Glycol
An antifreeze blended into the water of a hydronic loop to lower its freezing point and protect circuits exposed to cold.
What is glycol in a hydronic loop?
Glycol is an antifreeze blended into the water of a heating or cooling circuit to lower its freezing point. In the Québec climate it protects the parts of a loop exposed to cold — outdoor air coils, rooftop units, piping running through unheated spaces — against freezing and burst pipes when equipment goes down in mid-winter.
Propylene or ethylene: how to choose
Ethylene glycol transfers heat somewhat better at equal concentration, but it is toxic if ingested. Propylene glycol, far less toxic, is the choice wherever contact with potable water or food is possible. Either way the product must be formulated for HVAC service, with corrosion inhibitors — never automotive antifreeze, whose additive package does not suit the materials of a hot water boiler or the seals of a building circuit.
Effect on heat transfer
A glycol blend is more viscous and carries less heat per litre than water. For the same output, more flow is needed, so the heat exchanger and the pumps have to be selected accordingly. Concentration is set from the freeze protection actually required and the manufacturer’s limits: going beyond what is needed buys a performance penalty without useful added protection.
Monitoring the fluid over time
Glycol ages: it oxidizes, forms organic acids and sees its pH fall as inhibitors deplete. A degraded fluid becomes more corrosive than water. Periodic checks of concentration and pH belong to the same program as water treatment and fit naturally into the building’s planned maintenance schedule.