Antifreeze and Heating Systems

In response to a person on the Wall who asked, "Can I add antifreeze to my heating system?"

Brad White replies:

Antifreeze is commonly done but like anything else, you have to do it the right way. Do not use automotive antifreeze (ethylene glycol, which also has silicates). Use only propylene glycol such as Noble No-Burst or Downtherm products with inhibitors.

Glycol is a sugar-alcohol type chemical. It is a potential food for bacteria and can turn acidic if your inhibitors run out by ongoing leaks and fresh water replenishment. You have to monitor your system water chemistry and pH.

Glycol will find leak points you never imagined, even if the system is now "tight." The lower internal surface tension of glycol tends to find these pathways. Valve stems, screw caps on hose-end valves, weak solder joints. . . you will see it in living color if there are any leaks.

Glycol will reduce the system capacity. If the system is generously sized or the structure insulated beyond the original system design, great, you are likely covered. If not, you may lose between 5- and 10-percent. Your circulator will work harder to deliver the fluid.

Glycol can be pumped in by hand or my machine pump. I like my Silver King Force Pump but an Axiom feeder is a great way to monitor leaks and avoid dilution. (Disconnect your make-up water when you use the Axiom).

Bob "Hot Rod" Rohr replies:

Here are some tips and ideas for the use of antifreeze solutions in your heating system. First, when is it required? Certainly any system installed in an outdoor setting and in a freezing climate is a perfect candidate. Snowmelt systems, no doubt. Remote cabins in the mountains, yes. Possibly a home that is vacant for extended periods during freezing conditions.

I don't believe glycol is required in every hydronic system. It does require some extra time, money, and maintenance.

Glycol actually comes from crude oil. The refiners put it through several "cracking" process to produce the solution we call glycol.

There are two main types of glycol common to heating and cooling use: Propylene glycol (PG), and ethylene glycol (EG).

Propyleneglycol is considered non-toxic, or at least the FDA labels it at low-acute oral toxicity. In a food-grade form, PG is actually common in the food industry as a flavor- and scent-enhancer. Look at the labels on stored bakery goods to see PG or some variety used.

Being low-acute, or non-toxic, PG can be used safely in systems where domestic water could come in contact with the fluid, or where errant spills would not cause serious problems.

EG is still used, and common in larger commercial and industrial applications. It is generally less expensive and less viscous, which means it's cheaper and better at moving heat. EG is considered to have moderate-acute oral toxicity compared to PG's low rating. It is not FDA or USDA approved for use in the food industry.

EG can kill pets if ingested, and it's not a pretty death. Pets are attracted to the sweet smell and taste. Handle and dispose of these fluids (like automotive antifreeze) carefully. Most oil-change stores will dispose of drained antifreezes for you at little or no charge. They do recycle these fluids.

If you come across a system with an unknown glycol, the two types can be blended. However you end up with an unknown toxicity concoction. I'd rather the old fluid be flushed and disposed of and start fresh.

A few tips should you decide to glycol your system:

First, run a cleaner through the system. I prefer a good hydronic-specific cleaner. Dow suggests a 1-2% solution of TSP. I've had one supplier tell me that automatic dishwasher detergent can be used as it doesn't suds up and it cuts grease and oil well. All these products are basically strong soaps. Be sure any cleaners are completely flushed out before you add the glycol.

Next, be sure the system is free of any leaks. Glycol has a sneaky way of finding the smallest of leak paths. Threaded joints, packing around valve stems, etc. Plan on seeing some green "fuzzys" at some locations.

Third, select a quality brand and blend it carefully. Glycols should only be blended with filtered DI (deionized) or DM (demineralized) water.Plain tap water may or may not be good to blend with. Hydronic glycols have inhibitor chemicals added. These ingredients help buffer the pH, lock up hardness, scavenge oxygen, etc. If you blend with water of a questionable quality, you will compromise these important components of the glycol right off the bat.

I'd suggest purchasing pre-blended hydronic glycols. Most of the hydronic antifreezes are available in any size container from a five-gallon bucket to a rail-car, pre-blended. Beware of the freeze protection and the burst temperatures on the container label.

Stick with the brand-name products to assure it is first-quality and properly inhibited. If your system contains any aluminum components you MUST use a special AL (aluminum) blend. All of the major hydronic glycol manufacturers offer AL fluids now. Several manufacturers also offer HD (heavy duty) products. These have a better inhibitor package, I suggest HD fluids for solar applications, as they handle over-heating better, and longer.

I would highly recommend any boiler auto-fill systems be disconnected from a system with glycol installed. You can buy, or build a glycol fill tank to assure the system maintains pressure. Most of the newer modulating-condensing boilers require 10-12 psig to fire off. A glycol fill system will assure the system does not lockout due to low-pressure conditions. It is quite possible for a hydronic system to burp a little air as the heating season begins so be sure you cover that loss of pressure to prevent nuisance call backs.

Lastly, invest in glycol testers to maintain your systems. Buy at least a freeze-protection tester, such as a refractometer. Buy a good one  that will give you years of service. Also, get a pH tester because this will give you the first sign that the fluid has been compromised. I like the small, pocket-sized electronic type of tester.

Label all the systems where you install glycol! This can be as simple as a magic marker note on the boiler jacket. Many of the glycol suppliers have pre-printed labels for this purpose. Define the type of fluid and the percentage of mix, along with your company name and contact numbers.

Check the systems yearly for pH and protection level. if the glycol has been overheated it may be salvageable with an inhibitor boost kit if determined soon enough.

Most glycol manufacturer offer a more in depth analysis if you mail them a sample.

A few last tips:.

Glycoling adds cost to the system. Be sure to add to your price for the additional components and labor hours. An extra day's labor is common for the cleaning, flushing, filling, re-purging, testing, and documenting the installation.

Check the boiler installation manual. They will indicate a maximum percentage of glycol allowed.

Pumps may need to be up-sized when glycol is used because glycol is more viscous than water.

Expansion tanks may also need to be up-sized.

Tightly seal any unused antifreeze containers to limit to oxygen intake, which can deplete the oxygen scavenger in the fluid.

Dispose of old fluids properly. I use Safety Kleen to pick up large quantities. They will document the disposal to keep you covered from a liability position.

Never use automotive antifreeze. it contains silicates which WILL sludge your systems. Trust me on this one :)

For further information, contact the manufacturers for data sheets and Engineering Guides. Dow has an excellent Engineering and Operating guide available for PG and EG fluid, as well some software for design use.