How to Season Firewood for Your Outdoor Wood Boiler

If your outdoor wood boiler is full of wood but the water temperature keeps slipping overnight, the wood itself is often the problem. You might notice the backup heat kicking on more often, or wood that hisses, bubbles sap from the cut ends, or puts out heavy smoke. All of those point to firewood that is still holding too much water.

The fix is seasoning your firewood, which means drying split firewood until it tests below 20% moisture. Dry wood has less water to boil off, so the fire burns hotter and more of each load goes into heating the water. 

This guide covers how to season firewood, from choosing and cutting it to stacking and drying, plus a quick test for telling whether the problem is the wood or the boiler.

Why Wet Wood Makes a Wood Boiler Fall Behind

Freshly cut firewood holds a lot of water, and red oak can run 60% to 80% moisture right after it's felled. Before that wood can burn, the fire has to boil the water out, and the heat used for that never reaches the water jacket. Boiling off moisture also cools the fire, so the wood burns less completely. The result is less heat from each load, more smoke, and faster creosote buildup in the chimney.

On a mild day the boiler can usually make up the difference, but on the coldest nights it falls behind, which is when the water temperature starts to slip.

Green Wood vs. Seasoned Wood

That heat loss is the practical difference between green and seasoned wood. Green wood is freshly cut and still holds most of its moisture, while seasoned wood has been split and stacked long enough to dry to about 20%. Any type of wood can be either one, so a cord of green oak can give less usable heat than a cord of seasoned ash. Type of wood still matters, but mostly once the wood is dry.

Choosing Firewood for an Outdoor Wood Boiler

Any wood will burn in a wood boiler, but some species hold far more heat per cord and burn much longer than others.

Hardwood vs. Softwood 

Hardwoods are denser than softwoods, so a cord of hardwood holds more heat. For example, a cord of air-dried oak carries about 25 million BTUs, compared with about 15 million for a cord of pine. That density also makes hardwood burn slower and build a lasting bed of coals, which is what carries a boiler through the night between loads.

Softwoods like pine, spruce, and hemlock light fast and burn hot, which makes them useful for getting a fire going. A load of softwood burns down quickly, though, and the resin in conifers adds tar and creosote to the chimney. Some softwood mixed into a hardwood load works fine, while a boiler running mostly on softwood will need more frequent loading and chimney cleaning.

Good Hardwood Species for a Wood Boiler

The hardwoods below are common choices for boiler firewood, though drying time varies quite a bit from one to the next.

• Oak: Gives among the highest heat per cord and burns for a long time, but has one of the longest drying times.
• Ash: Splits easily and dries faster than oak.
• Cherry: Gives moderate heat, splits easily, and dries well.
• Mulberry and hackberry: Dense, good-burning options in areas where they grow.

How to Cut and Split Firewood for a Wood Boiler

Once you've picked your wood, how you cut and split it decides how fast it dries. Water leaves firewood mostly through its cut ends and split faces, so the sooner a log is cut to length and split, the sooner it starts seasoning.

1. Measure your boiler first. Check the firebox depth and the width of the door opening before you cut anything. Cut pieces a few inches shorter than the firebox so they load without jamming, and keep lengths consistent so they stack and load evenly.
2. Cut logs to length. On a sawbuck or a log supported at both ends, the middle sags as you cut and closes the kerf on the bar. Cut down about a third of the way, then finish with an upward cut from underneath so the log separates without pinching. For a log on the ground, cut most of the way through, roll it a half turn, and finish from the top so the chain stays out of the dirt.
3. Split every piece. Bark slows drying, so an unsplit log can stay wet for well over a year. Split each round so it fits through the door with room to spare, and keep a mix of sizes. Larger splits hold a fire longer overnight, while smaller ones dry faster and help rebuild a fire on hot coals. Start splits along the cracks already running through the end of the round, and split in cold weather when you can, since frozen wood splits more cleanly.
4. Stack it right away. Rounds and splits left in a heap lose weeks of drying time, especially the pieces buried in the middle. Move split wood straight to a stack where sun and air can reach it.

How to Stack and Store Firewood

Airflow does most of the drying work, so the goal is to let sun and wind reach as much of the wood as possible while keeping rain and snow off the top.

How to Stack Firewood

A good stack starts with where you put it and what it sits on.

1. Pick a sunny, open spot. Wood stacked in shade dries much more slowly, especially through the summer, when sun and warm air do most of the work. If you can, set the rows so the prevailing wind blows across them.
2. Get it off the ground. Stack on pallets or rails so air moves underneath and the bottom row doesn't soak up moisture from the soil.
3. Stack loosely, one or two rows deep. Tight, deep piles trap moisture in the middle, while loose stacks let air reach every piece.
4. Leave space between rows. Keep at least six inches between stacks so air can move through, and keep stacks a few feet from the house so insects and rodents in the woodpile stay out of your walls.

How to Cover Firewood

Cover only the top of the stack and leave the sides open so air can move through. A tarp wrapped over the whole pile, or pulled down into a tent, traps the damp air the wood is trying to release, which leads to mold and blackened wood. A sheet of metal roofing, or a tarp laid across the top and weighted down, keeps rain off without sealing the sides. A stack can stay uncovered through a dry summer, but cover the top before fall rain and snow set in.

Storing Firewood in a Woodshed

A woodshed keeps rain and snow off the wood while still letting air through, as long as the sides are open or slatted. A clear plastic or polycarbonate roof lets the sun heat the shed and speeds drying, though it also traps humid air on still days. Leaving any doors open as much as possible lets that moisture blow out.

How Long to Season Firewood

Most firewood needs 6 to 12 months of drying after it's split and stacked to reach about 20% moisture. How long firewood takes to dry within that range depends on the species, the size of the splits, and how much sun and wind the stack gets. Wood also dries more slowly in humid climates or shaded yards. Cutting and splitting wood a full year before you plan to burn it gives most stacks enough time, with some margin for a wet summer.

How Long Does Oak Take to Season?

Plan on at least a year for oak, and closer to two for large splits or stacks that don't get much sun. Oak's density and tight grain hold water longer than lighter hardwoods like ash or cherry, which is why it gives so much heat once it's dry and why it's so easy to burn too early. Test a few pieces before loading oak into the boiler, even if it's been stacked for a year.

When Is the Best Time to Cut Firewood?

The best time to cut is whenever it gives the wood a full drying season before you burn it. A living tree's moisture content changes very little through the year, so the clock that matters starts once the wood is split and stacked. Many owners cut in late winter, when frozen ground makes it easier to get equipment into the woods and frozen rounds split cleanly. Wood cut and stacked by spring then gets a full summer of sun and wind before fall.

How to Check Firewood Moisture Content

A moisture meter is the only reliable way to know whether wood is ready. The outside of a split dries first, so split a piece in half and press the meter pins into the fresh inside face. Check a few pieces from different parts of the stack, since wood in the middle or on the bottom row often dries more slowly. A reading below 20% means the wood is ready to burn.

Without a meter, seasoned wood usually shows a few of these signs:

• Cracks running out from the center on the cut ends
• Ends that have faded to gray
• Bark that is loose or falls off easily
• A noticeably lighter weight than a fresh piece of the same size
• A hollow knock when two pieces are struck together

How to Tell If Wet Wood Is Your Boiler’s Problem

When a boiler can't hold temperature, the cause is usually the wood or the equipment, and a simple test tells you which one to work on first.

Test Your Boiler With Kiln-Dried Firewood

Kiln-dried firewood is dried to below 20% moisture, so it shows what your boiler can do on dry fuel.

1. Buy enough kiln-dried firewood from a hardware or farm store for at least one full load.
2. Burn it on a cold day in place of your usual wood.
3. Watch whether the boiler reaches its set water temperature and holds it through the burn.

If the boiler holds temperature on kiln-dried wood, your own firewood is the problem. Check it with a moisture meter, give it more drying time, and look at how it's stacked and covered. If the boiler still struggles on dry wood, the problem is in the equipment. 

Common Questions About Firewood for Wood Boilers

If you're working with less-than-ideal firewood, these answers cover what to expect from green wood, unsplit rounds, and softwood in a boiler.

Can You Burn Green Wood in an Outdoor Wood Boiler?

You can, but the boiler will use more wood to make the same heat, and the chimney will build up creosote faster. If some green wood has to go in, mix it with seasoned hardwood so the coal bed stays hot enough to burn it.

Does Firewood Need to Be Split to Season?

Yes. A whole round dries mainly through its cut ends because the bark slows moisture from escaping through the sides, so it can take much longer to season than split wood. Splitting exposes the inside of the wood to air, and split pieces also load more easily and burn more evenly in a boiler firebox.

Can You Burn Softwood in a Wood Boiler?

Softwood is safe to burn in a wood boiler, though it burns fast and leaves more creosote than hardwood. It works well for starting fires or mixed into a hardwood load, and if you burn a lot of it, check the chimney more often.

Getting Your Firewood and Boiler Ready for Heating Season

Well-seasoned firewood is split, stacked off the ground with the top covered, and dried until it tests below 20% moisture. Most wood needs 6 to 12 months to get there, and oak needs at least a year. 

Dry wood works best in a boiler that's been cleaned and treated before the season starts. Our guides on pre-season boiler cleaning and wood boiler maintenance tips walk through both. 

Alternative Heating & Supplies stocks wood boiler parts for WoodMaster, HeatMaster, Heatmor, Central Boiler, and other major brands. If you have questions about a part or your setup, contact our team. For more repair walkthroughs, visit the Alternative Heating YouTube channel.