Hydronic Freeze Protection


Heat trace freeze protection heating cables have been installed under the insulation of the outdoor hydronic piping which has been connected to an air source heat pump.

For hydronic heating systems in colder climates, ensuring your system is properly set up to battle chilly nights will save the system in the long run.

By Roy Collver

Plain H2O is an amazing heat transfer fluid. It can be repeatedly charged up with heat—deliver it to where it is needed, release it, and then go back for more—indefinitely. Additionally, a given volume of water can absorb 3,400 times more heat than the same volume of air at the same temperature rise, allowing us to use little pipes to move a lot of energy. A 14-inch by eight inches duct can be replaced by a three-quarter-inch pipe—your choice.

Ensuring that pipes aren’t going to freeze during the chilly months is an important step in the installation process.

Unfortunately, one property of water that isn’t so helpful is that it expands by approximately nine per cent when it freezes. Because water is pretty much incompressible, that nine per cent is more than enough to bust a cold climate hydronic system apart. This is Canada and it gets cold after all.

Practical suggestions

Never let the space drop below freezing where there are hydronic components. Make sure to contain the whole system within the allotted space. This might include insulating piping components if they are close to outside walls and make sure there are no outdoor air intakes in close proximity. It could also include circulating heating fluid constantly even when the boiler or heat pump is not heating. Designing the system with redundant major components like boilers and pumps can keep the system running if there is a failure. With this approach, it will never run out of fuel or electricity. Back-up generators are recommended even in areas that normally have reliable power. Most readers will remember back in 1998 when the ice storms swept through Eastern Ontario and Quebec. Multiple days without power in the dead of winter is likely going to freeze most plain water systems.

Drain-down system design

If you’re going to go with the pure water route, you may want to make provisions to design the system for cold weather drain-down.

Drain-back is a popular option in some solar thermal systems where water is drained from the exposed solar panels into a holding tank inside the conditioned space. It is useful as an emergency procedure (or seasonal) for plain water hydronic heating systems. Careful design is required to make sure there are no water traps, and an adequate slope is maintained for gravity drainage. It only takes a cup of water remaining in the wrong place for this to fail. It is possible to design one of these systems with vacuum breakers, dump valves and a “freeze-stat” for automatic drain-down—most depend on a manual dump procedure. That means the system must be attended by someone trained in the procedure whenever freeze-up potential exists. Blowing out a system. Blowing out a system with compressed air may speed up the process, but it can damage some hydronic components and if the power is out, how will operators run the compressor?

Testing materials are an important part of determining glycol concentrations. One size does NOT fit all—each type requires a specific tester.

Plan for redundancy

If the system does include a back-up generator, electric heat can be used in a number of ways to protect your hydronic gear. The whole building could have a redundant electric back-up heating system.

This is actually a popular option in remote areas where continuity of fuel supply may be a bit dodgy, or when heating with handstoked solid fuel appliances. Most popular is a redundant electric baseboard heating system, and while I have seen electric boilers used as back-up to oil, propane and solid fuel boilers, a main pump failure can still take the system down. Spot heating of poorly insulated rooms with electric heaters is also done quite often to prevent local freeze-up during extreme cold events.

“Heat trace” cables are often installed under a layer of pipe insulation in colder rooms or in places where hydronic (and domestic water) piping travels through unconditioned spaces. Quality heat trace cables are temperature self-regulating, reliable, last for many years, and don’t cost a fortune to run—but pipes can cool off quickly during a power outage.

Danger zone

Insulate piping to contain heat, circulate water constantly and do not allow the system components to drop below the freezing point, even as the space is allowed to cool down.

In some buildings, the temperature in the conditioned space is allowed to drop close to the danger point in order to save energy. As long as the distribution components are insulated, the boiler can maintain the system at a freeze protection temperature. Still, be cautious when doing this—being too aggressive with this strategy is not recommended in the extreme climate areas of Canada (pretty much everywhere except southwestern, coastal B.C.).

Thermostatic radiator valves need to have a freeze protection setting that will allow a trickle of water through to keep radiators safe, and air handlers and fan coils should be equipped with freeze-stats to kick on pumps and disable fans during cold periods.

Ventilation air needs to be tempered by electric heating elements as it is drawn into the building. Deep temperature setback for plain water hydronic systems is not for the faint of heart. Which brings us to the most popular and safe freeze protection method—add some anti-freeze.


If you own, design, install or are responsible for hydronic heating systems in cold weather climates and want to sleep better at night— glycol is your best friend.

Many installers dislike adding glycol because it changes the viscosity of plain water and can all of a sudden reveal fluid leaks that didn’t show up before. Designers aren’t keen on it because it reduces the specific heat vs. plain water and requires higher water flows to compensate for the reduced heat transfer. The budget-conscious don’t like it because it is expensive (just about anything is expensive compared to plain water). Maintenance doesn’t like it because it has to be tested regularly and becomes an ongoing maintenance item. But it can save your butt if things go wrong.

Communication between the trades is important when it comes to the safety of the installer. Here, a label has been applied by the heat trace cable installer warning that there are live electrical wires hidden under the pipe insulation.

Ethylene and propylene glycols are the two types commonly used to protect hydronic systems from freezing, but because of its toxicity, ethylene glycol has largely fallen out of favour in the HVAC industry. Ethylene has some slight advantages in viscosity and heat transfer, but if you have a leak on the basement floor and the puppy laps it up because it tastes sweet—you are likely going to need to go out and get yourself a new puppy.

Use demineralized water

Propylene glycol heat transfer fluids are generally made of food-grade glycol and are considered low or non-toxic. Hydronic heating and HVAC applications use specially formulated glycol with other low toxicity chemical additives to help protect system components and buffer the fluid to neutralize glycolic acid as it forms. It can be ordered in drums, pre-mixed with demineralized water, or it can be ordered full strength in jugs and mixed on site with demineralized water. ALWAYS use demineralized water so you don’t mess with the inhibitor chemicals that have been carefully formulated to keep your system in good working order. A glycol feeder tank is a no-brainer for any system using glycol.

As always it is critical to the long-term health of your system to follow the manufacturer’s instructions. They will detail how to mix the glycol with water, what concentrations are required for different levels of freeze protection, and how to test the fluid once the system has been filled and mixed. Hydronic fluid manufacturers require a minimum concentration (25 per cent glycol to water is common), in order for the inhibitor chemicals to be effective against the bacteria and fungus that can grow and thrive inside your system and provide adequate buffering. Once tested and confirmed to be good, maintenance people should be supplied with the correct tools and trained to properly test at required intervals. A test sheet should be prepared and left on site.


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