Boiler replacement cost

Replacing a gas boiler costs $3,500 to $14,000, typically around $7,500. A standard efficiency boiler on an existing system sits at the bottom of that; a condensing boiler that needs new venting and a new flue liner sits at the top.

A boiler heats water and circulates it through radiators or floor loops. It is a different job from a furnace, in a different part of the country, and it is worth knowing which you have before you start reading prices: if there is a large metal box blowing air into ducts, that is a furnace, and this is the wrong page.

What it costs

The venting row is the one that surprises people. A condensing boiler does not vent up the old chimney, and if a water heater was sharing that flue, the chimney now needs lining for the water heater alone.

Standard or condensing

A condensing boiler pulls extra heat out of the exhaust by condensing the water vapor in it, which is where the efficiency comes from. It only does that when the water returning from the radiators is cool enough, below about 130°F.

That is the catch nobody mentions. A condensing boiler bolted to a system designed for 180°F water, with radiators sized for it, spends most of the winter not condensing and delivering closer to 87% than 95%. You paid for the higher number and are running at the lower one.

Condensing makes sense when:

• You have underfloor heating, which runs cool by design
• Your radiators are generously sized, or you are adding to them
• The installer will set up outdoor reset control, which lowers water temperature in mild weather
• Gas is expensive where you live and the payback maths works

Standard efficiency makes sense when the existing radiators are small, the budget is tight, and the chimney is in good order.

Combi or not

A combi boiler makes hot water on demand and does away with the cylinder, which frees a cabinet. It is common in Europe and increasingly sold in the United States.

It suits a small household with one bathroom. It struggles with two showers at once, because it is heating water in real time rather than drawing on a store. If your house has two bathrooms in regular simultaneous use, a system boiler with a cylinder is the better machine even though it takes more room.

What a proper installation includes

Boiler replacements are where corners get cut, because most of the work is invisible once the jacket goes on.

• The near-boiler piping redone to the manufacturer's diagram, not reused. The most common installation fault, and it causes short cycling and noise for the life of the boiler.
• A system flush before the new boiler goes in. Decades of sludge in old radiators will ruin a new heat exchanger.
• A new expansion tank and a checked pressure relief valve.
• Correct sizing, from a heat loss calculation rather than the output of the old boiler. Old boilers were routinely oversized by half.
• Outdoor reset control on a condensing boiler. Without it you are not getting the efficiency you bought.
• The permit, and an inspection.

Frequently asked questions

How long does a boiler last?

Fifteen to thirty years for a cast iron standard efficiency boiler, and fifteen to twenty for a condensing one. Cast iron boilers are famously long-lived; the controls and the circulators fail long before the block does.

Is my old boiler really that inefficient?

Probably less than you are told. A well-maintained 1990s boiler runs around 80%. Going to 95% saves about 16% of your heating gas, not half of it. The saving is real; the payback is usually eight to fifteen years, not three.

Can I switch from a boiler to a furnace or heat pump?

You can, and it means installing ductwork throughout the house, which is a substantial job in a finished home. The usual alternative is air-to-water heat pumps, which drive the existing radiators, or mini splits for cooling alongside the boiler for heating.

Do I need to replace the radiators too?

Not usually. They last generations. If you are going condensing and want the efficiency, you may want larger ones in the coldest rooms so the system can run at lower water temperatures.

Why is my new boiler noisier than the old one?

Most often near-boiler piping done wrong, air in the system, or a circulator pumping into the expansion tank connection rather than away from it. All three are installation faults and all three are the installer's to fix.