If you enjoy the articles in this web site, please consider supporting it by ordering the items you want by clicking on the recommended Amazon product links in the articles, which will just add those products to your Amazon shopping cart.

The product links contain a referral tag that allows me to earn a small commission on the sale of the products from Amazon.  This doesn't cost you anything extra but will help to offset the cost of running this web site and writing new articles.

Article Index

For those of us with access to low-cost firewood, it often makes sense to heat our homes with an outdoor wood furnace. When we built our house in 2001, we included a Security Chimney BIS Ultra Fireplace in our main floor living room to minimize the fuel consumed by our forced air system's natural gas furnace.  After having used this indoor fireplace for many years, I found that it was a lot of work to keep the fireplace continuously stoked. Our Security Chimney BIS Ultra Fireplace is rated to produce up to 60,000 BTU/hr for up to 8 hours with a load of wood.  In practice, with the wood we have in our woodlot (mainly ash, silver maple, and elm) a full load of wood in the fireplace doesn't last much more than 2 hours, 3 if we're lucky.  Waking up at 2 am to every night reload the fireplace gets old after a few winters.

With Federal Government's 2009 Home Renovation Tax Credit, in 2010 we decided to add an outdoor wood-fired hydronic boiler to our heating system. The payback of the wood furnace wasn't great because our heating costs were low for two reasons: 1) our house is well insulated (meets the R-2000 standard) and 2) we were already burning wood. The deciding factor was the potential cost of heating the in-ground pool planned for later that year. We weren't sure how much energy the pool would require but we figured it wouldn't be cheap to heat it with gas and, by using wood, we could extend the swimming season.

Because cutting wood is time-consuming and the amount of wood on our woodlot isn't infinite, we decided to go with a high-efficiency EPA-certified hydronic furnace. There are several such outdoor furnaces commercially available and, upon much deliberation, we decided to get Central Boiler's E-Classic 1400.

Central Boiler eClassic 1400


A hydronic furnace is an unpressurized boiler and the E-Classic 1400 furnace controller maintains a water temperature of 175°F to 185°F .  As far as I know, all high-efficiency EPA outdoor wood furnaces are "gasifier" furnaces.  This means that the wood in the firebox is converted to charcoal as the volatiles (ie, wood gas) "boil" out of the wood (i.e., gasify) and exit through the coal bed in the bottom of the furnace. You would see the "volatiles" in a fireplace as smoke and the smoke ignites when the fireplace becomes hot enough.  Essentially, burning smoke is the flame you see coming off the wood.  The orange glow of a flame comes from the glowing carbon in the smoke before it reacts with oxygen to form CO2.

Smoke is the hot fumes of wood.  If smoke condenses upon the interior of a cold chimney, it forms creosote, which is tarry substance.  It can self-ignite and cause a chimney fire if the chimney becomes hot enough.  Smoke emanating from a chimney is wasted fuel so do NOT let your wood smolder!

Although an EPA-certified furnace is much more expensive than a non-certified furnace, we chose that option for 3 reasons:

  1. Our time is valuable and, by having a more fuel-efficient furnace, we would spend less time cutting wood.
  2. Because we don't have a large woodlot, we wanted to minimize our wood consumption.
  3. Although not required by local regulations, our neighbours would be happier if we kept smoke emissions to a minimum.

After having operated the furnace for several years, we've found that getting an EPA furnace was a wise decision.  Even though the furnace burns very cleanly when operating properly, it can be quite smoky when it's not.  Seeing as the prevailing winds often blow towards the neighbours, I'm sure that they would be quite unhappy to be downwind of a conventional wood furnace.

As you can see in the following table, the E-Classic 1400 furnace holds almost 10 times more wood, has a higher furnace efficiency, and has a higher rated output over 8 hours than the BIS Ultra fireplace.  It has certainly been easier to load up the outdoor furnace once a day (or often twice when it's cold).  In practice, with our well-insulated house, we typically use the fireplace when it's above freezing outside because the fireplace has enough capacity to keep the house warm.  Using the e-Classic 1400 takes longer because of the time it takes to bring all of the furnace's water up to operating temperature and this light load tends to cause creosote build-up. Besides, a crackling fire in the fireplace is nice in the winter.

Although a 32" long log can fit into the firebox, the maximum log length of wood splitter will determine actual log lengths.  The dealer who sold me the E-Classic 1400 recommended 24" logs and this seems to be fairly common size for wood splitters too.

Specification  E-Classic 1400 BIS Ultra
Door 18.5" x 18.5" 25" x 14" (glass area)
Firebox 32" L x 24" W x 32.5" H
14.44 ft³ volume
25"W x 13.5"D x 13"H (nominal)
18"W x 12"D x 12"H (practical)
2.5 ft³ (nominal) / 1.5 ft³ (practical)
18"-19" log
Weight 2,085 lbs. 402 lbs.
Water Capacity 195 Gallons N/A
8-Hr Output Rating 107,459 BTU/hr †
(based on dry oak)
12-Hr Output Rating 72,000 BTU/hr  
Maximum Heat Output Capacity 209,316 BTU/hr †
(based on dry oak)
up to 60,000 BTU/hr
up to 8 hrs burn time
EPA Rating   4.8 grams/hr
Furnace Efficiency over 84% 75.2%
Note † Based on EPA qualifying test  tapered firebox is 18" wide
at the back

In 2006, we built a car port behind the workshop to house the tractor and store firewood. Many people keep their firewood uncovered outside, but we've found that the wood doesn't get completely dry this way. Covering the woodpile with a tarp is one option but this reduces the air circulation that helps with drying. More importantly, tarps disintegrate after a year or two (or maybe the cheap ones I buy), leading to tarp fibers getting into the pool if not promptly cleaned up.

The location of the outdoor furnace therefore was selected to be near the firewood to avoid unnecessary handling. This made the hot water line (InsulPex) to the house quite expensive because of the length of the run for the costly, insulated pipe. Although the pipe is well insulated (snow doesn't melt off from exposed line), we buried it several feet underground to minimize heat loss but mainly for aesthetics. Because the InsulPex pipe is 4" in diameter and extremely stiff, we had to bury it 4' deep at furnace to allow pipe to bend up into the bottom of the furnace.

Central Boiler eClassic 1400


The E-Classic 1400 is designed to maintain hot water between 175°F to 185°F by controlling the air supply to the furnace.  The furnace supplies air to the furnace with primary and secondary air ports.  Primary air is supplied to the furnace with air ports around the bottom perimeter (sides and back walls) of the firebox.  Secondary air is supplied to the furnace with air ports in the Charge Tube (Central Boiler term for flue gas passage at the bottom of the furnace).  When the furnace is running normally (bypass outlet is closed), the flue gas passes through the coal bed and then on to the Reaction Chamber (Central Boiler's term for the ash reservoir where combustion air further reacts with the flue gases) below the firebox via the Charge Tube.  Combustion air is supplied by a blower to a plenum housing the three solenoid-controlled primary and secondary air valves.

These temperatures are measured by a thermocouple in the Reaction Chamber and I believe the air valves open as follows:

  1. The 1st secondary air valve opens whenever the furnace controller calls for heat.  Air emanates from the Charge Tube when the furnace starts.
  2. The primary air valve opens at 550°F.
  3. The 2nd secondary air valve opens at 750°F.

These temperatures are measured by the thermocouple in the Reaction Chamber. When running correctly, reaction chamber temperatures can easily reach temperatures higher than 1400°F.  Visible smoke from the chimney disappears once the Reaction Chamber reaches 700°F.

A gasifying furnace is basically creating its own charcoal by driving off the volatiles in the wood.  By splitting logs into halves or quarters, the log's surface area increases which helps the gasification process.  Since the firebox's walls are water-cooled and are always therefore below the boiling point, some smoke condenses on the walls and door of the firebox and the tarry creosote residue runs down to the coal bed.  It is necessary to loosen the ash and coals in the coal bed and Central Boiler includes a 3/8" Cleaning Rod that they recommend using every time wood is added.  They also recommend using a Stanley Wonder Bar for loosening up the creosote around the perimeter of the firebox and around the firebox door.

The gasifying operation of the furnace causes the primary air supply holes in the furnace to plug up fairly quickly.  This is because the smoke has no place to go when the furnace shuts off after having reached its upper setpoint.  With no air being blown into the firebox, the hot wood fumes expand in the firebox and are drawn into the now-cooling primary air ducts all the way back to the air supply valves (mainly the primary air ducts).  A sure sign of plugged-up primary air ducts are slow-to-heat flue temperatures and a smoky output from the chimney.  Besides the unpleasantness of being downstream of a smoky chimney, plugged ducts cause increased wood consumption from inefficient operation.

It is important to regularly clean out the Reaction Chamber because excessive ash build-up could:

  1. prevent the flue gases from flowing out of the firebox
  2. prevent the thermocouple from reading the reaction chamber temperature correctly
  3. reduce the reaction time of the flue gases with the secondary air, thereby exacerbating smoky exhaust
  4. cause the steel plate covering the insulation in the reaction chamber door to warp
  5. cause ash build-up around the bypass door and chimney base.

Regularly cleaning the primary air ports is a very important a and should be done whenever the firebox is cool enough to enter.  It's much easier to rod-out the ports when the creasote build-up is light.  It gets progressively harder with increasing blockage.  Severely blocked primary air passages can take hours to clean and several broken rodding-out wires - definitely not a fun job.

I originally did not shut down during the winter because is possible to shovel out the Reaction Chamber without first shutting down the furnace.  My current practice to only start the furnace before 7 pm (when time-of-use electrical rates become Off-Peak) and only put in enough wood so that it will only last until 7 am (when TOU On-Peak rate begins).  No more than every few days or so, the primary air ports should be rodded-out with a stiff wire.  I use coat hangers cut up into even lengths but, while convenient, a coat hanger, isn't ideal because the rodding operation causes the wire to flex a lot and the cheap steel in coat hangers will break from metal fatigue. I've had to retrieve a broken piece of coat-hanger wire from inside the primary air passages more than once.

In addition to cleaning the primary air ports around the perimeter of the furnace, the primary air valve (made from a solenoid lifting a cap from a mitre-elbow) also needs cleaning.  The creosote will collect in this passage and will either be a tarry mess or a glassy residue, depending upon its temperature and residence time.  You will need a knife and chisel to scrape out the residue and a drill bit to clean out the air flow measuring tubes.  A shop vac would be useful to vacuum out the creosote particles loosened by the knife and chisel.

Not mentioned by Central Boiler is the need for dust masks to service the furnace. Even when asked for a recommendation, they had none and seemed to suggest that respirators are unnecessary.

Whenever the furnace is running and the firebox door is open, the furnace will emit fly ash and smoke and cheap dust masks will not keep dust out of your nose and lungs.  You need an N95 mechanical filter respirator and these have 2 straps unlike the single straps of the cheap respirators.  I've found that, even though my particulate respirator (3M 8511 from a local welding supply store) isn't rated for volatile organic compounds (VOCs), it seems to make being in smoke much less unpleasant.  I highly recommend wearing a respirator whenever the firebox door is open.

Although loading the furnace involves tossing wood into the firebox, it sometimes involves reaching into the firebox to move a log that didn't end up in its intended place. You can use the Wonder Bar or Cleaning to move the wood around but sometimes its just faster to reach in with your hands.  I've singed my sleeves a few times so be very careful about your coat catching fire.

I find that no matter how careful I am, I can't escape the smoky fragrance of tending the furnace. To minimize this effect, I wear old clothes and a hat (actually an old hard hat liner) to keep the smoke out of my hair.  I also recommend having furnace coat with an attached hood.  The hood keeps the creosote from falling down the back of my neck when I climb inside the firebox.  My furnace coat is an old Hydro Parka.

Now that I have had  a chance to gain some experience with operating a wood furnace, there are a few things that need improving.  Some things are not ideal but it's not worth changing right now.

If you're planning a new installation, I recommend that you understand how the various components are sized, both from a physical and a thermal point of view.  I would also have a plan for how the piping gets hooked up and discuss this with the installer before the work starts. Your installer might have ideas you haven't considered and you might have ideas your installer hasn't considered.

Heat Exchanger Manufacturer Model
Main Heating Central Boiler PN 106
(100k BTU)
Garage Central Boiler PN 287
(50k BTU)
Hot Water Heater Advanced Industrial
Components Inc.
(10 plates)
Pool Advanced Industrial
Components Inc.

Main Heating

The wood furnace dealer installed a PN 106 (100k BTU, 19"x16½") heat exchanger in the duct work between the gas furnace and the AC coil, which I would say is undersized.  The PN 107 (126k BTU, 20"x17¾") would have fit the 20"x18½" outlet duct of the gas furnace better and it would have been better for the furnace to heat the 3400 ft² house at a rate that is faster than ~0.8°C per hour. If you're planning a furnace upgrade and have some flexibility in duct sizing, I recommend that you size your furnace discharge ducting to fit the largest practical heat exchanger.  Central Boiler's largest is PN 111, which is a 220k BTU, 26"x24". Especially now that electrical Time-of-Use Billing now in place, a faster warm-up time would work better with a programmable thermostat.

To keep the hydronic system simpler, no zone valve was installed to control water flow to the heat exchanger in the gas furnace ductwork.  My system uses two thermostats: one for the gas furnace and air conditioning and on for the wood furnace.  The wood furnace thermostat only causes the furnace fan to come on when it calls for heat.  In cold weather, this isn't too much on an issue but the natural convection in the ductwork for a 2 story house causes the upper floor often to be too warm.  The only way to circulate air through out the house to even-out the temperatures between the floors would be to put that heat exchanger's manual valves into bypass mode.  A better solution would have been to use a 3-way zone valve to control hot water flow through the heat exchanger and this is on my to-do list.

Hot Water Heating

To further reduce our need for natural gas, the dealer installed a brazed plate heat exchanger for our hot water system.  The piping arrangement was that our hot water would be heated either by the plate heat exchanger or by our gas-fired hot water heater.  The problem we found with this set-up was that, with poor wood and/or need for furnace maintenance, the furnace would die overnight and the hydronic system would be too cold to adequately heat the hot water so we had several cold shower mornings - and a very unhappy wife. If you know what's good for you, your wife must never have to shower in ice-cold water. It would have been better to use the heat exchanger as a preheater for the gas-fired hot water heater which would ensure that gas use was always minimized while ensuring a constant supply of hot water.  Another job for my to-do list.

The dealer installed the heat exchanger for con-current flow rather than counter-current flow, which produces lower temperature hot water than the counter-current arrangement.  To ensure that the hot water was discharged to the house at a safe temperature, he also installed a tempering valve (Watts Series 70A Hot Water Extender). I have it set at the highest setting (160°F) rather than the lowest 120°F setting to prevent dilution with cold water.

Pool Heating

The pool water heat exchanger is piped up with a bypass valve arrangement.  Although this heat exchanger looks very compact, its rating (250,000 BTU/hr) is higher than the maximum output of the furnace (209,316 BTU/hr), which results in low pressure drops for both the heating water and the pool water.  Even so, the furnace will still cycle on & off while heating the pool with the crappy wood I was using up over the summer. With good firewood (like oak or hickory), I would expect the cycling to be even more pronounced.  For faster heating and to minimize creostote build-up in the primary air passages, it would be better to have a much higher capacity pool water heat exchanger and my recommendation would be to have closer to 50% greater rating (like a 300,000 BTU/hr) rather than the next size larger.  This would absorb all of the heat generated by the boiler by preventing the boiler from reaching its 185°F upper set point.

I originally thought it would be good to have a zone valve controlling pool water temperature but I've found that, with the use the pool's solar cover, the pool remains fairly warm during the summer without supplemental heat.  If the pool needs heating, its 103,500 L (27,300 gallon) volume requires that the furnace must run flat-out for easily 24 hours to raise its temperature 10°F (5.6°C).  It is a good idea to use a solar blanket to prevent evaporative cooling, especially when it's windy.

Outdoor Furnace Installation

We built the pad for the furnace according to the recommendations of the installation manual.  After having removed the chimney a few times for cleaning, it would have been better to extend the pad behind the furnace a couple of feet so a step ladder could rest on concrete instead of half-on concrete and half-on grass.  The pad was also right under the drip edge of the carport.  Without an eavestrough, the pad heaved because of the ground around the pad was saturated.  Adding an eavestrough to channel the water away from the pad has minimized heaving and has kept my back dry in rainy weather.