Why add antifreeze to hydronic systems?

Every once in a while it becomes necessary to use non-toxic antifreeze in a hydronic heating system. The reasons for using non-toxic glycol vary from job to job.

If it’s a home where the occupants may be gone for a long period of time, there’s a possibility that the home may lose its fuel source (propane or oil), there’s a possibility of the long-term loss of power (mountain homes) or there is piping exposed to potential freezing conditions (overhangs). I generally induce a solution of propylene glycol into the heating system for freeze protection.

There are some tricks to doing this right. Too little glycol and you’re not providing adequate freeze protection. Too much glycol and you’ll cost the consumer unnecessary operating costs and possibly create additional problems in the heating system’s performance. Guessing at how much glycol is needed generally results in too much or too little glycol in the system.

You can estimate fluid content in a number of different ways.

First, you can force-purge a heating system into a large container using compressed air and record the number of gallons you removed. This is a guaranteed way of determining the system’s fluid volume, but it is quite labor intensive and can be messy.

Another way is the sodium titrate method. With this method, you test the system for residual parts per million of sodium. Then you completely mix 5 lb. of salt into a 5-gal. bucket of water and pump this saline solution into the heating system. Allow it to circulate for at least two days, then perform another sodium titrate test.

The incremental increase in ppm will tell you how many gallons of fluid are in the system. This method is tricky inasmuch as very few technicians have access to titrate testing equipment. This type of testing is typically performed for free by water treatment chemical companies. Call your local water treatment company for details.

The third method works quite well with new systems but is no more than an educated guess in existing systems.

I have developed a spreadsheet using standard volumetric values for standard nominal tubing sizes from 3/8-in. through 2-in. The spread sheet calculates exactly how much fluid is in the system based on the linear feet of tubing in the system, plus the boiler’s volume.

Once this quantity is known, an accurate approximation of the glycol necessary can be calculated and the appropriate gallons of antifreeze purchased and taken to the jobsite.

A fourth method is to draw a sample of the glycol/water mixture and send the sample to the glycol manufacturer. It can tell you exactly how many gallons of glycol would need to be induced back into the system in order to bring the fluid up to a given percentage.

In order to properly fill the heating system, there must be a means of injecting the premixed glycol/water solution into the heating system, and a place for all of the air-entrained fluid to return back to your charging system. This basically requires the designer to install an additional drain cock, which is actually used as a fill port. This fill port can be incorporated into the make-up part of the system. The purge port can be installed in its usual position.

My friend and associate Robert “Hot Rod” Rohr reminded me that if you are using a boiler drain valve backwards, as in a fill port application, you might want to consider using a standard ball valve with a male hose bib adapter in lieu of the drain cock. The reason is that if you try to stop flow backward through a drain cock, it will strip the bib washer right off the screw. Boy, howdy, have you got problems now! Thanks, Hot Rod.

A word of caution here. If you have a heating system filled with glycol, it is not a good idea to leave the make-up in an automatic fill position. If something goes wrong, and the system starts taking on fresh water without being noticed, you will dilute the glycol to the point of having no freeze protection at all.

This won’t be noticed until it’s too late and the system has already frozen and can no longer provide heat. Unfortunately, in some cases, this can lead to additional freeze damage of the domestic plumbing systems.

Tune in next month for part two of this column.