To Freeze or “Antifreeze”

This winter season has set many records for severity across the entire US and Canada.  One record it’s unlikely you’ll find data on is freeze-ups and frozen pipes within buildings. Anecdotally, there are a record number of these unfortunate disasters this heating season.  Frozen pipes often burst, causing severe building damage and loss of use, and are extraordinarily difficult to thaw.  Not only have we experienced severely cold weather, but the coincident wind has made the “chill factor” of buildings much colder than usual.

Often there are conflicting views on protecting hydronic heating systems with antifreeze. Practitioners such as plumbers and heating technicians are often poorly informed, relying upon wholesalers for information.  Even system designers are frequently misinformed.  The debate is often emotional and illogical, so here are a few facts to help you make a good decision about its use.

The type of antifreeze used in HVAC systems is typically glycol.  Most of the glycol used for these applications has added corrosion inhibitors.  Some benefits of adding inhibited glycol to a system include:

  • Prevention of system freeze-ups and bursting pipes and coils when properly applied.
  • Allows for deeper temperature setbacks without a freeze risk resulting in reduced energy usage.
  • Reduced corrosion within piping, boilers, coils and valves leading to longer life.
  • Reduced scaling in boilers and heat exchangers thus maintaining higher efficiencies.
  • Minimal issues with toxicity (if propylene glycol is used).

All of these benefits presume that the glycol solution is properly maintained annually.

Some of the risks or disadvantages are:

  • Initial cost of adding and maintaining solution.
  • Slightly higher pumping power required.
  • Reduced heat transfer from coils, heat exchangers and boilers.
  • Larger expansion volume necessary.
  • More difficult air elimination from system.
  • System flushing and clean-up might be required on “dirty” systems before the addition of glycol.

Let’s explore these a bit more.  First, it’s necessary to determine which protection level you need.  Some prefer to protect to a very low temperature point by adding more glycol, say to as low as 0-10°F.  It’s also possible to protect the system to “burst point,” which is approximately 30° lower than the freeze point, say -30 to -20°F.  At the freezing point, the solution won’t flow, but the pipes and coils won’t rupture.  Once heat and/or pumping are restored, the “slushy-like” solution melts and becomes fully liquid again.  The higher the concentration of glycol, the higher the first cost and negative impact on heat transfer and pumping power.  Generally, it makes sense to protect to the higher burst temperature criteria.  For example, a 20% solution of propylene glycol by volume would yield a freezing point of 17°F, a burst temperature of approximately -10°F, require 3% more pump power for equivalent flow, and impede heat transfer by 3%.  This alone cannot be the total answer as to system performance.

The glycol used for heat transfer applications is generally propylene glycol (P/G) with corrosion inhibitors.  Ethylene glycol also has good performance characteristics, but due to toxicity concerns, we don’t recommend its use.  P/G is very different from automotive antifreeze, which contains silicates which tends to gel, impeding flow and causing problems especially in flow control valves.  P/G is not toxic and is widely used in many applications such as a solvent and carrier of flavor or color in the food and beverage manufacturing processes, to make drinks, biscuits, cakes, sweets, as a thickener, clarifier and stabilizer in consumables such as beer, salad dressings and baking mixtures.  P/G is also used to keep tobacco semi- moist.  Ever wonder what keeps Twinkies soft for so long?

The maximum working temperature of the P/G solution is 250°F.  For ordinary heating applications, this isn’t at all a problem, but care must be taken on closed-loop solar heating systems.  It is quite possible for the fluid flow to become stagnant in the collector plates if the controls aren’t properly working.  Temperatures can easily reach this upper limit causing the P/G to break down and become acidic.

Occasionally, P/G is added to hydronic systems that provide cooling (i.e. ice arenas). Obviously the working temperature for cooling is much lower than heating which results in a much higher solution viscosity. This more viscous solution is harder to pump and impedes heat transfer more than at higher temperatures. Both of these disadvantages must be planned for.

The corrosion inhibiting properties of glycol reduces scale build up, especially in boilers.  Scaling is quite common and reduces heat transfer significantly; for example, a 1/8” scale build up in a boiler results in 30% more fuel usage overshadowing the minor heat transfer loss from the addition of antifreeze.  As the performance and efficiency of boilers and heat exchangers has steadily increased over the past two decades, the surfaces have become greater and passages smaller making them much more difficult, if not impossible, to clean this scale buildup.

Many of the strong opinions about glycol in HVAC systems from designers, installers and service technicians are a reaction to some of the challenges working with the solution.  A few of the major challenges are:

  • A water/glycol (aqueous/glycol) solution has a lower surface tension than water alone and will leak where water doesn’t.  This is especially true on automatic air bleeding valves, causing “weeping.”
  • Glycol is an “oxygen scavenger,” making air elimination much more difficult during the initial system fill.  It can take several days to bleed all of the air out of a system.
  • Annual maintenance is required to assure that the concentration, pH and general fluid quality are acceptable.  Deficiencies generally can be corrected by adding more glycol, corrosion inhibitors and/or pH correction.  Neglected systems can turn acidic and deteriorate pipes, valves, fittings and equipment.
  • Care must be taken not to isolate sections of piping or equipment such as a valve; the solution needs to be able to expand into the expansion tank upon a system freeze event such as power outage.
  • The expansion tank needs to be slightly larger than a water only system.

Automatic fill valves should be eliminated to prevent inadvertent concentration dilution in case of a small leak.  These lists aren’t intended to be a complete list of do’s and don’ts or design considerations, but hopefully provide more information to help guide your decision.  If interested, consult one of the professionals at Thayer to determine the feasibility for you system.  There’s nothing worse than the expense, damage and the “coulda, shoulda, woulda” that often follows a catastrophe such as a building freeze up.  Act now, and “Call in the Experts.




Dan Thayer, P.E.