Outdoor Wood Furnace Size Calculator

As a planning estimate: multiply heated area by your climate-zone design factor (the 7-zone PickHVAC table: Zone 1 = 30 up to Zone 7 = 60 BTU/hr per sq ft). A 2,000 sq ft home in Zone 5 (50 BTU/hr per sq ft) needs about 100,000 BTU/hr of design heat load; add ~5% for underground line loss (105,000 BTU/hr) and round up to a 105,000 BTU/hr unit. Dealers also use a sizing value of 0.06 x sq ft: 2,000 x 0.06 = 120, which falls in the Central Boiler CL 4030 class (0-170 range). Confirm with a heat-loss calculation (Manual J) before buying.

Divide your annual delivered heat by wood energy and furnace efficiency. A 2,000 sq ft Zone 4 home needs about 235.6 MMBTU per year; at 65% furnace efficiency and 22 MMBTU per seasoned cord, that is about 16.5 cords per year. Real users report 8-12 cords for 2,500 sq ft in cold winters (e.g., a Central Boiler 6048 owner burning ~8 cords), and 10-20 cords in severe northern climates, depending on insulation and how much hot water is heated.

It depends on firebox size and heat load. One firebox load holds firebox volume x 0.75 fill x 171,875 BTU per stacked cubic ft (22 MMBTU per 128 cu ft cord) x efficiency. A 23 cu ft firebox (Central Boiler Classic Edge 865 HDX) holds about 1.93 MMBTU net per load at 65% efficiency: roughly 7.9 hours at its full 245,000 BTU/hr rating, or 16-24 hours at typical partial load. That firebox yields about 7.4 loads per cord.

Wood usually wins on fuel cost per MMBTU. At EIA prices from October 5, 2026 (heating oil $6.042/gal, propane $2.598/gal US average), delivered heat costs about $51.51/MMBTU for oil, $30.86 for propane, $49.82 for electric resistance, versus about $17.48 for wood at $250/cord and 65% furnace efficiency. For 235.6 MMBTU/year that is roughly $4,120 for wood versus $12,137 for oil: about $8,018 saved per year, paying back a $12,000 installation in about 1.5 years.

New units must be EPA-certified under the NSPS for residential wood heaters (Subpart QQQQ, hydronic heaters). Step 1 (May 15, 2015) limited particulate emissions to 0.32 lb per MMBTU of heat output; Step 2 (May 15, 2020) tightened that to 0.10 lb/MMBTU (0.15 if tested with cordwood), roughly 3 times cleaner. Phase 2 white-tag models are over 80% cleaner than old uncontrolled units. Many states and towns add their own rules or outright bans, so check your local code before buying; this is general information, not legal advice.

The EPA NSPS sets no federal setback; states and towns do, and they vary widely. Examples: Pennsylvania requires at least 50 ft from the property line with a stack at least 10 ft above the ground; Allegheny County PA requires 150 ft; Old Town, Maine requires 100 ft with operating months limited to October 15-April 15; Wawayanda, NY requires 100 ft setbacks on lots of 2+ acres and 25 ft from the structure being heated. Always verify your own municipality's ordinance; this is general information, not legal advice.

Stored heat equals gallons x 8.34 lb/gal x temperature swing in F. A 360-gallon unit (Central Boiler Classic Edge 865 HDX) with a 20F swing stores about 60,048 BTU. Circulator flow follows GPM = BTU/hr / (500 x delta-T in F): a 100,000 BTU/hr load at 20F delta-T needs 10 GPM, which calls for 1-1/4 in PEX or larger. Use insulated underground pipe; keep total head loss within your circulator's curve.

Yes, via a sidearm or plate heat exchanger on the water heater, and most owners do. Add about 25,000 BTU/hr of peak load to your sizing when including hot water (planning figure; actual peaks depend on simultaneous showers and laundry). Year-round hot-water-only operation typically adds 30,000-60,000 BTU per day, which is why many owners keep the furnace running through summer.

Oversized units idle and smolder, producing more smoke and creosote and wasting wood; undersized units cannot hold water temperature on the coldest nights and fall back on backup heat. Size to your design heat load with the underground line-loss adder, not to the biggest firebox you can afford, and have your dealer run a real heat-loss calculation. A slightly larger firebox is fine for longer burn times; a much larger rated output is not.

This tool sizes OUTDOOR wood boilers/furnaces (hydronic heaters sited outside the building): it adds underground line loss, firebox burn-time math, wood-cord consumption, and EPA setback guidance. For an indoor forced-air furnace use our furnace size calculator; for an indoor hydronic boiler use our boiler size calculator; for pure firewood math (cord volume, species weight, seasoning) use our firewood stack calculator.