Is an outdoor wood boiler cheaper than a gas furnace in Minnesota?
Usually not if natural gas reaches your house: counting bought cordwood or your own hours, a 95%-plus gas furnace is simpler and usually cheaper. A wood boiler pays best on rural land heated by propane, when you own the timber and are home to load it, and most owners keep a furnace as backup.
Key points
- An outdoor wood boiler heats water in the yard and pipes it to a coil in your furnace or to a boiler system, so the furnace usually stays.
- Wood is cheap heat only when you own the timber, the saw and the splitter, and you're home to load it.
- Smoke rules vary by city and county. Check them before you buy, and look for an EPA-certified model.
- If the fire goes out while you're away in January, the backup furnace is what keeps the pipes from freezing.
- In town on natural gas, a high-efficiency furnace wins on cost and effort almost every time.
If natural gas reaches your house, an outdoor wood boiler usually isn't cheaper than a 95%-plus gas furnace once you count bought cordwood or your own time. On rural land heated by propane, where you own the timber and someone is home to load the fire every day, wood can cut the heating bill hard. Either way, the furnace almost always stays, as the backup and as the blower that moves the heat.
An outdoor wood boiler is a firebox wrapped in a water jacket, set out in the yard with its own chimney. You see them on farms and rural lots past the edges of the metro and in Western Wisconsin. This compares the two the way they actually get used through a Minnesota winter.
How does an outdoor wood boiler heat a house?
You burn wood in the unit outside. The fire heats the water around the firebox, and a circulator pump sends that water through insulated pipes buried underground to the house. Inside, the heat gets handed off in one of two ways:
- A water-to-air coil in the furnace plenum. Hot water runs through a coil above the furnace, and the furnace blower pushes house air across it. The furnace's burners stay off unless the wood heat can't keep up.
- A heat exchanger into a hot-water system. In a house with a boiler and radiators, in-floor heat or baseboard, a plate heat exchanger passes the heat into that system.
Many setups also heat domestic hot water. The point to take away: in most homes with a wood boiler, the furnace or indoor boiler doesn't go away. It becomes the backup and the air mover. If you're choosing between the two, you're really choosing between a furnace alone and a furnace plus a wood boiler.
Fuel and labor: what you're really comparing
A high-efficiency gas furnace asks nothing of you except a filter change and a yearly tune-up. You pay the gas bill.
A wood boiler trades the fuel bill for work:
- Getting wood. Cutting, splitting and stacking cords of it, or buying it by the cord and still stacking it.
- Seasoning. Wet wood burns cooler, makes more smoke and creosote, and gives you less heat per load. Wood needs time under cover to dry.
- Loading. Once or twice a day, more in a cold snap, every day from fall to spring.
- Ash and water. Regular ash removal, and checking the water level and treatment so the jacket doesn't corrode.
If you own the timber, the saw and the splitter, and someone is home every day, wood can be very cheap heat. If you're buying cordwood, the savings shrink fast, and they shrink more if you value your time at anything.
The fair comparison depends on your fuel. In town with natural gas, a 95%-plus furnace is hard to beat on cost and effort. On a rural property heating with propane, the math tilts toward wood, and it also opens up other options. Our propane cost guide shows how to price propane heat per delivered BTU, and our post on switching from propane to a heat pump covers the electric route.
Run the numbers for your place
Price every option per million BTU of heat delivered into the house. For the fuels a furnace burns, the math is simple:
| Fuel and furnace | What you buy per million BTU delivered |
|---|---|
| Natural gas, 95% furnace | about 10.5 therms |
| Propane, 95% furnace | about 11.5 gallons |
| Propane, 80% furnace | about 13.7 gallons |
Those use the U.S. Energy Information Administration's standard figures: 100,000 BTU in a therm and about 91,500 BTU in a gallon of propane. Multiply by your price per therm or per gallon.
Wood is harder to pin down. EIA uses a rough figure of 20 million BTU per cord, but the real number swings with the species and how dry the wood is, and the share that reaches your house depends on the boiler's efficiency and the heat lost from the buried lines. Ask the manufacturer for the model's tested efficiency, then divide your price per cord by 20 times that efficiency (as a decimal) for a rough wood cost per million BTU. If you cut your own, the price per cord is your saw, fuel, splitter and hours, which is the part most comparisons leave out.
Why do cities regulate outdoor wood boilers?
Outdoor wood boilers sit low and burn day and night, so smoke drifts toward neighbors in a way an indoor woodstove's usually doesn't. That's why many cities, townships and counties regulate them with setbacks from property lines, minimum stack heights or outright bans, especially in the metro and on smaller lots.
Before you buy:
- Call the city or county zoning office and ask what's allowed on your parcel.
- Buy an EPA-certified unit. EPA's wood heater standards cover hydronic heaters, the category outdoor boilers fall under, and certified models burn much cleaner than older designs. EPA's Burn Wise program explains the standards and how to find certified models.
- Think about the wind. Put the unit where the prevailing winter wind carries smoke away from your house and your neighbors' windows.
What winter risks come with an outdoor wood boiler?
- The fire goes out while you're gone. A weekend away in January means someone has to load it, or the backup has to carry the house. If the backup isn't sized to heat the whole house alone, pipes are at risk.
- Cold snaps. At -20°F the house needs the most heat and the firebox burns through wood fastest, so loading gets more frequent right when drifted snow makes the trip to the unit hardest. Keep a shoveled path and a covered wood supply close to it.
- Power outages. Pumps, draft fans and furnace blowers all stop without electricity. If outages are common on your road, plan a generator connection whichever way you heat.
- The furnace side. A water coil in the plenum adds restriction to your furnace's airflow. Too much, and the furnace runs louder and can trip its high-limit switch when it fires on gas. Our furnace limit switch guide explains that shutdown.
- Underground lines. Poorly insulated or waterlogged buried lines lose heat to the ground all winter.
What each setup costs
Our furnace replacement range is $4,500–$9,500 installed: roughly $4,500–$6,000 for an 80% furnace and $6,500–$9,500 for a 96–98% condensing model. Wood boiler prices vary too much with size and site to quote here, so get the full installed price in writing, including trenching, the insulated lines, the coil or heat exchanger, wiring and the chimney. Then keep the furnace in the budget. You'll likely keep one as backup, and if yours is old, a wood boiler price that leaves it out isn't the whole bill.
If you're on propane and the reason for wood is the fuel bill, price a cold-climate heat pump too. Our installed range is $7,500–$14,500, and paired with the propane furnace it cuts gallons without the daily loading.
When does a wood boiler make more sense than a gas furnace?
- In town with natural gas: a high-efficiency gas furnace. Wood rarely pays for the labor, and local rules may not allow it anyway. Our page on furnace replacement in Minneapolis shows what a typical city furnace job involves.
- Rural, heating with propane, own timber, home every day: an outdoor wood boiler can offset a lot of propane, as long as the furnace stays sized to carry the house alone.
- Rural, heating with propane, no time for wood: look at a cold-climate heat pump paired with the propane furnace. Our dual-fuel guide explains how that works.
- Houses already heated by an indoor boiler: keep the boiler healthy either way. Our boiler service page covers what we do on hot-water systems.
Warning signs, and when to call
| What you notice | Likely cause | What to do |
|---|---|---|
| Furnace shuts off on its high limit when it fires on gas | The plenum coil restricting airflow, or a clogged filter | Change the filter, then have airflow and static pressure checked |
| House cools off with a good fire going | Circulator pump, air in the lines or low water in the jacket | Check the unit's water level and pump; call if it doesn't recover |
| Backup furnace doesn't start when the fire dies | Thermostat setup or a furnace fault | Get it fixed before the next cold night |
| Water on or around the furnace | A leaking coil or fitting in the plenum | Have it checked before water reaches the furnace controls |
| Smoke smell inside, or a CO alarm | A venting or combustion problem | Get everyone out and call 911 from outside |
If the backup furnace won't run on a cold night, our no-heat checklist covers what's safe to check before a technician arrives.
What to do next
Whatever you decide about wood, the backup furnace or boiler has to be ready to carry the whole house on the coldest night. If yours is old, noisy or tripping out, have it checked before winter. Our $79 trip charge is waived when you approve the repair, a furnace tune-up is $129, and replacements are sized with a Manual J load calculation. Call 612-429-3465, open 7 days, 7am–9pm.
Questions
Do outdoor wood boilers work during a power outage?
No. An outdoor wood boiler needs electricity to run its circulator pump and draft fan, and the furnace blower that moves the heat into the house needs power too. A gas furnace also needs power for its blower, inducer and controls. Either way, a properly installed generator connection is what keeps heat on during an outage.
Can an outdoor wood boiler heat my whole Minnesota house by itself?
A correctly sized one can, as long as someone keeps it loaded. The weak point is people, not the boiler: a weekend away, a sick week or a -25°F night with a half-loaded firebox. That's why most owners keep a gas, propane or electric furnace sized to carry the whole house on its own.
Does adding a wood boiler coil affect my furnace?
It can. The water-to-air coil sits in the supply plenum, so the blower pushes all of the house's air through it every cycle. A coil that's too restrictive raises the static pressure the blower works against, and the furnace can overheat and trip its high limit when it fires on gas. Ask whoever sizes the coil to measure static pressure with it installed and compare it with the furnace's rating.
Are outdoor wood boilers allowed in the Twin Cities suburbs?
It depends on the city. Many cities regulate them with setbacks, stack-height rules or outright bans because of smoke, and small suburban lots are where they are hardest to place. Rules differ between cities, townships and counties, and some change over time. Call your city or county zoning office before you buy anything, and get the answer in writing.
Sources
- Burn Wise program epa.gov