Boiler Replacement Guide: Types, Sizing and What to Expect

A boiler replacement is removing an old hot-water or steam boiler and installing a new one sized and piped for your home's radiators, baseboard or radiant floors. This boiler replacement guide covers when to replace, the main boiler types, how sizing differs for steam and hot water, venting, controls and what a good North Shore installation includes. Many older homes in Medford, Melrose, Lynn and Chelsea still run boilers from the 1980s and 1990s.

Key Takeaways

• Replace like for like in most homes. Hot water with hot water, steam with steam.
• Condensing boilers are the efficiency leaders. They work best with lower water temperatures.
• Steam boilers are sized differently. By connected radiation, not just heat loss.
• Combi boilers save space. They heat domestic hot water too.
• Piping and controls matter. A great boiler with poor near-boiler piping disappoints.

When Should You Replace a Boiler?

Boilers often last 20 years or more, so age alone rarely decides. Look at the full picture.

• Age past about 20 to 25 years with rising repair needs.
• Leaks from the boiler itself, especially a cracked section or heat exchanger.
• Rising fuel bills with no change in use.
• Frequent no-heat calls or parts that are hard to find.
• Safety concerns such as venting problems or soot.

Not every problem means replacement. Read common boiler problems first. Want a straight answer on yours? Request a boiler inspection and we will tell you whether to repair or replace, in writing.

What Are the Main Boiler Types?

Choosing between steam and hot water, or wondering about converting? See steam vs hot water boiler.

How Efficient Are New Boilers?

Efficiency is measured in AFUE. Per DOE's guide to furnaces and boilers, older low-efficiency systems often run at 56% to 70% AFUE, while high-efficiency condensing systems reach 90% to 98.5%. ENERGY STAR certified gas boilers must reach at least 90% AFUE.

A condensing boiler only condenses, and reaches its rating, when return water is cool enough. Systems with generous baseboard or radiant floors condense often. Systems with small radiators that need very hot water condense less. An outdoor reset control lowers water temperature on mild days to help.

How Is a Boiler Sized?

• Hot-water boilers: size to the home's heat loss from a room-by-room calculation, then check that radiation can deliver it.
• Steam boilers: size to the total connected radiation, measured in square feet of equivalent direct radiation (EDR). A technician measures every radiator.
• Do not copy the old nameplate. Many old boilers are oversized.

Venting, Piping and Controls

• Venting: condensing boilers vent through PVC or polypropylene to the outside. Non-condensing boilers use a chimney, often with a new liner.
• Near-boiler piping: modern boilers often need primary-secondary piping or a hydraulic separator to protect flow rates.
• Steam piping: steam boilers need correct header piping per the manufacturer to avoid wet steam and banging.
• Controls: outdoor reset, zone valves or circulators, and low-water cutoffs on steam.
• Domestic hot water: an indirect water heater or a combi boiler can replace an old tank.

How Much Does Boiler Replacement Cost?

Cost depends on boiler type, fuel, venting, piping changes and whether you add hot water. We break down typical industry ranges in boiler replacement cost. Massachusetts homeowners may qualify for Mass Save incentives on some qualifying equipment; check masssave.com for current amounts as of 2026.

Should You Add a Heat Pump Instead?

Many boiler homeowners add ductless heat pumps for cooling and efficient heat, and keep the boiler as backup. Some replace an aging boiler with a smaller one plus mini-splits. See heat pump with boiler and baseboard heat and our ductless mini-splits in Massachusetts guide.

What Happens on Boiler Replacement Day?

Most boiler replacements take one to three days, depending on piping and venting changes.

1. Shut off fuel and power, then drain the system.
2. Disconnect and remove the old boiler, which can weigh several hundred pounds.
3. Set the new boiler and build the near-boiler piping.
4. Install venting: PVC for condensing units, or a chimney liner for others.
5. Refill, purge air and add water treatment where required.
6. Fire the boiler, run a combustion test and check for carbon monoxide.
7. Set controls and walk you through operation.

In winter, plan for the heat to be off for at least part of a day. Good installers schedule to keep that short.

Caring for a New Boiler

• Book a yearly service with a combustion check.
• Watch the pressure gauge on hot-water systems.
• On steam, check the sight glass and flush the low-water cutoff as the manufacturer directs.
• Keep the condensate drain clear and protected from freezing on condensing boilers.
• Bleed air from radiators when needed.

A seasonal heating tune-up covers these checks.

What a Good Boiler Installation Includes

1. A heat loss calculation or radiation survey.
2. Correct venting and a chimney liner where needed.
3. Manufacturer-correct near-boiler piping.
4. System flushing and, for condensing boilers, water treatment.
5. Expansion tank, relief valve and fill valve checked or replaced.
6. Combustion analysis and a CO test at startup.
7. Old boiler removal.

Safety basics: keep CO detectors on every floor. Never cap a leaking relief valve; see boiler pressure relief valve leaking. If you smell gas, leave and call the gas company or 911.

Plan Your Boiler Replacement

This boiler replacement guide comes down to a few choices: the right type for your radiators, the right size, and a clean install with proper piping and venting.

Jose has worked in HVAC since 2015. HomePros installs high-efficiency Burnham and Weil-McLain boilers with a load calculation on every job. Get a written boiler replacement estimate, or call (781) 426-8428.