Boiler Replacement Cost: Line Items Every Quote Needs
A homeowner's 25-year-old cast iron boiler fails in January. They call three contractors. Quotes come back at $5,800, $9,200, and $13,500. The low quote looks like a steal until you ask what is in it. Here is every line item that belongs in a complete boiler replacement quote.
Equipment: Modulating vs. Single-Stage
A single-stage non-condensing gas boiler (82–85% AFUE) runs $1,800–$3,000 for 100,000–120,000 BTU. A single-stage condensing boiler (92–96% AFUE) runs $2,500–$4,500. A fully modulating condensing boiler (95–96% AFUE with 5:1 or 10:1 turndown) runs $3,500–$6,500.
Modulating boilers are the right call on radiant floor systems, mixed radiator/baseboard loads, or any system where the heat load varies widely. On a straight single-zone baseboard system, the modulating premium is harder to justify.
Near-Boiler Piping Update
Pre-2000 boilers often lack modern near-boiler piping. A condensing boiler installation requires:
- Primary-secondary piping arrangement (prevents low return water temps that condense flue gases on non-condensing-rated components)
- Air separator and automatic air vent
- Expansion tank sized for system volume
- Pressure-reducing valve (PRV) for fill
- Backflow preventer
- Mixing valve or primary loop pump for condensing boiler protection
Full near-boiler piping update: $800–$1,500. Skipping it and bolting the new boiler to the old piping is how a condensing boiler fails in 3 years.
Venting Scope
Condensing boilers vent through PVC or polypropylene direct-vent — two pipes (intake and exhaust) through a sidewall or roof. Materials: $200–$400. Labor for a 15–25 ft run with elbows and penetrations: $800–$1,500. Total venting scope on a condensing conversion: $1,500–$3,000.
Non-condensing boiler flue: existing B-vent chimney liner stays if sized correctly and in good condition. If the flue is shared with a water heater, verify both appliances meet draft requirements after the swap. Common issue: new high-efficiency boiler drafts less than the old boiler and the shared chimney liner becomes oversized for just the water heater — leading to condensation and corrosion.
Electrical and Control Updates
Modern boilers include outdoor reset sensors, zone control relays, and often require a dedicated 15A circuit (vs. 120V plug on older units). Low-voltage wiring to new zone valves or circulator pumps may need upgrading. Thermostat compatibility: most modern boilers want a 24V thermostat with 2-stage support if you are running a modulating unit. Add $200–$500 for electrical and control work.
Permits and Code Compliance
Mechanical permit ($150–$400), gas permit if venting changes ($100–$200). Combustion air requirements have tightened in 2024 updates — tight homes may need a dedicated combustion air duct ($150–$300). Code also requires carbon monoxide detectors near sleeping areas on any fuel-burning appliance install. Verify presence or include CO detectors in the quote.
Removal and Disposal
A cast iron boiler weighs 400–800 pounds. Breaking it down for removal: 2–3 hours labor. Scrap value: $40–$120 depending on local scrap steel prices. Net cost to the customer: $200–$400 in labor for removal, offset partially by scrap. Oil-fired boiler disposal is more involved (tank cleaning if being replaced) and runs $400–$800.
The Full Quote
Low end: Non-condensing single-stage replacement, like-for-like venting, minimal piping work: $6,000–$8,500.
Mid: 95% AFUE condensing, new PVC vent, near-boiler piping update: $9,500–$12,000.
High end: Modulating condensing, full piping update, zone control upgrade, high-rise or complex venting: $12,500–$15,500.
Use the Boiler Replacement Cost Calculator to price by BTU, efficiency rating, venting type, and piping scope. The calculator produces a defensible quote that explains why your number is $9,200 and the other guy's is $5,800.
Frequently Asked Questions
How much does a boiler replacement cost in 2026?
A 100,000 BTU condensing gas boiler replacement runs $8,000–$12,000 installed. The boiler itself is $2,500–$4,500. Add $1,500–$3,000 for new venting (PVC on condensing units), $500–$1,500 for near-boiler piping updates, and $1,500–$2,500 in labor for a 1.5–2 day install. High-efficiency modulating units push $12,000–$15,000.
Should I replace with a condensing boiler or standard?
Condensing (90–96% AFUE) is the modern standard in most of the US. On hydronic systems with low return water temperatures (<140°F), condensing units achieve full efficiency. On old high-mass radiator systems with 170°F+ return temperatures, you lose the condensing benefit and non-condensing (82–85% AFUE) is a legitimate choice at lower upfront cost.
What is near-boiler piping and why does it matter?
Near-boiler piping is the hydronic plumbing within 10 feet of the boiler — primary/secondary loops, pumps, air separators, expansion tanks, and backflow preventers. On a replacement, near-boiler piping is often undersized or missing modern components (like a mixing valve for condensing boilers). Updating this piping is $500–$1,500 and is required for condensing boilers to work correctly.
Do I need a new flue for a boiler replacement?
If you are going from atmospheric or power-vented to condensing, yes. Condensing boilers use PVC or polypropylene direct-vent systems ($1,500–$3,000 installed) and cannot share a flue with a water heater. On a like-for-like non-condensing replacement, the existing B-vent chimney liner can usually stay — verify sizing and condition first.