R-value

Step 4: Venting and Combustion Air

Venting configuration depends on equipment AFUE:

Non-Condensing (80% AFUE) Equipment

• Masonry chimney — usable if chimney is in good condition, properly lined, and sized for the equipment’s BTU/hr input. Many older Kirkwood homes in Meramec Highlands and Central Place have masonry chimneys that need relining to accept Category I venting from modern 80% AFUE equipment.
• Type B vent (double-wall metal) — standard for 80% AFUE residential equipment, typically routed through attic to roof penetration.
• Combustion air — typically drawn from indoor air in older installations, or from outdoor air through a dedicated combustion air supply in newer installations meeting Missouri Mechanical Code requirements.

Condensing (92%+ AFUE) Equipment

• Sidewall PVC venting — Schedule 40 PVC or polypropylene venting routed horizontally through a sidewall to outdoor termination. Total length and number of elbows must meet manufacturer specifications.
• Direct vent (separate combustion air intake) — second PVC pipe brings combustion air from outside, providing complete combustion isolation from indoor air. Recommended for tight modern homes and basement installations.
• Condensate drain — condensing furnaces produce 0.5–1 gallon of acidic condensate per hour at full fire. Drain must route to nearest sanitary drain, typically through PVC with appropriate slope. Neutralizer may be required by local code on older sewer service to prevent acidic damage to cast iron drain lines.

Installation Day Sequence

A typical residential furnace replacement is a one-day install for like-for-like replacement, 1.5–2 days if changing equipment type (80% to 96% AFUE requires venting reconfiguration; gas furnace to heat pump requires substantially different scope).

1. Floor protection and walkthrough. Canvas drop cloths, plastic sheeting, shoe covers.
2. Existing equipment shut-down and removal. Gas shut-off at the meter or service valve, electrical disconnect, vent disconnection, removal from cabinet space. Old equipment goes to St. Louis County waste management or licensed scrap.
3. New equipment placement. Cabinet leveling, vibration isolation pad placement, alignment with existing supply and return ductwork.
4. Gas connection. CSST or black iron gas line from existing service to new furnace inlet. Drip leg installation. Manual shut-off valve. Initial pressure test with manometer.
5. Venting installation. Sidewall PVC routing for condensing equipment, or chimney/B-vent for non-condensing. Combustion air supply if applicable. Sealing at penetrations.
6. Condensate drain (condensing equipment only). Routing to nearest drain, slope verification, neutralizer if required.
7. Electrical connection. 115V supply, ground verification, thermostat wiring to new equipment terminals.
8. Initial startup and verification. Gas pressure verification at the meter (Spire Missouri 7" WC nominal) and at the manifold (3.5" WC for single-stage natural gas, varies by manufacturer for two-stage and modulating equipment). Bacharach Fyrite Insight Plus combustion analysis with target under 100 ppm CO air-free at high fire. Stack temperature verification. Inducer motor amperage verification.
9. Commissioning. Static pressure measurement across the air handler (target under 0.5" WC standard, 0.8" WC ECM). Supply and return temperatures. Blower operation verification across all stages or modulation range.
10. Customer walkthrough. Thermostat operation, filter location and MERV rating, recommended filter change interval, location of emergency gas shut-off, manufacturer warranty paperwork, Purisync labor warranty card.

Permits and Inspection

Furnace installation permits are pulled through the applicable municipal authority within one business day of contract signing — Kirkwood Public Works at (314) 822-5800, St. Louis County DPW at (314) 615-8530 for Affton and Sappington, Webster Groves PW at (314) 963-5300, Crestwood PW at (314) 729-4790, or Town and Country PW at (314) 432-6606. Municipal inspector visits within 5–10 business days of installation completion. Inspector verifies: gas line installation, venting installation, condensate handling, electrical connection, and final commissioning documentation.

Furnace Installation Pricing

• 80% AFUE single-stage: $2,800–$4,500 installed, depending on equipment capacity and venting scope.
• 92–96% AFUE condensing single-stage or two-stage: $4,800–$7,200 installed, including sidewall PVC venting installation.
• 97%+ AFUE modulating variable-speed: $6,400–$10,800 installed, including communicating thermostat compatible with the equipment series.

Pricing includes equipment, labor, permit fee (typically $80–$240 depending on jurisdiction), standard venting installation, gas connection, electrical connection, condensate drain (condensing equipment), and Purisync 2-year labor warranty. Extended labor coverage (5/10/12 year) is additional.

Frequently Asked Questions

How much does a furnace installation cost in Kirkwood?

Pricing depends on equipment tier and installation scope. 80% AFUE single-stage installations run $2,800–$4,500 installed. 92–96% AFUE condensing single-stage or two-stage runs $4,800–$7,200 installed. 97%+ AFUE modulating variable-speed runs $6,400–$10,800 installed. All pricing includes equipment, labor, permit fee, standard venting, gas connection, electrical, condensate drain (condensing equipment), and Purisync 2-year labor warranty. Sidewall PVC venting installation for condensing equipment retrofits adds $400–$900 over like-for-like replacement of existing 80% AFUE equipment that already had appropriate venting.

How does Purisync size a new furnace?

ACCA Manual J load calculation using Wrightsoft Right-J software with St. Louis Lambert climate data (ASHRAE 99% winter design 3°F, Climate Zone 4A). Inputs include square footage by floor, window U-factors and SHGC with orientation, wall/ceiling/floor insulation R-values, infiltration rate, indoor design temperature, and duct losses through unconditioned space. Manual J output gives heating load in BTU/hr at the 3°F design temperature. Manual S equipment selection matches AHRI-rated furnace capacity within 0–15% of calculated load. Manual D duct verification confirms existing ductwork can handle the new equipment’s blower CFM. A typical 2,400 sf Kirkwood home from the 1970s with average insulation calculates to 55,000–80,000 BTU/hr heating load.

What’s the difference between 80% AFUE and 96% AFUE furnaces?

AFUE (Annual Fuel Utilization Efficiency) measures how efficiently the furnace converts gas fuel to heat delivered to the home. 80% AFUE means 80% of the fuel’s energy becomes heat; 20% leaves as flue exhaust. 96% AFUE means 96% becomes heat; only 4% leaves as exhaust. The difference comes from condensing operation in 92%+ AFUE equipment: the flue gases cool below the dewpoint of water vapor in combustion products, condensing the vapor and recovering its latent heat (about 17% of total fuel energy). Practical effects: 96% AFUE equipment needs sidewall PVC venting (not masonry chimney) because the cooled flue gases won’t draft up a vertical chimney. 96% AFUE produces condensate that requires drainage. 96% AFUE costs roughly $2,000–$2,700 more installed than 80% AFUE, with payback period of 4–9 years on the operating cost difference depending on natural gas pricing and heating degree days.

What’s involved in switching from a non-condensing to a condensing furnace?

Three main scope additions over like-for-like replacement: sidewall PVC venting installation (typically $400–$900 additional, depending on routing complexity), condensate drain routing to the nearest sanitary drain ($150–$300 additional), and potentially a neutralizer cartridge if local code requires acidic condensate neutralization on older cast iron sewer service. The existing masonry chimney or B-vent that served the 80% AFUE equipment is no longer used (chimney can be capped or left in place for future flexibility). Total additional cost over like-for-like 80% replacement typically $550–$1,200. The efficiency gain and improved indoor air handling typically justify the upgrade for residential applications.

How long does furnace installation take?

Like-for-like furnace replacement is a one-day install (8 AM to mid-afternoon). Changing equipment type (80% to 96% AFUE with sidewall PVC venting) typically runs 1.5 days due to additional venting work. Complete heating system change (gas furnace to heat pump, or new furnace plus new AC plus ductwork modifications) typically runs 2 days. Pre-install equipment delivery and permit pull typically takes 5–10 business days from contract signing.

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Contact Purisync Heating and Air

For furnace installation quotes, equipment recommendations, or Manual J load calculation consultation, contact our 325 N Kirkwood Road office at (314) 338-5111. Installation scheduling typically targets 5–10 business days from contract signing, with emergency same-day replacement available during Polar Vortex weeks when equipment can be pulled from distributor inventory inside 4 hours.

• Emergency Line (24/7): (314) 338-5111
• Address: 325 N Kirkwood Rd #245, Kirkwood, MO 63122
• Email: info@purisyncheatingairconditioning.xyz
• St. Louis County Mechanical Contractor License: #MC-2014-08439-STL
• Kirkwood Business Registration: #BL-2014-1187
• EPA Section 608 Universal: #608U-2014-385721

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Office Hours

• Emergency Service: 24 hours a day, 7 days a week
• Office Staff: Monday – Saturday, 9:00 AM – 5:00 PM
• Closed: Sundays and State/Federal Holidays (emergency line always active)