Smoke Gets in your Eyes
It’s time to do wood-fired heating right.
Every August, I make a short trip to a local event call the Woodsmen’s Field Days. This gathering is all about cutting down trees and then slicing them up into either firewood or lumber. Although it’s held in the small village of Boonville, N.Y., it has grown to one of the largest events of its kind in the nation. Several thousand people attend on all three days. If you like chainsaws, hydraulics, firewood processors and elk burgers, this is the place to be in late summer (Figure 1).
Plenty of wood-burning products are on display at this show. They range from kits for making barrel stoves to high-tech, pellet-fueled boilers from Europe. In the middle of this spectrum are several brands of outdoor wood-burning furnaces and single-stage wood-burning boilers.
If you look around the booths, you’ll probably find a gas/oil boiler such as the one in Figure 2 piped up and operating. You also might see some type of recommended installation drawings. Chances are high that the installation drawings don’t take advantage of the latest hydronic hardware or design methods. Here are some of the possibilities:
- A drawing that shows the wood-fueled boiler piped in series with a conventional boiler.
- A system where the wood-fueled boiler and a conventional boiler are piped in parallel, but where heated water flows through both boilers, regardless of which one is adding heat to the water.
- A 3/4 in. or 1 in. thermostatic mixing valve, the type designed for tempering domestic hot water, being used as the mixing device for a generous number of radiant floor circuits.
- The lack of any legitimate means of protecting the wood-fired boiler from low return water temperatures.
- The use of cast-iron circulators in nonpressurized “open” systems.
- The lack of a heat exchanger to separate a nonpressurized, wood-fired heater from the balance of the hydronic system.
- Various (sometimes seemingly random) placements for hardware such as pressure-relief valves, expansion tanks and air purgers.
Hydronics know-how needed
You will detect a strong sense of self-reliance in most of the people at the Woodsman’s Field Days. Many of these folks not only harvest their own firewood, they also install the device that burns the wood. As you might expect, a fair percentage of these people have little, if any, guidance on how to do the hydronics right. It’s also likely that no one else will inspect the resulting installation. They buy the unit, install it as they think it should be installed and live with the results.
My view is that any wood-burning boiler or outdoor furnace intended to heat water should take advantage of the best hydronics technology available. What’s the point in spending $12,000 to $15,000 on an outdoor wood-fired heater only to realize marginal comfort, or discovering that you now have to live with a variety of undesirable operating or maintenance issues?
Here are seven recommendations to improve these systems:
- Don’t use cast-iron circulators in “open” hydronic systems, such as those associated with most unpressurized outdoor wood-fired heaters. Instead, use stainless-steel or bronze circulators designed for open loops. Sure they cost more, but you know what — a chain saw costs more than an axe.
Install a properly sized stainless-steel heat exchanger that separates the open circuit associated with the wood-fired heater from what can be a fully closed hydronic distribution system. Size that heat exchanger for a maximum approach temperature difference of 10º F.
Designing the load side of the system as a closed loop helps eliminate problems such as air getting sucked into piping components that are under negative pressure, steam flash under low pressure, air gurgling through pipe and corrosion of ferrous metal components.
Take a look at the “insulated underground piping” shown in Figure 3 (page 15). This was one of the offerings on display at the Woodmen’s Field Days. Seriously, does anyone really think this will perform properly when buried in wet soil with rocks, tree roots and burrowing rodents?
Use high-quality, pre-insulated underground pipe between the outdoor unit and the building(s) being heated. That piping should have a durable, watertight jacket to keep the heat in and the ground water out. Be sure to use a watertight wall penetration sleeve that’s specifically designed for the pre-insulated pipe. Squeezing multiple tubes of silicone caulk into the gap between a hole chiseled in the foundation and the pipes passing through it just doesn’t make for a long-lasting seal.
- Install a means of absorbing the expansion movement of the buried PEX tubing where it connects to rigid tubing in the basement. A 100-ft. length of PEX can expand almost 17 in. when changing temperature from 50º to 200º. Without expansion compensation, that PEX can generate enough force to bend and kink copper tubing. I suggest use of reinforced silicone rubber hose rated for at least 250º to make up an expansion compensation offset or loop between the PEX and rigid piping.
Don’t freeze-protect an outdoor heater by “stealing” heat from a furnace plenum, using the heat exchanger coil that would normally supply heat to that plenum. Doing so will significantly cool the forced air supplied to heat the building. Other ways can be found to keep the outdoor plumbing from freezing that don’t directly affect interior comfort. Antifreeze is one solution. A small shunt circulator combined with a temperature setpoint controller, as shown in Figure 4 (page 15), is another.
When the temperature sensor in the outdoor heater reaches a temperature that’s near freezing, yet still a few degrees above, 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 the circulator in the outdoor heater is operating.
Flow through the load side of the heat exchanger should be adjusted using the flow setter so that excessive heat is not transported to the outdoor heater and 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.
Design systems so they can seamlessly transition from the wood-fired heater to an auxiliary heat source. Comfort should not be compromised during this transition. Ask the owner if he would like you to install an indicator light so that he knows when the auxiliary heat source is operating.
This way, the owner can decide if he wants to rekindle the fire at 5 a.m. when it’s -10º outside, or roll over in bed and let a couple gallons of fuel oil or propane pass through the auxiliary heat source.
- Finally, I can’t stress how important it is to burn only seasoned wood. For a gasification boiler, the maximummoisture content should be 20%. I suggest that the same criteria should apply to any firewood used for space heating. Use a moisture meter to check the firewood and don’t just stick it on the end grain. Split open a few pieces and check their core moisture content.
A market exists for wood-fired outdoor heaters, at least for the time being. If you are going to install one, why not combine it with modern hydronics and professional installation to make the most of what it provides?
By the way, if you want to attend this year’s Woodmen’s Field Days, they will take place Aug. 16-18 in beautiful Boonville, N.Y. Maybe I’ll see you there.