There are two general types of Propylene Glycol:
a) Inhibited propylene glycol which has corrosion inhibitors included by the glycol manufacturer

b) Un-inhibited propylene glycol which has no corrosion inhibitors included
(to which you add your own or boiler manufacturer recommended or provided inhibitors.)

Both types come in either 100% concentrations or in premixed diluted (30%-55%)concentrations.
For the most part it becomes quite allot more expensive to use the premixed when filling an
outdoor boiler as the majority of outdoor boilers will require 3 drums of premixed concentrate
to fill the system and comparably only 1 to 1-1/2 drums of 100% concentrate to fill the same
boiler mixed with good quality water.

Propylene glycol possess many characteristics that makes it ideal for use in heat transfer
systems where protection from freezing is required. Desirable properties include high
boiling points, low freezing points, stability over a wide range of temperatures, and
high specific heats and thermal conductivities. Furthermore when used with an appropriate
inhibitor, demonstrates a non-corrosivisity that may substantially prolong system life.

Propylene glycol should be used for freeze protection where direct contact with foodstuffs
or incidental contact with drinking water may occur. Although propylene glycol is generally
recognized as safe (GRAS) by the FDA, it is not intended for human consumption.

Freeze Protection:
The glycol mixture design concentration must be determined with due regard to the minimum
temperature that the system is expected to encounter. The concentration should guarantee
adequate freeze protection, and avoid using overly concentrated solutions, which add
unnecessary expense and reduce system efficiency.

The design concentration for an outdoor boiler should be 30% to 50% glycol by volume.
(minimum flow protection levels are estimated and are dependant on system & equipment,
attempting to circulate fluid below freeze point may overload pump and could cause pump failure.)

Propylene Glycol Concentration Testing
Propylene concentration cannot be tested with a conventional anti-freeze tester!
Concentration can be easily and accurately checked using a handheld refractometer.
System concentration should not vary significantly from test to test.

Fluid Maintenance:
The anti-freeze solution must be checked at least once a year
in accordance with the manufacturer's recommendations.

Solution pH Testing
While high quality glycol solutions may last in excess of 20 years, hard use, improper
maintenance or chemical contaminants will significantly shorten fluid life.
Fluid pH serves as a good barometer for the condition of the glycol and is best measured with
a field pH meter, although commonly measured with disposable pH test strips.