Burning Too Much Wood In Your Outdoor Wood Boiler?

Do you believe you are burning too much wood in your outdoor furnace? If so, let's talk about this for a minute.

High wood consumption can be caused by a number of factors. I will discuss here some of the more common causes of high wood consumption – many of which can be easily resolved.

First, keep in mind that wood contains energy in the form of BTUs. Your furnace converts that BTU heat energy from one form (wood) to another (hot water) so the heat can be used in your home or building. If your furnace is consuming the wood, those BTUs are going somewhere.  If you think you are burning too much wood, then the question you will want to answer is “where did those BTUs go?” You want that heat energy to go into your home, not somewhere else!  Let’s look at some common culprits:

1. A big culprit to burning too much wood is this: The wood has not been properly seasoned. If your wood is not properly seasoned, technically you are burning almost 40% of your wood pile just to burn off the moisture in your wood! Just because your wood was cut 1 year ago doesn't mean it's been properly seasoned; in other words, wood that's been cut for a long time does not equate to being dry enough to burn. The conditions have to be right. I have some wood at my shop that was cut 18-24 months ago and I split it last week and the moisture content of the wood is still 42%! Check out this blog post on BTU content of various species of firewood.

For a gasification furnaces and high efficiency outdoor boilers, your hardwood must be properly seasoned for at least 12-24 months in order to get it to the moisture level necessary for proper operation. In other words, by Autumn each year, you should be seasoning your firewood for the winter burning season that begins one year from that time.

2. So what is the big deal about seasoned firewood? Why is it so important?  Imagine your wood pile and think of the work and/or cost you incurred to purchase, haul, cut, split, stack, and cover that wood pile. Now imagine somebody coming in the middle of the night and stealing 40 percent of your wood pile. Not a nice thought. When you burn green unseasoned wood, you lose as much as 40 percent of the heating value in your wood to burn off the moisture in the wood. Where do those BTUs go? They exit your furnace in the form of steam, or create condensation in your furnace! But just think about the enormous waste! Personally, I don’t like the idea of wasting 40 percent of my wood pile, and all just because I didn't plan ahead.

3. Burning wood that isn't properly seasoned also may affect the proper operation of your furnace.  Most furnace manufacturers design their products to burn seasoned firewood, so any time you burn improperly seasoned firewood in your furnace, it is like driving a car that has water in the gas tank.

If your wood isn't properly seasoned, you may have thick layers of creosote inside your boiler. You should remove those layers of creosote by tossing a creosote stick into your firebox once per week.  Get creosote sticks HERE.

4. The next culprit is insulation at your furnace. If heat escapes your furnace, you will feel warmth if you touch the skin or roof of your furnace. Of course, the chimney and door frame will be too hot to touch but those spots are designed by the manufacturer to not be insulated. All other parts of the furnace should be insulated no heat energy is lost to the outdoor atmosphere. The great outdoors will absorb an infinite amount of heat so make sure your insulation is high quality and all in place. We recommend furnaces that are the best-insulated furnaces in the world – we recommend only high-grade R-30 insulation…the best in the industry.

5. Next, check your underground pipe for parasitic heat loss. You may want to perform a “Delta-T” test which is simply a measurement of how much heat is lost in your underground pipes. The instructions for the Delta-T test are attached below. Again, OutdoorBoiler.com only sells the highest grade underground pipe – and at a cost that is less than half the price of many lower-grade competitor pipes.  Make sure your underground pipe has a heavy outer casing (but not too heavy that it cracks!), triple wrap insulation and a true radiant barrier.

6. If your Delta-T test shows that you are losing significant heat, you should follow the Delta-T instructions on locating the source of the heat loss. These include the following:

1. Uninsulated pipe in the back of the furnace
2. Uninsulated pipe in other cold areas – unheated basements or crawl spaces, etc.
3. Water in the underground pipe

7. If your Delta-T test shows only normal minimal heat loss, but you are still burning more wood, than you feel you should, consider the following other possible causes. Let’s continue:

8. Ash Maintenance – If the level of the ash in your furnace is too high, this kills the efficiency of your furnace and far too much of your heat will just go out the chimney.  As always, follow proper maintenance procedures outlined in your owner’s manual, and maintain your furnace properly.  OutdoorBoiler.com recommends furnace owners not allow their ash level to rise above the bottom door frame. It is smart to only allow your ash to be about one inch deep in the bottom of your firebox.  Also, ALL ashes and coals must be removed at least monthly and the inside of the firebox completely scraped out. 

9. Another word on Ash Maintenance – Ash in its dry powdery state is harmless, but if it builds up and gets too thick, it will absorb some of the moisture from the wood and create an acidic paste that will corrode your steel.  Do not let this happen.

10. Proper Cleaning Maintenance – All outdoor furnaces require some cleaning maintenance.  Be sure to follow the manufacturer’s guidelines for proper cleaning so that your furnace will operate at its ideal efficiency.  I once visited the home of a customer who was complaining that his gasification furnace was not performing well.  I looked at his heat exchange tubes and they were almost completely filled with creosote!  No wonder his furnace was not functioning.  I asked him when was the last time he had cleaned the tubes and he said he had NEVER cleaned the tubes.  He was burning far too much wood because his furnace could not get out of first gear. 

For more information on the proper use of cleaning tools, click HERE.

11. Operating Temperature – This is VERY important.  You must NEVER operate your boiler at temps below 140 degrees.  No boiler (regardless of the brand) can be operated at temps below 140 degrees – the only exception to this is during “Start-up” but that is only allowed ONCE PER YEAR in the autumn. The reason for this is because wood contains moisture – lots of moisture!  When the boiler water is below 140 degrees, the moisture in the wood condenses on the inside walls and tubes of the boiler and that turns into large volumes of creosote.  That creosote plugs up the air tubes and this PREVENTS proper air flow. When that happens, you will not get enough heat out of the furnace, and then no matter how good your wood quality is, you cannot get a good hot burn because air cannot flow through the boiler tubes!  

It is important to note that when the boiler water temp is at its normal operating temp range of 172 to 182 degrees, all the moisture from the wood that is burned off will EXIT THE BOILER HARMLESSLY AS STEAM.  But again, when the boiler water temp is below 140 degrees, that moisture condenses on the walls and tubes of the boiler. You will note that during the fall “start-up”, some water will leak out the back of the furnace upper rear door. 

Those water drips will stop once the boiler reaches 140 degrees and the remaining moisture that had condensed will dry up harmlessly during this once per year “start up”.  Note that some customers will save some burned wood charcoal coals from the previous winter to use for start up in the fall.  Such coals have already had the water burned out of them so the amount of potential creosote is greatly reduced.  Then once the boiler water temp is above 140 degrees, they add seasoned wood.

We have a fun video on the topic that you can watch at this link: Frozen? 3 Reasons to Keep your Furnace temperature Above 140 Degrees.

If your Aquastat controller is not functioning properly, it must be replaced. Call our technical support team for a proper diagnosis, and click HERE for replacement parts.

If you have a GX-Model furnace, you must have a Thermostatic Load Valve Kit ("TLV Kit") installed on your outdoor boiler system. This is most commonly installed inside the house, and it will prevent the boiler temp from dropping to dangerous levels by bypassing the home heat exchangers when the water jacket temp gets too low. Without the TLV Kit, your outdoor boiler will not function properly. Every time we get calls from GX owners who did not install a TLV Kit, their furnace ownership experience is pretty miserable so don't subject yourself to that and make sure the TLV Kit is installed. More info is available HERE.

12. Level – Makes sure your furnace is level. Furnace manufacturers design their products to operate ideally in a level position.

13. Water temps must remain in the designated range of 172 to 182 (160 to 180 for Hawken models HE-1100 and HE-2100).  If not, this will create significant problems. If your heating load pulls more heat than the rated output, your operating temperatures will drop below the recommended temperature range, and this will cause the furnace to lose efficiency and accordingly, burn more wood than necessary.

If you are operating your furnace at temps below the designated temp range, here are some potential causes:

1. Your wood is not properly seasoned so the boiler never has a chance from the start.  All gasifiers require properly seasoned wood.
2. The boiler must be cleaned every two weeks.  Regardless of how dry and seasoned your wood is, all gasifiers require this bi-weekly cleaning.  For tips on how to clean your GX10 properly, watch this fun video:  Mindy Cleaning a Furnace IN HEELS!!!.  If your boiler is not cleaned regularly, or if ANY of the cleaning steps are missed, the tubes will get clogged and you will also not get proper combustion.  This will lead to the boiler operating below temps of 140 degrees, and the problems that that causes will result, thus becoming a self-fulfilling prophecy!
3. If your boiler is too small for your heating requirements, then it is likely that the boiler will require wood more often than you care to fill it.  This can lead to running out of fuel, the boiler temp therefore drops below 140 degrees, and again, the problems that that causes.
4. If your furnace ever runs out of wood, you must schedule more frequent loading times.
5. Other factors affecting airflow can also be culprits, but these will generally not cause you to burn too much wood.  Regardless, I will mention them: the fan could be defective. This can be evaluated by simply observing the fan and feeling airflow. Also, the solenoid that lifts the damper plate can fail.  Test this by turning on the blower fan switch on the front of the furnace and you should hear a click. Turn it off and hear a similar sound.  If not, remove the fan for a visual inspection. The fan removes easily with a clip on the right side of the fan assembly – remove it slowly and be careful to note the slots on the left that the fan cover fits into. (For Hawken models HE-1100 and HE-2100, the fan assembly and solenoid/damper are located inside the box on the front door.)

14. Naturally, you will need to follow all other instructions in your owner’s manual, including keeping the furnace full of water and as always, make sure you follow the proper water treatment procedures – Add water treatment each year and submit a water sample to the Hawken Laboratory for the free water test to ensure that your warranty remains valid! 
15. One final thought: If your HE-Model furnace has a pull rod, the normal operating position of that rod is pushed in. If you operate the boiler with it pulled out, or if the plate has become disconnected from the pull rod, most of the heat generated in your firebox will escape too quickly up the chimney and you will waste wood.
16. Likewise with your GX-Model furnace, make sure that in the upper rear chamber that the furnace is only operated with the air deflector in the proper position in the left side. This air deflector is designed to hold the catalyst disk, but also serves the important function of keeping airflow in the proper path, and protecting the surrounding steel. Without this deflector, you will waste wood, and possibly damage the steel surrounding that chamber.

Hopefully this was helpful! We promise that if you follow these instructions that you will be able to burn wood without wasting any of the actual burn power!

Warm regards,

The Outdoor Boiler Support Team