Should You Use Antifreeze in Boiler?

One of the first questions that pops up soon after installation is whether you should be putting antifreeze in your hot water boiler or just stick with straight water. Like most questions in life, it depends!

Every situation is different, so one pat answer would oversimplify a difficult decision. So instead, I'll give you the issues in order of importance to guide in making a decision that is right for your particular situation.

Advantages of Antifreeze for Home Boilers:

  • Burst Protection: if the power goes out and the liquid in your boiler is no longer circulating you don't need to worry about the pipes freezing and blown lines if you have used antifreeze for your home boiler. We all know what happens to water and that's not pretty.

    This is the number one reason why most clients are even considering putting in antifreeze instead of just hot water. Boilers are expensive and many don't want to risk the damage that can be caused by freezing if they are not around to load it with firewood or the power goes out.

    This is the number one justification in putting antifreeze in hot water boilers.
  • Lubrication: It is true that various forms of antifreeze act as lubricants and protect your plumbing and piping, but this is a rather insignificant issue in the boiler + water vs antifreeze question.

Disadvantages of Antifreeze for Home Boilers:

  • Insurance! This is the biggie! And this is the questions to ask before you buy anything. If you absolutely MUST use glycol and your insurance company revokes your house insurance, now is the time to find this out. Even using the correct non-toxic food-safe form of glycol is no guarantee that they will insure! Ask and Ask again and get it on paper with a price and conditions before you dig!

    Answer: Find another insurance company or

    Answer: find another method to get burst protection without using glycol antifreeze (a generator for electrical backup or alternate heat sources if the fire goes out that works in reverse to keep boiler water from freezing may be a better choice than using antifreeze)

  • Toxicity: Many forms of traditional antifreeze like the type frequently used for automotive applications is highly toxic, but you can somewhat address this issue by using the right type of glycol. This is discussed in the next section.

    Answer: Use non-toxic propylene glycol antifreeze.

  • Heat Transfer: The ability of glycols of various forms to transfer heat from your home boiler to your home heating system is obviously of great importance. After all that is the purpose of the furnace. Glycol antifreezes certainly do not do this as efficiently as just putting in hot water. Boilers where you intend to use glycol might have to be larger to compensate.

    Answer: When sizing a outdoor boiler to the size of building to heat, factor in approximately 15% reduced heat transfer capacity if thinking of using glycol antifreeze. (or better yet use the documentation for the heat transfer fluid you are considering)

  • Oxidation, Limescale and Bacterial Growth: All glycols oxidize on exposure to heat and air creating an organic acid that if you leave it to its own devices will rust out the inside of your boiler and cause scale or buildup on your pipes. Think about cholesterol and heart attacks.. same thing!

    Answer: use "inhibitors", chemicals added to you boiler to maintain a pH balance and neutralize the acids. Note that inhibitors are specially mixed to accommodate your home boiler's needs both from the perspective of the type of metal it was made up of and the characteristics of your top-up water. (see section on water chemistry, and keep in mind that this is also an issue with plain water, but a little less complicated than with glycol)

  • Expansion: Glycol fluids expand MORE than straight water

    Answer: increase size of expansion tanks to reflect these higher needs

  • Cost: The glycol antifreezes that are specifically formulated for use in home boilers are quite expensive, where a full installation can run you $1000 - 1500 to fill your boiler depending on the size of your unit.

    Answer: Win the lottery!

All said, if at the end of the day antifreeze is the only answer you can find to offer essential burst protection then the insurance issue is the only key stumbling block. So research that first and find ways to reduce their risk in your outdoor wood boiler plans and they will be more likely to want to consider your application.

  • Use closed loops to limit the need of antifreeze to only the most at-risk plumbing
  • use non-toxic glycol (see the next page in this series)
  • consider a cement-well to mount your outdoor boiler in as a catchment basin in the case of cracking in the outer jacket (though this won't help if the plumbing breaks at the other end and it drains downhill)

So should you use antifreeze in boiler applications? Not if you can avoid it. Your first attempt should be to find alternatives that offer the freeze-thaw protection you need, and only then consider glycol as the answer of last resort.

Read on to Find out what is the "right"  Type of Glycol Antifreeze.

Main Index: Antifreeze for a Home Boiler