How Much Wood Does a Wood Boiler Use? Calculator and Cord Estimates

A wood boiler can burn a little wood on a mild fall day and several times as much during a deep freeze. So the useful question is not "how many logs a day". It is "how many cords for the whole winter".

The answer depends on your house, your climate, your firewood and your boiler.

How to read your result

• Cords per winter is the most likely amount of wood your home will use.
• The range is there because no two winters are the same. A colder winter lands near the top.
• "Plan to stock" adds a reserve of about 15% so you do not run out in January.
• Typical heating day is the winter total spread over the days you actually heat. Spring and fall days use less.
• Coldest days are about twice a typical day, so you know how much wood to keep close to the boiler in a deep freeze.
• One cord lasts is how many heating days a cord gets you at the typical rate.
• The boiler cards are matched by the size of your home. Coverage is approximate, so final sizing still needs a heat-loss check.

Typical cords per winter by home size

This table uses the same assumptions as the calculator: average insulation, dry red oak and a UNI wood gasification boiler.

These are planning numbers, rounded to the nearest half cord. A home with new windows and good insulation will use less. A drafty older home will use more.

How much wood does a wood boiler use per day?

A wood boiler does not burn the same amount every day. A typical day in the heating season is much lighter than the coldest day, and spring and fall days are lighter still. A wood gasification boiler burns in batches, so a typical day is one or two loads and the coldest days are two or three.

The table assumes about 200 heating days, average insulation and dry red oak. The days add up to the winter total. A 2,000 ft² home burning 80 lb a day for 200 days uses about 16,000 lb, which is about 4.4 cords.

Wood type barely changes the weight you burn each day, because pounds of dry wood give about the same heat. Wet wood does change it. You burn about a third more.

Why the number changes from house to house

Two homes of the same size can burn very different amounts of wood. These are the biggest reasons:

• Climate: longer, colder winters need more heat.
• How leaky the house is: insulation, windows and drafts matter a lot.
• Thermostat setting: keeping the house at 74°F takes more wood than 68°F.
• Your firewood: denser wood gives more heat per cord.
• How dry the wood is: dry wood gives more heat from every log.
• Your boiler: a more efficient boiler wastes less of each log.
• Hot water: if the boiler also makes your hot water, you will burn more.

That is why square footage alone cannot give an exact answer.

Which firewood gives the most heat?

A cord is a measure of space, not heat. A cord of dense hardwood holds more wood, and more heat, than a cord of pine. Heat is measured in BTU. A bigger number means more heat.

The figures come from USDA Forest Service data for wood at about 20% moisture.

Pine works fine in a wood boiler. You just need to stack about half as much again as you would with hickory to get the same heat.

Dry wood saves wood

Wet wood wastes heat. The boiler has to boil off the water in the log before the wood can heat your house. It also burns cooler and smokier.

Burn wood at 20% moisture or less. For best results:

• Split the wood before you stack it.
• Keep the stack off the ground.
• Cover the top, but leave the sides open for airflow.
• Check a freshly split face with a moisture meter.
• Wait for 20% or less before burning.

Softwoods often need about six months to dry. Hardwoods often need closer to a year. It depends on your climate, the size of the logs and where you stack them.

Does a more efficient boiler use less wood?

Yes. A wood gasification boiler burns the smoke from the wood in a second, very hot stage. Older boilers let much of that heat go up the chimney.

MBTEK's UNI wood gasification boilers are rated at 86% or better. The TITAN Lambda is rated at up to 95%. A sensor adjusts the air for you, so the burn stays clean. At those ratings, the TITAN Lambda uses about 10% less wood than a UNI for the same heat.

A high rating does not fix wet wood, a boiler that is too big or a poorly planned heating system. Those can erase the savings.

Does a bigger boiler burn more wood?

No. Your house decides how much wood you need over the winter. The boiler size only decides how fast it can make heat.

A boiler that is too big can make things harder, because it makes more heat than the house can use. These are the UNI sizes and the approximate area each covers:

• UNI 15: up to 2,500 ft²
• UNI 40: up to 4,000 ft²
• UNI 60: up to 5,000 ft²
• UNI 95: up to 8,000 ft²

Coverage is approximate. Final sizing should be based on your home's heat loss, climate and insulation.

UNI wood gasification boilers

UNI 15 wood boiler, 65K BTU$5,111View product UNI 40 wood boiler, 130K BTU$7,106View product UNI 60 wood boiler, 200K BTU$8,553View product UNI 95 wood boiler, 330K BTU$11,209View product

Browse wood boilers

Can a buffer tank help?

A buffer tank stores the extra heat from a full load of wood and releases it later. The boiler can then burn hot and clean instead of idling when the house does not need heat. It is worth a look with any wood gasification boiler. See our guide to the why a boiler needs a buffer tank for how it works.

How to burn less wood and how much to buy

The best way to use less wood is to waste less heat.

• Burn dry wood, at 20% moisture or less.
• Seal drafts and add insulation.
• Keep the boiler and chimney clean.
• Insulate the hot water pipes.
• Do not overheat the house.
• Have the boiler sized to your home's heat loss.

If this is your first season, do not rely on a rule like "a house needs four cords". Instead:

1. Run your numbers in the calculator. 2. Use the wood you actually plan to burn, and be honest about how dry it is. 3. Add a reserve. Having exactly the number you calculated leaves no room for a cold winter. 4. Write down how much you burn in year one. Your own history is the best guide after that.

How much wood does a wood boiler use? The bottom line

Nobody can give one number for every house. The simple rule is:

Cords per winter = the heat your home needs ÷ the heat you get from each cord.

Dry wood, dense hardwood, an efficient boiler and a well-sealed house all push the number down. Use the calculator for your own estimate, then add a reserve.