Two Ways to Keep a Solar Loop from Freezing

What is the difference between drainback and glycol?

In a glycol system, a mix of water and propylene glycol sits in the collectors at all times. The loop is sealed and pressurized, and a heat exchanger passes heat to the household water. Freezing is prevented by the chemistry.

In a drainback system, the loop holds ordinary water. When the pump turns off, gravity pulls the water out of the collectors and exposed pipes into a drainback tank that sits in the heated space. With the collectors empty, there is nothing to freeze. When the pump restarts, it refills the loop.

What are the advantages of a glycol loop?

Glycol layouts are familiar to most installers and tolerate pipes that do not slope perfectly. They can be used where the storage tank is above the collectors or where the pipe run is long. The cost is maintenance: the fluid ages, becomes acidic if overheated and loses its freeze protection, so it should be tested and replaced on a schedule. The maintenance guide lists the checks.

Glycol is also more expensive to install than a drainback layout once you add the heat exchanger, pressure gauge, relief valve and expansion vessel.

What are the advantages of drainback?

Water is a better heat carrier than glycol mixtures, so the pump can be smaller. There is no fluid to degrade. And because the collectors empty during a power failure or when the tank is hot, drainback layouts handle overheating well. The trade is geometry. Every collector pipe must slope continuously back to the tank, no sags allowed, and the drainback tank must sit below the collectors. A loop installed with a sag can trap water and freeze and split the pipe. Good workmanship matters more than the choice itself, so look for installers with several drainback jobs behind them.

Because drainback uses water, a small leak is a nuisance rather than a chemical spill, and the water can sometimes be topped up by the owner. Glycol leaks need the right fluid and a pressure check. A drainback pump must also be strong enough to lift the water to the roof and refill the loop, which can mean a larger pump than a glycol system on a tall house, so ask about pump head and noise at start-up. If you hear a gurgle as the loop fills, that is normal, but a gurgle that never stops can point to an air problem.

Which is better in Canada?

Both work in very cold climates, and both are common. Choose on site conditions. If the mechanical room is under the roof and the pipe run is short and straight, drainback is simple and cheap to run. If the tank has to be higher than the collectors or the run is long and awkward, a glycol loop is often the practical answer. Cost differences show up in the cost guide, and the explainer on how the loop works is a useful companion.

Collector type also interacts with the choice. Tubes and flat plates behave differently when stagnant, as the collector comparison explains.

What about a large system?

At Drake Landing in Okotoks, 800 collectors fed a district loop that had to survive Alberta winters without any household attention. Large systems of that kind put emphasis on controls, pressure management and ease of servicing. The Energy Centre page shows how the loop connected to the short-term tanks.

Whichever loop you pick, ask the installer to describe in plain words what happens on a hot, sunny day when nobody is home and the tank is full, and what happens in a blackout in January. A good answer names the safety valve, the pump behaviour and the drain or dump path. A vague one is a warning sign, because overheating and freezing are the two ways a poor solar loop gets damaged.

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