Let me extend my gratitude to those of you who’ve written to me over the past few months in response to the business-centric columns I wrote in the first half of 2024.

I’m excited to learn that many of you share some of my perspectives, and I admire how you’re navigating the challenges we currently face. I’m not a business expert, but to use a cliché, I attended the school of hard knocks, and I’m happy to share those with you — and even happier to learn from anyone willing to share with me. 

With that said, I’ll take a step back from economics and business development for a while. Let’s get technical. Why not start with antifreeze?

Using antifreeze in hydronic systems is a necessary evil when the application calls for it. My company uses antifreeze more often than most heating contractors, and we’ve learned a thing or two along the way.

By using antifreeze, we provide freeze protection for commercial chiller systems, snowmelt loops, old houses with poor insulation, vacation homes — literally any application with a possibility of freezing. Antifreeze in a hydronic system is an insurance policy, but only if done correctly. If it’s done improperly, it can quickly become a liability.

Two things I cannot stress enough when using antifreeze: systems must be specifically designed for the use of antifreeze; and systems with antifreeze must be maintained.

We’ll cover maintenance in more detail later, but it’s worth mentioning now that all hydronic system antifreeze chemicals require maintenance and testing. 

Hydronic antifreeze additives have come a long way in the past decade, thanks to product research and development on the part of chemical companies; antifreeze isn’t as maintenance-heavy as it once was. Regardless, the freeze protection provided by any chemical product can decline over time if not maintained, and antifreeze can turn caustic (acidic) if neglected. 

The pH of the system fluid must be tested and balanced. If left to its own devices, antifreeze can destroy a hydronic system. 

Types of Antifreeze

Most of the antifreeze my company uses is either Hercules Cryo-Tek or Fernox Alphi 11. 

Generally, the refractometers sold at your local auto parts store work with propylene glycol solutions, so you don’t need a proprietary refractometer. This hand-held device tests the level of freeze protection a water/glycol solution provides. While all antifreeze chemicals require maintenance, propylene glycol probably requires the least.

Other antifreeze chemicals, such as ethylene glycol or potassium formate, can be used in a system. These alternatives are sometimes used in chiller installations or where food-grade designations are needed. 

In our experience, ethylene glycol and potassium formate require proprietary testing equipment or more frequent maintenance, so we avoid using them if possible. In fact, we’ve found that potassium formate is caustic at higher concentrations, even when properly maintained, and I’m not aware of a refractometer that works with it. I’m sure there are applications for this antifreeze, but it’s not for the average heating contractor. 

Ethylene glycol is used in some food industry applications. We’ve found that it turns acidic more quickly than propylene. I’ve seen a cast-iron boiler rot out in less than a decade because the ethylene glycol solution within the system wasn’t maintained. 

Also, I have not come across an ethylene product with a rust inhibitor, which is almost standard across the propylene chemicals available to our industry. Be aware that ethylene glycol is toxic if ingested.

System Design

Hydronic systems must be designed to use antifreeze; it’s as important as knowing how much pipe and radiation is in the system.

Using antifreeze requires two big considerations during the design process:

Propylene glycol has a lower BTU carry capacity than water.

Glycol is more viscous than water, making it harder to circulate.

Using a glycol solution in a hydronic system lowers the BTUs the system fluid can carry. This must be considered when sizing all system components, especially the radiation. Once you exceed a concentration of 25 percent, the reduction in BTU capacity is rather significant; losing five to 10 percent is common.

We use LoopCad software to design most of our systems. This program allows the user to input whether glycol is used and at what concentration. It sizes all system components accordingly. 

As I mentioned, using antifreeze increases the viscosity of system fluid. Water has a viscosity of 1.0. Typical hydronic fluid with a glycol mixture could have a viscosity of 1.4 or 1.6, depending on the concentration. 

Because this increases head requirements, pumps must be sized accordingly. Compare the pumping requirements of your system to the pump curves provided by circulator manufacturers to select an appropriate model. If you need the pump curve for specific circulator models and can’t find them online, contact your manufacturers’ representative.

There are several other design considerations. Make sure all the parts in your system are rated for use with a higher glycol solution; some may not be. For example, some press fittings are not rated for use with glycol solutions over a certain concentration, but this varies depending on the application. 

Also, automatic air vents and glycol do not play well together, regardless of brand, style or size. In my experience, automatic air vents rarely self-seal once they’ve come in contact with antifreeze. When laying out your boiler system, be mindful of where your bleed points are. They should be readily accessible, and all vents should include isolation valves so they can be easily replaced or isolated after the system is purged of all air.

Air elimination is more difficult on a system filled with antifreeze. We painstakingly bleed these systems and then close the air vents. We also replace the air vents more frequently than in systems without antifreeze. 

Finally, we install as many nonferrous components as possible, such as stainless-steel circulators. If neglected, acidic antifreeze will attack all metal components, but ferrous metals are damaged more rapidly.

Picking a Concentration

It’s important to know how much antifreeze a system needs. This is largely determined by how cold your area gets and the level of exposure the system piping has. 

Most antifreeze buckets feature two numbers corresponding to different concentrations: the temperature at which the system fluid will freeze, and the temperature at which the system fluid will burst piping and components. The freeze point is the temperature at which ice crystals first form. This does not mean the solution will freeze solid at that temp, but it can happen.

Our systems in New York and Pennsylvania are at lower risk of freezing than a system in, say, Duluth, Minn., so we generally select a glycol concentration for a worst-case scenario of 20-degree freeze point. This generally offers burst protection to -10 degrees in southern areas of our territory and slightly lower freeze and burst points in more northern areas of our territory. 

Many installers use concentrations to protect against freezing rather than bursting. We prefer to protect against bursting. The higher the concentration, the more pronounced the negative effects of antifreeze (the need for maintenance, higher head, lower BTU transfer, etc.). Our goal is to protect against damage to the system while also offering the highest system performance. Protecting against a burst and subsequent property damage is our goal. 

Makeup Water

Using any antifreeze product in a boiler system creates an additional consideration for designers and installers: the need to provide makeup water appropriately.

Imagine if a boiler filled with antifreeze solution was equipped with a conventional feed valve and developed a slow leak. Over time, the concentration of antifreeze in that system would grow weaker, lowering its ability to protect the system. This is one reason that systems filled with antifreeze should not feature a traditional feed valve.

The other issue is that most municipalities require a system with antifreeze to be equipped with a double backflow preventer between the system and the potable water supply. That said, all municipalities prefer to see a system feeder completely disconnected from the water supply. 

Glycol feeders are vessels hand-filled with a glycol solution. We always install a feeder so we can visually inspect the system fluid level, fill the system with antifreeze solution instead of pure water and have an alarm on the system. The Axiom feeders we use feature an alarm contact, which we wire to the boiler control to shut the boiler down if the feeder runs dry. 

Maintenance

I can’t overstate the need to maintain a boiler system with antifreeze. Our annual maintenance regimen includes pH testing of system fluid. A pH of 8.5 is generally recommended, but refer to the boiler installation and operating manual for a safe pH range. Testing is conducted with litmus strips sold by the antifreeze manufacturer. 

If we find that the fluid is acidic, we add a corrosion inhibitor to the system. After allowing the system to circulate for half an hour or more, the pH test must be conducted again.

We also use a refractometer to determine that the freeze protection level is still on target. If we discover we need more protection, we’ll add antifreeze to the system. 

Once we’re certain the system fluid is in good shape, we check that no air is in the system and close all the system’s auto air vents. 

Using antifreeze in a hydronic system adds upfront and maintenance costs, but those are grossly outweighed by the benefits in situations with freeze potential. 

As with all HVAC systems, creating a functional, long-lasting antifreeze system starts in the design phase and relies on diligent maintenance. Do this, and your customers will be happy with the product, even in the event of a severe weather event. l 

Tom Soukup is the principal of Patriot Water Works Co., with more than 20 years as a hydronic designer and installer.