Plain water is the default fill for most indoor hydronic systems because it transfers heat efficiently and is inexpensive, while a glycol and water mixture is used specifically where freeze...
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Plain water is the default fill for most indoor hydronic systems because it transfers heat efficiently and is inexpensive, while a glycol and water mixture is used specifically where freeze protection is needed, such as an outdoor loop, an unheated space, or a system in a building at real risk of extended heat loss during a power outage.
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Water is an excellent heat transfer fluid, readily available, and inexpensive compared with glycol, which is why the vast majority of indoor hydronic systems in heated Metro Vancouver homes run on plain water with a corrosion inhibitor added rather than any glycol mixture. Glycol has a lower specific heat capacity than water, meaning a glycol mixture carries somewhat less heat per unit volume moved through the system, so adding glycol where it is not needed is a real efficiency trade-off, not a free upgrade.
The exception is any part of a hydronic loop that could realistically drop to freezing temperature, since plain water freezing inside piping or a coil can crack fittings and split pipe, which is a far more expensive problem than the mild efficiency cost of using glycol where it is genuinely needed.
A hydronic loop fully inside a continuously heated home rarely needs glycol, since the building itself, and the boiler running normally, keeps the loop well above freezing under ordinary conditions.
| Factor | Plain water | Glycol mixture |
|---|---|---|
| Freeze protection | None | Protects to a set concentration-dependent temperature |
| Heat transfer efficiency | Higher | Somewhat lower per volume |
| Compatibility with boiler | Universal | Some boilers restrict glycol; check manufacturer guidance |
| Maintenance | Periodic inhibitor check | Periodic concentration and inhibitor check |
Not every boiler is designed to have glycol circulated directly through it, which is why systems like snow melt loops commonly use a separate heat exchanger to keep the glycol mixture in its own loop, isolated from the boiler's own water, protecting the boiler's warranty and its internal components from a fluid concentration it was not necessarily designed around.
Before adding glycol to any system, or to a new loop off an existing boiler, a technician confirms whether the boiler manufacturer allows glycol in direct contact with the boiler or requires a heat exchanger to isolate it, calculates the concentration needed for the actual freeze risk (a stronger concentration for a genuinely cold, unheated space, weaker for a mild freeze risk), and uses a glycol product formulated for hydronic heating systems rather than an automotive antifreeze, which contains different additives not appropriate for a home heating loop.
Safety note
This is part of the closed water loop and controls tied to a gas-fired boiler, so it should be assessed and adjusted by a licensed technician. If you ever smell gas near the boiler or mechanical room, leave the home first, then call FortisBC at 1-800-663-9911.
Usually not, since an indoor loop inside a continuously heated home rarely faces freezing temperatures; glycol is typically reserved for outdoor loops or unheated spaces.
No, automotive antifreeze contains additives formulated for engine cooling systems, not hydronic heating loops, so a glycol product made specifically for hydronic systems should be used instead.
A glycol mixture carries slightly less heat per volume than plain water, so where glycol is not needed for freeze protection, plain water with a corrosion inhibitor is the more efficient default.
Many boilers are not designed to have a glycol mixture circulated directly through their internal components, so a heat exchanger keeps the glycol loop separate from the boiler's own water while still transferring heat between them.
It depends on how cold the specific loop could realistically get, which a licensed technician calculates based on the location and exposure of that piping rather than a one-size-fits-all percentage.
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