Interest in wood-fired hydronic heating systems continues to rise along with the price of conventional fuel. One heat source used for such systems, especially in rural areas of North America, is the outdoor wood-fired furnace.

Even though heat is carried away from them by a stream of water, they are called furnaces rather than boilers because they have unpressurized water compartments that surround their large fireboxes.

Although intended to be fueled with firewood, especially by the Environmental Protection Agency (EPA), I’ve learned that for some people, just about anything that fits through the firebox door is fair game for fuel. I even heard how one of these furnaces allowed a certain person to make a deer (which had been shot in violation of location regulations) disappear when he heard the game warden was on his way.

Outdoor wood-fired furnaces bring some baggage to system design. First, because they are unpressurized, water in piping connected to the furnace, and above the water level inside it, will be under subatmospheric pressure when the circulator is off. Second, because they are open-loop heat sources, no ferrous metal components, such as cast-iron circulators, should be used in portions of the system piped directly to the furnace.

Big Bang Theory

Figure 1 shows how subatmospheric pressure exists in piping above the water level in the furnace. This is the case when the circulator is off. When the circulator is on, the pressure at a given location in the piping may be positive or negative relative to the atmosphere. It depends on the head added by the circulator, the pressure drop caused by head loss along the piping, as well as the elevation of the piping.

In theory, water can remain in piping under subatmospheric conditions, provided that certain conditions are met. The water has to remain above its vapor pressure. When water reaches its vapor pressure, it boils. This vapor pressure depends upon the temperature of the water.

In Figure, the graph shows the relationship between boiling point and absolute pressure. For example, at an absolute pressure of 14.7 psi, which corresponds to normal atmospheric pressure at sea level, water boils at 212˚F. However, let’s say water finds itself at a pressure 5 psi below normal atmospheric pressure (e.g., 14.7–5 = 9.7 psia). According to Figure 2, it will boil if it reaches a temperature of about 190˚.

This could occur the instant a circulator shuts off in a distribution system where the 190º water is about 12 feet above the water level in the outdoor furnace. The sudden drop in pressure, relative to when the circulator is on, could lower the local pressure below the vapor pressure. The result will be strong banging sounds from the piping as the water flashes to steam. It’s not a sound anyone wants to hear coming from their heating system.

The higher the top of the distribution system is compared to the water level in the furnace, the more negative the water pressure, and the lower the temperature at which the water will boil. The formula below can be used to estimate the negative static pressure at the top of the system each time the circulator turns off.

Pstatic = -(0.433 x H)


Pstatic = static gauge pressure of the water at a given location (with circulator off) (psi)

H = height from top of system piping down to water level in outdoor furnace (feet)

For example, determine the negative static pressure of water located 16 feet above the water level in the outdoor furnace. If the temperature of this water is 190˚, will it boil at the top of the system when the circulator turns off?

First, determine the extent of the negative pressure using Formula 1:

Pstatic = -(0.433 x 16) = -6.9 psi

The water will boil if its static gauge pressure is lower than (more negative) the vapor pressure corresponding to 190˚. Figure 2 shows the vapor pressure of 190˚ water to be 9.5 psi absolute. This corresponds to a gauge pressure of 9.5–14.7 = -5.2 psi. Because the static gauge pressure at the top of the system (-6.9 psi) is lower than the vapor pressure of the water (-5.2), boiling will occur. This is a situation that must be avoided.

Another nuance of open hydronic systems is that air will enter through any possible leakage path located where the local pressure is subatmospheric. Such paths include float-type air vents, valve packings, circulator flange gaskets, or less-than-perfectly sealed threaded connections.

As air enters the piping, water slides back to the outdoor furnace. Over time, the water level could drop several feet, depending on the elevation of the furnace. When the circulator turns on, it may not be able to push this air pocket back around the circuit and refill the piping. Even if it can, who wants to listen to air bubbles gurgling through piping as the circuit refills?

Multiple Solutions

There are techniques that can help prevent nuisance boiling in open-loop systems supplied by outdoor wood-fired furnaces. One is to lower the water temperature. Another is to lower the height of the distribution system relative to the water level in the furnace. Still another possibility is to locate the furnace at a higher outside elevation given the constraints of the property, building locations, etc. All these options have their limitations.

In my opinion, the best solution is to avoid both the nuisance boiling and air admittance problems by using a stainless-steel heat exchanger to isolate the unpressurized outdoor furnace from what will then be a true closed/pressurized indoor distribution system. The concept is shown in Figure 3 (above).

The closed-loop portion of the system can contain the auxiliary boiler, cast-iron circulators and the standard trim that would be present in any modern system. Just think of the heat exchanger as the boiler in the system and design accordingly.

When this approach is used with an auxiliary boiler, make sure the circulator in the outdoor furnace is turned off whenever the auxiliary boiler is operating. It’s also a good idea to install a check valve so that heat produced by the auxiliary boiler doesn’t thermosyphon back outside.

Finally, please don’t scrimp on the piping or insulation system between the outdoor furnace and the interior of the building. Use a quality pre-insulated piping system specifically intended for buried installation. Size the piping for a reasonable small head loss, and be sure it’s compatible with the temperatures that outdoor wood-fired furnaces can create.

The emissions from outdoor wood-fired furnaces have been a contentious issue for years. So much so that some municipalities in my area have either banned any further installations or put significant restrictions on such installations, such as distance from property lines and required chimney heights. EPA, working in conjunction with manufacturers of outdoor wood-fired furnaces, has developed a white-label emissions standard that holds emissions to higher standards (e.g., lower particulates).

Even with these changes, it’s hard for me to see outdoor wood-fired furnaces surviving the ever-tightening standards for higher efficiency and lower emissions. Other technologies, such as wood-gasification, are likely to prevail in the long term. Still, tens of thousands of outdoor wood-fired furnaces are installed each year in the United States and Canada. If you’re going to get involved with one, be sure to stay mindful of what we’ve just discussed.

Publication date: 03/12/2012