"Why Won't My Outdoor Furnace Get Hot? Top 4 Reasons"

"Why Won't My Outdoor Furnace Get Hot? Top 4 Reasons"

Have you noticed that your outdoor wood furnace water temp stays low? Does your blower fan run continuously? 

If these are sy mptoms you are experiencing, you may have a serious airflow problem that will continue until you TAKE ACTION. 

If you feel you are burning too much wood, then your problem is different, and it is addressed in an upcoming newsletter. This article addresses the problem of airflow which produces the following symptoms:

1. Water jacket temp NEVER gets up to the max temperature (180 degrees for HE Model furnaces, and 182 degrees for GX model furnaces). 

2. Instead, the water jacket temp stays low, sometimes hovering around 120 degrees or lower.

3. Blower fan runs continuously but doesn't seem to keep the fire going very well.

4. When you open the firebox door, you DON'T see a nice strong fire burning, even though your wood is dry and sufficient, but rather you see a firebox full of thick smoke and almost no flames.

5. After you open the firebox door, the fire seems to take off and burn much better with the door open.

6. Your house or building cannot get sufficient heat from the outdoor wood burning boiler.

These symptoms almost always prove to be the result of a major airflow problem. The likely causes of this are as follows:

1. Your blower fan is about to die and must be replaced immediately. Read the article: "Top 4 Signs Your Blower Is Near Death". Click HERE for fan product info.

2. Your solenoid is failing and needs to be replaced. The solenoid is the device that when energized, becomes a magnet that lifts the damper plate that allows air to flow from the fan into the boiler. If it is failing, this is easy to diagnose - just remove the cover and turn the fan switch on and off to see if the solenoid is lifting the damper plate. (This must be done while the furnace is in the mode with the fan running, not the idle mode.)

3. You have an airflow blockage. This is a serious situation that must be resolved immediately, but the solution depends on whether you have an HE Model furnace or a GX Model furnace. These two methods are described below.

4. Finally, your outdoor wood boiler may be the wrong size to adequately meet your heating needs. This may only be the case in the coldest part of Winter, in which case you just need to fill your outdoor wood burner more often.

How to Clean Airflow Problems - HE Models

For GH and HE Model furnaces, an air blockage can only occur in the chimney, or in the deflector plate. The chimney only rarely gets completely blocked, but it should be cleaned annually by removing the chimney cap, and wire brushing out the chimney followed by a complete vacuuming.


The deflector plate or "D-Plate" is located above the cross tubes that you see at the top of your outdoor wood. In normal operation, the pull rod is pushed closed, and the heat and smoke must circulate inside the boiler to the front top entrance to the deflector plate area, and then flow to the back to the chimney through this deflector plate channel. This is an innovative design feature that increases efficiency and prevents all your heat from just flowing straight up and out the chimney.


The D-Plate can be cleaned in two ways: We recommend using a D-Plate Brush available HERE. Also, you can pull your Pull Rod towards you to the stop, then turn the handle clockwise about a one-eighth turn which lifts the plate over the stop. Then the handle can be pulled out several feet.

If you do this with the firebox door open, then you will see a cloud of white powder falling out of the deflector plate area. BE GENTLE! Some customers have pulled too aggressively and broken the bolt that holds the cleaning plate to the Pull Rod.

How to Clean Airflow Problems - GX Models

Cleaning a GX Model furnace is more complicated:

1. Make sure you have a Thermostatic Load Valve Kit ("TLV Kit") installed on your outdoor furnace 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 wood stove will not function properly.

2. First make sure the slots in the firebrick are clear

3. Next clean ash out of the secondary burn chamber from the bottom door.

4. Clean the vertical tubes using the appropriate size cleaning brush.

5. The wire whip head/nylon rod tool is also very useful with the vertical tubes (available HERE). Watch this video HERE for the procedure.

6. For GX15 and GX30, repeat steps 3. and 4. with the horizontal tubes.

7. For GX10 furnaces, use a shop vac and the steel cable cleaning method described below. 

Once you have confirmed that all airflow passages are clear, the furnace should operate perfectly!

GX10 Steel Cable Cleaning Method:

Purchase a steel cable 3/8" x 54" and a pipe 3/4" x 30". Then watch the video entitled "Mindy Cleaning Furnace In Heels" and advance to 3:10 for the specific demonstration on using the Steel Cable Cleaning Method. Remember that the shop vac is a critical component to this procedure. Here is the process described in the GX Info Guide (call for a free copy of this Guide):

1.      Heavy Duty Wire Cleaning Tool – Sometimes cleaning the tubes of the GX10 will require more strength than the 1.5” tube brush can deliver. In cases like this we offer this simple solution. A length of steel wire rope about 4' 6” (54”) long makes a great cleaning tool. Depending on the size of your drill, you can buy the right size wire to match your drill’s chuck, preferably 3/8". 

One end goes into the drill and the other end goes into the tubes of the GX10 to clean. Spin the drill backward to avoid unwinding the wire. It is essential to feed the wire rope through a 30” piece of steel pipe (for example, a ¾” x 30” black pipe nipple fits a 3/8” wire rope nicely).

This will help stabilize the wire rope as it spins and give you something to hold on to.