BOILER WATER CHEMISTRY :
Automotive or RV Antifreeze

Automotive antifreeze almost always comes up in a conversation of outdoor wood boilers + antifreeze. We are familiar with its freeze-thaw characteristics from its application in our cars so we think that it MUST be appropriate for this application... let me change your mind.

Why You Should NOT use Automotive Antifreeze in Outdoor Wood Boilers!

  • Toxicity: Automotive antifreeze is a commodity product most often imported from Asian countries. I have attempted to get MSDS sheets for this or even a "list of chemical contents" and found it impossible. Suppliers suggest it consists of some sort of glycol mix, most likely Methanol and Ethylene, both highly toxic chemicals.. so be forewarned!
  • Cooling: Automotive antifreeze is designed for the cooling needs of a car NOT as a heat transfer fluid.
  • Silicates are added to this antifreeze to inhibit increases in pH and thus help slow corrosion. It is designed for high flow environments and aluminum radiators. Its doesn't work as well for low circulation systems.

    These inhibitors are NOT designed for the copper and steel components common in heating loops... the chemistry is different!

    The silicates precipitate out of solution and sludge up, plugging your plumbing and reducing efficient thermal transfer... again think of cholesterol and heart attacks. Silicates are also somewhat abrasive and can affect your pumping seals.

    Silica build-up is very resistant to acid cleaning so there isn't even a good "repair" solution after-the-fact!

  • Leak stopping additives: this is common in car applications but may foul up your pumps and valves. Again they are designed for very high temperatures and a fluid that is always on the move.

    At the very least, the expectation is always that you'll flush your radiator fluids every few years so if it is does develop sludge it can always be replaced... not so easy when you're talking about 150 gallons of toxic waste in an outdoor wood boiler.

So ..I hope that I have convinced you that an outdoor wood boiler + automotive antifreeze is NOT the way to go and you'll source some more ecological and safer options.

A quick note about RV antifreeze. It is most often propylene glycol so certainly a safer option and it is formulated to work in potable water systems so toxicity should not be an issue, but...

  • It is used to winterize plastic pipes, a less demanding application. It is NOT formulated to transfer heat and has no metal corrosion inhibitors so if you decide to use it, it will rot your outdoor wood boiler from the inside out!.
  • Most RV antifreeze is imported from Asian so again hard to get spec's but most seem to suggest that it is pretty diluted and would not have the strength for Northern climates where minus degree weather implies a greater need than a small sweater.

    Glycol concentrations in RV antifreeze are NOT sufficient to offer adequate burst protection.

So we've discussed the various types of glycol and whether you even should use antifreeze in your boiler, now lets move on to the last most important topic. wood boiler water treatment and water chemistry... a key consideration whether you are putting water OR antifreeze in a hot water boiler/furnace.

Main Index:: Outdoor Boiler Plans: Antifreeze or Water Considerations