If you manage a commercial building in Massachusetts, New Hampshire, or Rhode Island, you may have run into a frustrating situation.
You added glycol to your hydronic system to prevent freezing. Everything seemed protected. Then a cold stretch hits, and suddenly you are dealing with frozen lines, poor flow, or even burst piping.
It raises a fair question.
If glycol is supposed to prevent freezing, why are your pipes still freezing?
At Harold Brothers Mechanical Contractors, we see this issue across New England every winter. In most cases, glycol is present, but the system still fails under real operating conditions.
In this article, we will explain why hydronic lines can freeze even when glycol was added, what is actually happening inside your system, and how to prevent it from happening again.
Glycol is added to hydronic systems to lower the freezing point of water and protect piping from damage during cold weather. It is a critical part of freeze protection in many commercial buildings across New England. However, this is where many building owners get misled. Glycol does not guarantee that your system will never freeze.
For glycol to work properly, several conditions must be met. The concentration needs to be correct, the fluid must be properly maintained, and the system has to be designed and operating the right way. If any one of these factors is off, the protection glycol provides can break down, and your system can still freeze. This is why ongoing HVAC maintenance is crucial in commercial buildings.
This is the number one issue we see.
Many systems start with the correct glycol percentage, but over time:
This dilutes the mixture.
When that happens, your freeze protection rises closer to 32°F, which puts your system at risk during a cold snap. To avoid this, make sure that levels get checked during maintenance visits.
What it looks like:
Your system can freeze solid and still not burst. That does not mean it is protected
Some glycol mixtures are only designed for burst protection.
That means the fluid can still turn into a slushy or partially frozen state, restricting flow inside the piping.
What it looks like:
Even with proper glycol levels, your system can freeze if water is not moving.
Common causes:
When flow stops, cold areas like rooftop piping or perimeter loops can freeze quickly. Glycol helps, but moving water is still your best defense.
In New England, this is a major issue. Wind chill has no effect on fluid temperature, but it accelerates heat loss from piping.
Even with glycol, piping that is:
can drop below the fluid’s protection temperature.
Once that happens, freezing can begin at specific points in the system.
Where we see this most:
Glycol is not a set-it-and-forget-it solution.
Bacteria and contaminants can also degrade glycol and reduce its freeze protection ability.
What it looks like:
This is a bigger issue than most people realize, and why knowing how your hydronic system is designed is so important.
Glycol is thicker than water and does not transfer heat as efficiently.
If your system was originally designed for water:
That combination can create cold spots and lead to freezing in certain areas.
If your hydronic lines froze even though glycol was added, it is rarely caused by a single issue. In most cases, it is the result of several factors working together within the system.
These problems often include incorrect glycol concentration, poor flow or circulation, system design limitations, and a lack of ongoing maintenance. When these conditions exist at the same time, the system becomes much more vulnerable during cold weather. This is why many buildings experience the same freezing issues year after year instead of resolving the problem for good.
Watch for these early warning signs:
These are all indicators that your system may not be properly protected
The solution is not just “add more glycol.”
It requires a system-level approach:
Use a refractometer to confirm proper concentration.
Even small leaks can dilute your protection over time.
Ensure pumps, valves, and air elimination devices are working correctly.
Protect vulnerable piping from cold air and wind.
Older systems may need adjustments to handle glycol properly.
Hydronic lines freeze with glycol not because glycol failed, but because the system around it is not operating the way it should.
Glycol is only one piece of freeze protection. Without proper flow, concentration, and system design, it cannot do its job.
If your building struggles with freezing, it may point to a deeper issue in how your hydronic system is designed and operating. To understand why some systems perform better than others, read our guide on hydronic system design.
At Harold Brothers Mechanical Contractors, we help building owners across New England diagnose and correct these system-level issues before they turn into costly failures.