Is Using Antifreeze Good for Your Outdoor Wood-Burning Boiler?

Is Antifreeze Good For Your Boiler?

”Should I put antifreeze in my outdoor boiler?"

This is a common question that we get from customers who own an outdoor wood boiler.

The answer is almost always the same – “NO!”

But why not?

There are three simple reasons why we don’t recommend using an anti-freeze.

1. You most likely don’t need it.
Naturally, you believe you require an anti-freeze because you are concerned about freezing. But, freezing the water in an outdoor wood-burning boiler can be avoided by doing the following things:

  • Have a friend put wood in the boiler and keep it running
  • Keep the pump running and allow the forced air system to back feed some heat into the circulating boiler water.

But of course, there are exceptions. If you will be away from the outdoor boiler, or cannot continue to use the boiler, antifreeze may be your best option. You may also have health issues or in rare cases, you face the risk of having no electricity for an extended period of time – these are also considerations.

2. Antifreeze is very expensive.
Remember that we are burning wood to save money, right? So using an expensive anti-freeze is impractical for someone who wants to save money especially if there is another option other than using it. We use an outdoor furnace to burn wood not burn money!

3. Antifreeze reduces your efficiency by approximately 13 percent.

Water with an anti-freeze has a lower heat transfer efficiency than just plain water. Anti-freeze lowers the thermal conductivity of water, making it less effective in transferring heat. Thus, it will require the system to work harder to obtain the same degree of heating. This could result in higher energy consumption and running costs.

3 Rules for Using an Anti-Freeze

1. Antifreeze as boiler water treatment? NOPE! Antifreeze is NOT a substitute for an outdoor boiler water treatment and will not protect your boiler from corrosion. Remember the rule: “Always keep your boiler full of properly treated water!” Always use Liquid Armor Water Treatment!

This is the most important point and is often misunderstood – even if you use antifreeze, you must STILL use Liquid Armor water treatment. This boiler chemical (corrosion inhibitor) that you would apply to your furnace water coats the inside of the furnace to reduce the effects of corrosion on the metal.

2. Which type of antifreeze is best? NEVER, EVER USE automobile antifreeze, commonly known as ethylene glycol - ever. This is a hazardous product that will void your warranty and should never be kept in your home. (If you used this in your outdoor wood burner, remove it immediately and properly dispose of it.)

You may only use food-grade propylene glycol. Do not use RV antifreeze – that is intended only for toilets and drains. You will need to use a concentrated propylene glycol product similar to Dow Frost or Noble NOBURST. Click HERE for info on NOBURST. Also, remember that some brands of antifreeze need to be periodically recharged. Use only high-quality propylene glycol!

3. How much antifreeze is needed? Since every product is different, and your need for freeze protection will depend upon where you live and the circumstances of your installation, we recommend you follow the manufacturer’s guidelines.

Some antifreeze products will give you freeze protection to 10 degrees at a 40 percent mixture, some products will give you freeze protection only to 20 degrees at a 75 percent mixture.

If you plan to NOT use your outdoor furnace this winter, and you want to shut it down for the entire winter and protect it, and you are therefore asking the question “How do I do this?”, then these are some important things to consider.

For more information on this topic, see our blog article How Do I Winterize My Outdoor Wood Furnace?”


The use of antifreeze in a heating system should be carefully examined. There may be advantages of using an anti-freeze, especially for freeze prevention but it must be balanced against the disadvantages of reduced heat transfer efficiency and greater complexity. Proper insulation and other freeze prevention methods may be preferable to using antifreeze in locations where freezing is a concern.