Sizing the systems
Boiler replacement cost
A cast-iron gas boiler replaced like for like on an 1,800 sq ft house in a cold zone is about $4,750 installed — a 105,000 BTU unit, the expansion tank and circulator that go with it, the old one out, the permit, 12 licensed hours. Condensing is $8,108, with the plastic vent, the condensate and the liner the water heater needs once it is alone in the chimney. But the number on the box is only real if the radiators let it be: with 150 ft of baseboard this house needs 169°F water on its coldest day, nothing condenses at that temperature, and the 95% boiler runs at 91%. This sizes the boiler to the house and the water to the radiators, prices gas, oil, steam, combi and electric, and runs each of them for ten winters.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-24 · Nothing you type leaves your browser
Boiler replacement cost calculator
The boiler, the house, and the radiators it already has
The house sets the heat loss; the radiators set how hot the water has to be to cover it; and that temperature decides whether a condensing boiler earns its number. Steam is sized to the radiators alone.
The boiler most houses have: a cast-iron block, an atmospheric burner, the chimney it came with. Simple, long-lived, and the federal minimum.
All the floors the boiler heats.
Chicago, Denver, Boston, Detroit
Insulated walls and attic to the standard of its decade. The baseline these figures assume.
Around 15% of floor area, double glazed.
The copper-and-aluminium strip under the windows. 550 BTU per foot with 180°F water, 300 with 140°F — which is why a condensing boiler needs almost twice the baseboard to condense.
Add up every run of fin-tube in the house.
Only matters when the boiler goes condensing.
Only a condensing boiler makes condensate.
Hot water stays as it is.
Same fuel as before.
Used only to pick a regional labour rate. It stays in your browser.
Gas, cast iron, 84%, 105,000 BTU input, installed
$4,750
72,000 BTU/h of heat loss · 82,500 BTU/h of radiation at 180°F · about $1,599 a year in gas
169°F water on the coldest day
82,500 BTU/h of radiation against 72,000 of loss: the boiler sends 169°F on the coldest day and less the rest of the winter. Sized to the loss with a 15% allowance for the pipes, 105,000 BTU input is 7% over — the nearest size made.
Running it
$1,599 a year, 1,184 therms.
At national prices, on this house’s load in zone 5, at the 84% this boiler actually reaches with these radiators. Plus about $180 for the annual cleaning. The chart below runs every boiler on this house for ten winters.
Every boiler, ten winters
This house, these radiators: installed, then run ten years at national fuel prices.
What the contractor charges for
- Gas, cast iron, 84%, 105,000 BTU input
The boiler most houses have: a cast-iron block, an atmospheric burner, the chimney it came with. Simple, long-lived, and the federal minimum.$2,700 - Near-boiler kit
Expansion tank, air separator, fill valve, relief valve and a circulator: replaced with the boiler, because the old ones are the boiler's age.$450 - The old boiler out
The old boiler drained, broken down and out. A cast-iron block comes apart in sections; a steam boiler is the heaviest of them.$260 - Permit and inspection
Mechanical permit and inspection. Required almost everywhere for a boiler.$200 - 12 hours, licensed
A licensed heating contractor's rate. A boiler is not a homeowner job in any jurisdiction that inspects.$1,140
What it costs
National average. A gas, cast iron, 84% at 105,000 BTU input replacing a boiler of the same system in a 1,800 sq ft house with 150 ft of fin-tube baseboard. Not included: new radiators or baseboard, zone valves and thermostats beyond the one, asbestos on old pipe, or converting steam to water.
The heat loss comes from the BTU page; if the water heater is going at the same time, the water heater page prices the tank the indirect replaces.
Worth knowing
- The house loses about 72,000 BTU an hour on the design day, and 150 ft of fin-tube baseboard can deliver 82,500 with 180°F water — so the boiler has to send 169°F to keep up. The boiler is sized to the loss — 105,000 BTU input, 88,200 out — not to the radiators, which have 15% to spare.
- Running it: about $1,599 a year in gas on this house at national prices — 1,184 therms — plus the annual cleaning at about $180.
On this page — 5 sections
Sized to the radiators, not the house
The short version
- Cast-iron gas boiler, like for like: $4,750. Condensing: $8,108. Oil $6,140, steam $6,883.
- The house sets the size; the radiators set the water temperature — and that decides whether 95% is real.
- Just-enough baseboard: a 95% boiler runs at 91%. Radiators to spare, or a radiant floor: it runs at 95%.
- Steam is sized to the radiators alone. The house’s heat loss does not enter into it.
A furnace is replaced by the house. A boiler is replaced by the house and by what is already bolted to the walls. The heat loss — 72,000 BTU an hour for an 1,800 square foot house in a cold zone — says what the boiler must put out, and a 105,000 BTU input unit at 84% does it with the 15% the pipes take. But the radiators say what gets into the rooms: 150 feet of fin-tube baseboard is 82,500 BTU an hour with 180°F water and 45,000 with 140°F, so this house needs 169°F on its coldest day, and no boiler changes that. A house whose rooms were always cold has too little radiator, not too small a boiler.
72,000
BTU/h the house loses
Design day, zone 5
82,500
BTU/h the baseboard delivers
150 ft at 180°F
169°F
Water needed on the coldest day
Above the condensing line
105,000
BTU input, the size made
7% over the sizing
Air-Conditioning, Heating, and Refrigeration Institute / U.S. Department of Energy, Residential boiler ratings — input, DOE heating capacity, net AHRI rating and AFUE · checked 2026-09-24
Condensing only if the radiators let it
A condensing boiler earns its number by cooling the flue gas until the water in it condenses, and that happens when the water coming back from the radiators is under about 130°F. Send 170°F out and it comes back at 150, nothing condenses, and the stainless heat exchanger is doing the job a cast-iron one does for two thousand dollars less. Outdoor reset — lowering the water as the weather warms — gets it condensing through the mild half of the season, which is why the effective figure on a just-enough house is 91% and not 84% or 95%. Give it radiators to spare, or a radiant floor at 110°F, and it condenses every hour it runs.
| Cast iron, 84% — $4,750 | Condensing, 95% — $8,108 | |
|---|---|---|
| Equipment | $2,700 | $3,913 |
| Venting | The chimney it had | Plastic through the wall, $380 |
| Condensate | None | Neutralizer, $150 |
| Water heater in the chimney | Stays as it was | Relined, $950 |
| Fuel a year, 150 ft of baseboard | $1,599 | $1,476 — 91% real |
| Fuel a year, 250 ft of baseboard | $1,599 | $1,414 — 95% real |
| Payback on the difference | — | 27 years · 18 with the baseboard |
| Life | 25 years | 18 years |
Hydronics Institute / Slant/Fin and Burnham ratings, Heat output of fin-tube baseboard and cast-iron radiators by water temperature · checked 2026-09-24
Steam, oil, combi, electric
Steam is a different animal. The boiler is sized to the connected radiation — 25 square feet of EDR for a typical radiator, 240 BTU a square foot, and a third again for the pipes — and the house’s heat loss does not come into it: too small and the far radiators never fill; too big and it short-cycles. The near-boiler piping in black iron is the job, and the reason a steam boiler costs more to install than a water boiler of the same size. Oil is the same cast-iron block with a pressure burner, cleaned every year, and the fuel bill is the argument for switching where there is gas. A combi is one box that heats and makes hot water for one bathroom. An electric boiler is the cheapest to install and the dearest to run.
| 1,800 sq ft, zone 5, 150 ft of baseboard | Installed | Real AFUE | Fuel a year | Ten winters |
|---|---|---|---|---|
| Gas, cast iron, 84% | $4,750 | 84% | $1,599 | $20,740 |
| Gas, condensing, 95% | $8,108 | 91% | $1,476 | $22,868 |
| Gas, condensing combi | $8,116 | 91% | $1,476 | $22,876 |
| Propane, condensing, 95% | $8,245 | 91% | $3,106 | $39,305 |
| Oil, cast iron, 87% | $6,140 | 87% | $3,137 | $37,510 |
| Gas steam boiler, 82% (12 radiators) | $6,883 | 82% | $1,638 | $23,263 |
| Oil steam boiler, 85% (12 radiators) | $7,945 | 85% | $3,211 | $40,055 |
| Electric boiler, 100% | $3,958 | 100% | $4,811 | $52,068 |
National fuel prices: gas $1.35 a therm, oil $3.80 and propane $2.60 a gallon, electricity 16.5¢ a kilowatt-hour. The water heater is relined where the boiler leaves the chimney.
MaterialsEstimator, Boiler replacement and installation price survey · checked 2026-09-24
The day of the swap
Before the old one is drained
- The old plate photographed: system, fuel, input. The new one is the same system; the input is not a spec.
- The heat loss on paper, and the radiators added up. The contractor who sizes from the old plate is guessing.
- Water or steam: a steam boiler comes with a piping drawing, and the quote follows it.
- Chimney: what else is in it. A water heater left behind a condensing boiler is relined, on the quote.
- A floor drain within reach of the boiler, or a condensate pump on the quote.
- Hot water decided: combi for one bathroom, an indirect tank for two.
- Switching from oil: gas line, tank out, and the utility's schedule for the meter.
- The cleaning booked: yearly for oil, every year or two for gas, about $180 to $280.
What goes wrong
01Sizing to the old plate
Old boilers were oversized as a rule, and a new one the same size short-cycles.
Instead — The heat loss with 15% for the pipes; the radiators for steam.
02Buying 95% for a just-enough house
With 150 ft of baseboard the water runs at 169°F on cold days and nothing condenses: 91% real, 27 years to pay back.
Instead — More baseboard first, or cast iron kept — then the condensing boiler earns its number.
03Forgetting the water heater in the chimney
It is left alone in a flue sized for both. It condenses, corrodes the flue, and can backdraft.
Instead — A stainless liner, $950, on the quote and not mid-job.
04Cheap near-boiler piping on steam
A header without the drawing's rises and the equalizer carries water into the mains: banging, surging, wet steam.
Instead — Black iron to the manufacturer's drawing, and a contractor who does steam.
05A combi for two bathrooms
Three gallons a minute of hot water is one shower. The second one runs cold.
Instead — A boiler and a 40 gallon indirect tank.
06A bigger boiler for cold rooms
The rooms are cold because the radiators cannot deliver the loss, not because the boiler cannot make it.
Instead — Baseboard added where the rooms are cold, then the boiler sized to the loss.
07Condensate into a cast-iron drain
It is mildly acidic, and it eats the drain.
Instead — A neutralizer cartridge before the drain or the pump.
08Skipping the oil cleaning
Soot on the exchanger is insulation on the wrong side: fuel up, output down, and a sooted chimney.
Instead — Every autumn, about $280.
- 01Read the old plateWater or steam, gas or oil, and the input. The replacement is the same system; the input is a clue, not a spec — old boilers were oversized.
- 02Size the houseSquare feet, climate zone, insulation and glazing give the heat loss. Steam skips this step: it is sized to the radiators.
- 03Add up the radiatorsFeet of fin-tube, or the count of cast-iron radiators, or the square feet of radiant floor. This is what the boiler can actually deliver into the rooms.
- 04Find the water temperatureThe loss divided by the radiation says how hot the water must be on the coldest day. Under 140°F, a condensing boiler condenses; over 170°F it does not.
- 05Look at the chimney and the floorA water heater sharing the flue needs a liner if the boiler goes condensing; no drain nearby means a condensate pump.
- 06Run every boiler for ten wintersThe installed price is the small end of the bar. The calculator puts each option on the same house at national fuel prices.
The sizing this page starts from — square feet, zone, insulation, glazing — with the cooling side as well, for a house deciding whether the boiler is the thing to replace at all.
The heat loss on its own- Boiler vs furnace
- A boiler heats water (or makes steam) for radiators, baseboard or a floor; a furnace heats air for ducts. The house has one or the other, and the replacement is the same kind.
- Input / DOE capacity / net rating
- The three numbers on the plate. Input is fuel burned; DOE capacity is what comes out; net rating is capacity less an allowance for the pipes — 1.15 for water, 1.33 for steam.
- AFUE
- Annual fuel utilization efficiency: the share of the fuel that becomes heat in the house over a season. 84% is the federal minimum for gas hot water; 95% is a condensing boiler that is allowed to condense.
- Condensing boiler
- One whose heat exchanger pulls the last heat out of the flue gas by condensing the water vapour in it. Only happens with return water under about 130°F.
- Outdoor reset
- A control that lowers the water temperature as the weather warms. It is how a condensing boiler condenses on a house that needs 170°F on the coldest day.
- EDR
- Equivalent direct radiation, in square feet: how a cast-iron radiator is rated. 170 BTU per square foot on hot water, 240 on steam. A steam boiler is sized to the total.
- Pickup factor
- The allowance for heating the pipes before the radiators: 15% for water, 33% for steam. Built into the net rating.
- Near-boiler piping
- On steam, the header, equalizer and Hartford loop in black iron that separate water from steam. Done to the drawing, the system is quiet; done cheaply, it bangs.
- Orphaned water heater
- A gas water heater left alone in a chimney sized for it and the boiler, after the boiler goes condensing and vents through the wall. It needs a liner.
- Indirect water heater
- A tank heated by the boiler's water through a coil, on its own zone. The most efficient hot water a boiler house can have.
- Combi boiler
- A condensing boiler with a plate exchanger that makes hot water on demand. One box, no tank; about one shower at a time.
- Short cycling
- A boiler too big for the load firing and stopping every few minutes. Wears it out and wastes fuel; the reason not to round up a size.
Common questions
- How much does it cost to replace a boiler?
- About $4,750 for a cast-iron gas boiler replaced like for like on an 1,800 sq ft house in a cold zone: a 105,000 BTU unit, the expansion tank, circulator and valves that go with it, the old one out, the permit and 12 hours of a licensed contractor. Going condensing is $8,108 with the plastic vent, the condensate line and the chimney liner the orphaned water heater needs. Oil is $6,140, steam $6,883, a combi $8,116, an electric boiler $3,958.
- How much does a gas boiler installation cost?
- $4,750 for a standard 84% cast-iron boiler on the chimney, $8,108 for a 95% condensing boiler vented through the wall — $6,873 if there is no water heater left in the chimney to reline. The equipment is $2,700 against $3,913; the rest of the difference is the vent, the condensate, the liner and the extra hours. Whether the 95% is real depends on the radiators, which is the next question.
- Is a condensing boiler worth it?
- Only if the radiators let it condense. A condensing boiler reaches its 95% with return water under about 130°F; this house, with 150 ft of baseboard against a 72,000 BTU loss, needs 169°F water on the coldest day, so it runs at 91% over the season — $1,476 a year against $1,599 on cast iron, a 27 year payback on the $3,358 extra. With 250 ft of baseboard the water runs at 138°F, the boiler condenses all winter, and the payback is 18 years. On a radiant floor it is the only boiler to buy.
- What size boiler do I need?
- A hot-water boiler is sized to the house's heat loss with a 15% allowance for the pipes: 72,000 BTU an hour for this house, so 105,000 BTU input at 84%. Not to the old boiler's plate — they were oversized as a rule — and not to the radiators, which only say how hot the water must be. A steam boiler is the exception: it is sized to the connected radiation, 300 sq ft of EDR × 240 × 1.33 for twelve radiators, and the house's loss does not enter into it.
- How much does an oil boiler replacement cost?
- About $6,140 like for like — an 87% cast-iron boiler with a pressure burner, 14 hours, the old one out. It burns about $3,137 a year on this house at $3.80 a gallon, against $1,599 on gas, and it is cleaned every year at about $280. Switching to a condensing gas boiler instead is $10,493 with the gas line and the tank out, and saves about $1,661 a year in fuel.
- How much does a steam boiler cost to replace?
- About $6,883 for a gas steam boiler on twelve cast-iron radiators — more than the water boiler for the same house because of the near-boiler piping: header, equalizer, Hartford loop and low-water cutoff in threaded black iron, $750 of it, done to the manufacturer's drawing or the system bangs. It is sized to the radiators — 300 sq ft of EDR — not to the house, and there is no condensing steam boiler for a house.
- Combi boiler or boiler with an indirect tank?
- A combi is $8,116 and makes hot water on demand at about three gallons a minute: one shower at a time, a slow bath, and it is the right answer for a small house with one bathroom. A condensing boiler with a 40 gallon indirect tank is $9,988, gives the boiler a hot-water zone and gives two bathrooms all the hot water they want. Past one bathroom, the tank.
- Why does the water heater need a chimney liner?
- Because the old boiler kept the chimney warm. A condensing boiler vents through the wall in plastic and leaves the gas water heater alone in a masonry flue sized for both: its small, cool exhaust condenses on the way up, the acid eats the mortar, and on a cold day it can backdraft into the basement. A stainless liner sized to the water heater, about $950, is on every condensing conversion where a water heater stays behind, and on the quotes that forgot it, it turns up mid-job.
- Can I keep the boiler and add baseboard?
- Often the better move. With 150 ft of baseboard this house needs 169°F water on the design day; with 250 ft it needs 138°F, which is what lets a condensing boiler condense. Baseboard is cheap next to a boiler, and a house whose radiators were always a little short — $22,500 BTU short with 90 ft — is not fixed by a bigger boiler.
- How much is an electric boiler?
- About $3,958 installed, the cheapest box and the simplest job — no flue, no fuel line, a 60 to 100 A circuit. And about $4,811 a year to run on this house at national electricity prices, against $1,476 for condensing gas. It makes sense where there is no gas, the house is small, or the power is cheap; otherwise the chart makes the case against it.
- How much does boiler cleaning cost?
- About $180 a year for gas — burner, heat exchanger, flue, controls, every year or two — and $280 for oil, which is every year without exception: the nozzle, the filter, the exchanger brushed, the draft set. A condensing boiler adds a flush of the condensate trap. Skip an oil cleaning and the soot costs more in fuel than the visit.
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About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
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