Outdoor Furnaces are open to the atmosphere, which allows air into the furnace which causes oxidization of the metal. This corrosion inside of the furnace water jacket causes leaks and premature failure. The only answer is to pressurize and remove the air.

Real pressurized Boilers are rated in BTU output. Therefore Wood Doctor Boilers has moved to BTU output.

The size of any regular outdoor wood furnace or any EPA outdoor gasification furnace should be matched to the requirements. All wood furnace manufacturers use either a BTU rating or square footage capabilities.

Base Calculations

Wood Doctor Boilers are now rated in BTU so do not have to be concerned with this.

However if you purchase an Outdoor FURNACE In Canada and Northern USA, start your calculations by using 60% to 75% of the stated maximum square footage. This is usually a safe place to start under most conditions. When you exceed this amount, be very careful in your calculations. Upgrade to the next size of any outdoor furnace or EPA outdoor wood gasifying furnace long before you reach the maximum heating capability.

Ceiling Height

Calculations of an outdoor wood furnace heating ability is always based on new construction with an 8 ft. ceiling. For a 16 foot ceiling consider adding up 40% to 50% to that area.

Basement & Upstairs

VERY IMPORTANT Also add the square footage of the basement and the upstairs and any other area to be heated.

Building Age

The older the building the less likely it is to have adequate insulation and be airtight. New R2000 buildings have lower heating demands on the outdoor water furnace.

Distance From Furnace to Buildings

The better the underground insulation then the less impact from long distances. Some of the better underground insulations loose almost no heat in 300 feet.

Easy Rule of Thumb Solution

If your purchase is an Outdoor Furnace, and not a BOILER, once you pass the 3/4 mark on square footage, consider moving up to the next larger size of outdoor furnace. Over sizing never hurts; it will allow more time between fills. You do not want to regret buying a small wood stove that is maximized.

Problems Caused by Under Sizing.

When filling a properly sized boiler, you would normally add wood either morning and evening, or once per day, depending on the season. If the firebox is too small and it is a really cold night and you are using lots of heat, and most of the fire wood is already burned up; your outdoor furnace may not be able to continue making more BTU per hour than you are using. When you are low on wood in a high demand time, the temperature of the water in your outdoor furnace may drop from 180F to 160F to 140F to 120F. Anytime you are 150F or lower in any outdoor furnace the difference in the temperature between the water jacket and the firebox will cause condensation in the firebox. This condensation can cause steam in the firebox, which will cause a poor fire burn. It may seem like you are gobbling up wood and getting very little heat. Sometimes the quickest way to recover from this is to turn off the circulator until the water reaches 180F, and then the temperature will stay up there. With a properly sized firebox, this will never happen.

Free Calculation Service Provided

If you have an older home, multiple buildings, or simply would like to know for sure give us your measurements and we will calculate the proper sizing for you. Include your name, zip code and phone number as we may need to ask for more information.

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