Should a Saint Paul home with a boiler switch to a furnace?
Usually not: if a Saint Paul home's radiators and piping are sound, a new boiler keeps the quiet, even heat and avoids cutting ducts into plaster walls. Switching makes sense mainly when you're already opening walls in a remodel or the piping is failing. Homes that already have ducts should stay with a furnace.
Key points
- The distribution system decides more than the heat source: pipes and radiators favor a boiler, existing ducts favor a furnace.
- Boilers give quiet, even heat and easy zoning. Furnaces bring central AC, filtration and a humidifier through the same ducts.
- Our furnace replacements run $4,500–$9,500. National cost guides put a gas boiler replacement at $4,000–$9,000.
- Radiator homes add cooling with ductless mini-splits or a high-velocity small-duct system, not a full duct retrofit.
- Xcel Energy supplies gas and electricity in Saint Paul; the federal 25C credit ended for equipment installed after 2025.
Most Saint Paul homes with a boiler should replace it with another boiler. If the radiators and piping are sound, a new boiler keeps the quiet, even heat you already have and avoids cutting ducts through plaster. Switching to a furnace makes sense mainly when the walls are open for a remodel anyway or the piping itself is failing. And if your house already has ducts, stay with a furnace.
What Saint Paul houses already have
Walk Summit Hill, Cathedral Hill or Mac-Groveland and a lot of the houses heat the way they did when they were built: a boiler in the basement feeding cast-iron radiators, mostly hot water, some steam. Many of those boilers have been replaced once or twice while the original radiators and piping stayed. Move into the parts of Saint Paul that filled in after World War II, including much of Highland Park and the East Side, and forced-air furnaces with ductwork take over.
That history decides most boiler-or-furnace questions before anyone talks about equipment. The heat source is the cheap part to change. The distribution system (pipes and radiators, or ducts and registers) is the expensive part. So the practical question is usually "what do I replace my boiler with," and the answer is usually another boiler.
How each one heats, side by side
| Hot-water boiler | Gas furnace | |
|---|---|---|
| How heat moves | Water through radiators, baseboard or floor tubing | Air through ducts and registers |
| Feel | Even, quiet, no drafts | Faster response, some blower noise |
| Central AC | Needs a separate system | Shares the ducts |
| Filtration and humidifier | Not through the heating system | Mount on the furnace |
| Zoning | Zone valves or circulators, simple | Dampers and controls, more involved |
| Condensing option | Yes for hot water, not for steam | Yes, 90%+ AFUE |
A furnace heats air and blows it through ducts. A boiler heats water (or makes steam) and moves it through pipes. Each can be high-efficiency. A condensing furnace or boiler vents through PVC out a side wall and needs a condensate drain. A condensing boiler saves the most when it runs cooler water, which radiant floors allow and cast-iron radiators allow on mild days. An outdoor reset control lowers water temperature as the weather warms so the boiler condenses more of the season.
Steam is the outlier. Steam boilers don't condense, so a steam home can't reach the efficiency of a condensing hot-water boiler without converting the system, and that's a big job.
Upkeep differs too. A furnace needs a filter change every few months through the heating season and a yearly check of the burners, flame sensor, venting and heat exchanger. A hot-water boiler has no filter, but it has system pressure, an expansion tank, a circulator and radiator air to watch. A steam boiler needs its water level and low-water cutoff checked. Either one deserves a yearly look and a carbon monoxide check.
Which way to go: a quick guide
| Your house | Usually the better call | Why |
|---|---|---|
| Hot-water radiators, sound piping | New boiler, condensing if the radiators allow | Keeps the even heat; no duct work |
| Steam radiators | New steam boiler | Converting a steam system is a big job |
| Radiators, and you want central air | Keep the boiler, add ductless or small-duct cooling | Far less plaster damage than full ducts |
| Gut remodel with walls open | Price a furnace and ducts | Ducts can go in while the walls are open |
| Failing steam mains or returns | Price both | The piping bill can approach a duct system |
| Ducts and a furnace already | Stay with a furnace | The ducts are already paid for |
How much does it cost to replace a boiler or a furnace?
Our installed furnace range is $4,500–$9,500: roughly $4,500–$6,000 for an 80% AFUE furnace and $6,500–$9,500 for a 96–98% AFUE model. Our page on furnace replacement in Saint Paul explains where a house lands in that range, and our Saint Paul HVAC cost guide puts every system side by side.
We don't publish a boiler range, because steam versus hot water, the amount of radiation in the house and the venting change the job too much. We look first and quote in writing. For scale, national cost guides such as HomeGuide put a typical gas boiler replacement at $4,000–$9,000 and a high-efficiency boiler at $4,000–$10,000. Installs carry our 2-year labor warranty on top of the manufacturer's parts warranty.
Keeping a boiler still leaves choices. A new cast-iron boiler suits large radiator systems and every steam system. A wall-hung condensing boiler frees floor space and saves fuel on hot-water systems. A combi boiler, such as the Navien combi boilers we service, also makes your domestic hot water, which fits smaller homes, while a busier household usually does better with a boiler and an indirect water tank. Our boiler and radiant heat service page covers those options.
One Saint Paul detail shows up on both kinds of jobs. When a new condensing boiler or furnace moves to PVC venting, the water heater can be left alone in an old masonry chimney that's now far too big for it. That usually needs a chimney liner or a different water heater venting setup, and it should be in the quote from the start. Replacements also need a mechanical permit, and our permits guide walks through that.
What rebates can Saint Paul homeowners get for a boiler or furnace?
Xcel Energy supplies both gas and electricity in Saint Paul, so utility rebate questions for either option go through one company. Xcel's Minnesota rebate list has included high-efficiency gas furnaces and boilers, and some of its measures require an Xcel-registered contractor, so check the current terms and ask about that before you buy. We help with the paperwork where the program allows it. The federal 25C tax credit ended for equipment placed in service after December 31, 2025, and Minnesota's HEAR and HOMES rebates had not launched as of mid-2026. Our Xcel and CenterPoint rebate guide keeps the details current.
How do you add air conditioning to a radiator house?
The main reason people consider tearing out a boiler is summer. Radiator homes have no ducts for central air. There are two good ways around that without losing the radiators.
- Ductless mini-splits. An outdoor heat pump feeds wall, ceiling or floor units in the rooms that need them. Our installed range is $4,200–$12,500 depending on how many rooms you cover. Because they're heat pumps, they can also take mild-weather heating off the boiler. See ductless mini-splits in Saint Paul.
- High-velocity small-duct systems. Flexible tubing snakes through closets and joist bays to small round outlets. National cost guides such as HomeGuide put these at $3,500–$15,000+ installed. Our post on high-velocity HVAC for historic homes covers the trade-offs.
Either option usually costs far less, and damages far less plaster, than running a full duct system through a finished two-story.
When does switching from a boiler to a furnace make sense?
- You're gutting the house or large parts of it, so walls and ceilings are open anyway.
- The steam mains or return lines are failing and the piping repair bill starts to look like a duct system.
- You want central AC, whole-house filtration and a furnace-mounted humidifier, and you're willing to give up some closet and soffit space to get them.
- The house is small and simple enough that ducts can run in the basement to the first floor and up one chase.
Going the other way, from a furnace to a boiler, almost never pays unless you're adding radiant floors in a remodel or addition.
Getting both options priced
- Confirm whether the system is steam or hot water: a sight glass on the boiler means steam.
- Have the heat exchanger, near-boiler piping and radiator valves checked for leaks.
- Size the replacement properly: hot-water boilers to the house's heat loss (a Manual J load calculation), steam boilers to the radiation connected to them, never by copying the old nameplate.
- Get the chimney, water heater venting and permit into the written quote.
If your boiler is noisy or giving you trouble, start with a repair visit; our post on boiler banging and kettling explains the common noises, and our page on boiler repair in Saint Paul covers local service. A repair visit carries a $79 trip charge, waived when you approve the repair. If it's time to replace, we service and install both boilers and furnaces, so we can price a new boiler, a furnace conversion and a cooling add-on side by side in writing. Call 612-429-3465 to set up a visit.
Questions
Is a boiler more efficient than a furnace?
Not inherently. Both come in standard and condensing versions, and AFUE is measured the same way for each. A condensing boiler saves the most when it can run cooler water, which suits radiant floors and big radiators on mild days. Steam boilers don't condense, so a steam home tops out lower. In practice, the condition of the pipes or ducts matters as much as the rating.
How long do boilers last compared with furnaces?
A well-kept cast-iron boiler often outlasts a furnace, and the radiators and piping can outlast several boilers. Modern wall-hung condensing boilers are more like furnaces: efficient, with more electronics and more to maintain. Age alone shouldn't decide a replacement. The heat exchanger, parts availability and repair history matter more.
Do I need a permit to replace a boiler or furnace in Saint Paul?
Yes. Replacing a boiler or furnace normally requires a mechanical permit from the city, followed by an inspection. Older Saint Paul homes can also turn up venting or combustion-air issues at inspection, which is one reason to have them looked at during the quote rather than after the install.
Can I get central air in a house with radiators?
Yes, without tearing out the radiators. Ductless mini-splits put indoor units in the rooms that need them, and a high-velocity small-duct system runs flexible tubing through closets and joist bays. Many owners keep the boiler for winter heat and let heat pump mini-splits cover cooling and some shoulder-season heating.
Sources
- HomeGuide cost guides (2026 editions) homeguide.com