HPAC Magazine

Room For Improvement

February 1, 2013 | By John Siegenthaler

Outdoor wood-fired heaters bring some baggage to system design.

Interest in wood-fired hydronic heating systems continues to rise, particularly in areas where wood competes against fuel oil. One heat source used for such systems, especially in rural areas of North America, is the outdoor wood-fired heater. An example of such a product is shown below.

Heat is carried away from these appliances by a stream of water. Despite this, they are called “heaters” rather than boilers, because non-pressurized water compartments surround their large fireboxes.

Although firewood is the intended fuel, I have 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 outdoor heaters allowed a certain person to make a deer, shot in violation of local regulations, disappear when they heard that the game warden was on his way. For the record: I am not suggesting the use of deer as fuel.

There are system design issues with outdoor wood-fired heaters. First, because they are non-pressurized, water in piping connected to the heater, and above the water level inside it, will be under sub-atmospheric pressure when the circulator is off. Second, because they are open heat sources, no ferrous metal components, such as cast iron circulators, should be used in portions of the system piped directly to the heater.


The water level in the outdoor unit establishes the point of 0 gauge pressure within the boiler circuit when the circulator is off. Figure 1 shows how sub-atmospheric pressure exists in piping above that water level. When the circulator is on, the pressure at a given location in the piping may be positive or negative relative to atmospheric pressure. 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 sub-atmospheric conditions, provided that certain conditions are met. One of those conditions is that the pressure on the water has to remain above the water’s vapour pressure. When the pressure exerted on water reaches its vapour pressure, it boils. This vapour pressure depends upon the temperature of the water as shown in Figure 2.

This graphic 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, and 0 gauge pressure) water boils at 212F. However, let’s say water finds itself at a pressure five 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 190F.

This could occur the instant a circulator shuts off in a distribution system where the 190F water is about 12 feet above the water level in the outdoor heater. The sudden drop in pressure, relative to when the circulator is on, could lower the local pressure below the vapour pressure of the water. The result will be an immediate steam flash inside the piping. This will produce strong banging sounds that rival solid blows from a 24-ounce mallet hammer. It is not a sound people want to hear coming from their heating system.

The higher the top of the distribution system is compared to the water level in the heater, the more negative the water pressure and the lower the temperature at which the water will boil. Formula 1 can be used to estimate the negative static pressure at the top of the system each time the circulator turns off.

Formula 1



Pstatic – static gauge pressure of the water at a given location (w/ circulator off) (psi)

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

Here is an example: Determine the negative static pressure of water located 16 feet above the water level in the outdoor heater. If the temperature of this water is 190F, 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: 


Now, convert the negative gauge pressure to absolute pressure. Just subtract 6.9 psi from atmospheric pressure (14.7 psi).


 Water will boil whenever the absolute pressure on it is lower than its vapour pressure. Figure 3 shows the vapour pressure of 190F water to be 9.5 psia. Thus, boiling will occur.

If you start with the absolute pressure at the top of the system (7.8 psia), find this pressure on the vertical axis of Figure 2, draw a line from there to the curve, and then another line down to the lower axis, you will find that water at about 182F or higher would boil under these conditions. This is a situation that must be avoided.


Another nuance of open hydronic systems is that air will enter through any possible leakage path that is located where the local pressure is sub-atmospheric. 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 drains back to the outdoor heater. Over time, the water level could drop several feet. When the circulator turns on, it may or may not be able to push the 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 itself on every call for heat?


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

In my opinion, to avoid nuisance boiling and air admittance problems the best solution is to use a properly-sized stainless steel brazed plate heat exchanger to isolate the non-pressurized outdoor heater from what will then be a true closed/pressurized indoor distribution system. The concept is shown in Figure 3.

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 hydronic system. Just think of the heat exchanger as the boiler and design accordingly. 

When this approach is used with an auxiliary boiler and the outdoor circuit is operating with an antifreeze solution, be sure your controls are set up to turn off the circulator in the outdoor circuit to prevent that circuit from scavenging heat from the heat exchanger. The circulator that carries heat from secondary side of the heat exchanger to the closely-spaced tees can also be shut off. It is also a good idea to install a check valve so that heat produced by the auxiliary boiler does not thermosyphon back outside.

If the outdoor unit operates with water, the detail in Figure 4 can be used to “trickle” just enough heat from the remainder of the system to keep the outdoor unit from freezing.

When the temperature sensor in the outdoor heater reaches a temperature that is near, but still a few degrees above freezing, a small “shunt” circulator routes warm water returning from the distribution system through the load side of the heat exchanger. The same temperature controller that handles this function also ensures that the circulator in the outdoor heater is operating. Flow through the load s
ide of the heat exchanger should be adjusted using the flow setter so that excessive heat is not transported to the outdoor heater and so that indoor comfort is not compromised. With the proper controls, the piping in Figure 4 allows the auxiliary boiler to fire to provide heat to the outdoor unit, even if there is no other call for heat in the system. 

Finally, do not scrimp on the piping or insulation system between the outdoor heater and the interior of the building. Use a quality pre-insulated piping system specifically intended for buried installation. Two one-inch PEX tubes wrapped up with bubble foil and buried 12 inches under the lawn is not going to do the job.

Size the piping for a reasonable small head loss. In some cases this means 1.25″ or even 1.5″ pipe, rather than the common one-inch PEX that is often sold in combination with these heaters. Remember, the fundamental hydronic system design principals that determine flow rates, head loss, and heat transfer prevail, wherever the Btus are produced. Why invest thousands of dollars in such a heater and then bury improperly sized or improperly insulated piping, under three or four feet of dirt, from the heater to the building?

If you plan to install an outdoor wood-fired heater, do it right with modern hydronic details. Use a heat exchanger between the outdoor circuit and remainder of the system. Also consider some of the other details discussed here. These modern materials and methods will help deliver more heat and better comfort from outdoor wood heaters. 

John Siegenthaler, P.E. is the author of Modern Hydronic Heating (the third edition of this book is now available). For reference information and software to assist in hydronic system design visit www.hydronicpros.com.



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