Why Is My Outdoor Wood Furnace Providing Low Heat Output?

Why is My Outdoor Furnace Providing Low Heat Output?

Why would your outdoor boiler not provide sufficient heat for your home?

Heating your home or other structures with an outdoor wood furnace is an efficient and cost-effective way. However, imagine if your outdoor wood furnace isn't producing enough heat. That can be frustrating! And of course, so inconvenient for you and your family.

Well, the thing is, it can happen. Especially if you overlook some of the important steps in the maintenance and regular operation of your outdoor furnace.

There are various things that can cause this problem. This article will look at some typical causes of insufficient heat output from outdoor wood furnaces as well as possible remedies.

Here is a quick list of possible causes:

1. Using Not Properly Seasoned WoodSeasoned Wood

One of the primary reasons for inadequate heat output is an insufficient fuel supply. And one reason for this is using not properly seasoned wood. Keep in mind that green or wet wood won't burn well and may provide less heat. Ensure that there is enough dry, seasoned wood available for burning.

Read our article “3 Steps In Preparing Wood for Your Outdoor Wood Boiler” for more information on how to prepare firewood for your outdoor boiler.

2. Airflow Problem

In an outdoor wood stove, proper ventilation is essential for effective combustion and heat transfer. Inspect that there are no blockages, such as creosote buildup or debris, in the chimney or flue. Reduced heat output and incomplete combustion can result from insufficient airflow.

3. The Boiler Pump is Starting to Fail

This one is simple to understand and diagnose. If your pump is starting to fail, remember that it can die a slow death; pumps will sometimes fail slowly and start to circulate less and less water. This will reduce the heat provided to your home. You will burn less wood, but your home will be harder and harder to heat, and it will seem that your indoor forced air furnace (if you have one) runs all the time.

pump failThis can be diagnosed by observing the flow of water in your filter after the cartridge is cleaned. Is the flow strong or weak? You can also detect this by simply touching the supply and return lines – the supply line should be 180 degrees and you cannot keep your hand on it (too hot), but the return line will be very cool. Water that is flowing very slowly will experience a drastic drop in temp going through your heat exchanger.

The solution is to replace the pump – soon! Because you may only have a few hours of pump life left.


4. No Boiler Filter Installed

If a Filter Kit was never installed, then your entire system is at risk. All outdoor wood boilers are open to the atmosphere and therefore are susceptible to sediment damage. Sediment is the “Secret Killer” of outdoor boiler systems.

Approximately 98 percent of our customers have the Filter Kit installed, but those who don’t end up calling us after 3-5 years and need to replace their pumps and heat exchangers because sediment will kill the pump, and build up in the heat exchangers causing them to become restricted and fail.

If you do not have a Filter Kit installed, the solution is simple – just install a Filter Kit. It takes less than 30 minutes. Once the Filter Kit is installed, it is like getting a new kidney – the sediment will get cleaned out of the system, and you may not have to replace the heat exchanger.

If you have a filter, it is also recommended to regularly clean it and replace the cartridge when it wears out. Watch this video on how to clean your fil ter in 5 minutes.

Filter Kit Video

5. Damaged or Plugged Boiler Heat Exchanger

The effectiveness of the furnace can be decreased if the heat exchanger is damaged or unclean, which may block heat transfer. Plugged heat exchangers can be avoided by using a filter. But if the heat exchanger shows any signs of damage, such as leaks or rust, think about fixing or replacing it. Click HERE to purchase a new heat exchanger for your system.

6. Outdoor Boiler Aquastat or Control Issues

Inadequate heat generation can be a result of defective Aquatats or control systems. Check the settings on your Aquastats to make sure they are calibrated and programmed for the right temperature. The ideal temperature of your boiler is 180 degrees F. This is also where you set your Aquastat with a differential of 15-20 degrees to shut off the blower fan. But there are some boilers that are set at 175 - 185 degrees F. Please check your owner’s manual or Check the electronic controls on your furnace for any issues or error messages.

7. Failed Underground Pipe

If your underground insulated PEX pipe is losing too much heat between the furnace and the house, then your underground pipe has failed and must be replaced. I had this experience at my home – I decided to attempt to use a cheap black underground pipe and of course, it filled with water, and the wood I was putting into my boiler was heating the earth between my boiler and my home – there was a green patch of grass directly over the path of the underground pipe!

Read our blog article “Why Does Snow Melt Between My House and Outdoor Furnace?” for more information.


8. System Maintenance

The best operation of your outdoor wood burner depends on routine maintenance. To eliminate ash, soot, and debris, routinely clean the firebox, flue, and heat exchanger. Regular maintenance increases the longevity of your furnace and aids in preventing problems that could reduce heat production.

We also have a blog article on the routine maintenance items that you should check when you have an outdoor wood boiler. These "Routine Maintenance Items" and "Other Resources" are parts of your complete guide to owning an outdoor wood furnace.


Remember, regular maintenance and correct operation are key to ensuring your outdoor wood furnace operates efficiently and delivers the desired heat output for a cozy and comfortable environment.

There are a number of reasons why your outdoor wood furnace may not be producing enough heat. And you can troubleshoot and fix the issue by addressing these possible causes. However, it is recommended to get assistance from outdoor wood boiler experts if you run into complicated problems or are unsure about completing repairs.

As always, our goal here at OutdoorBoiler.com is to help you maximize your savings with your outdoor wood stove. Let us help you with any customized circumstances – just contact us and explain your situation and we will help you.