STAR BRITE Premium -100° Boiler System Antifreeze - Propylene Glycol Heating & Cooling Fluid - Made in USA - 1 Gallon

Why I reached for this boiler antifreeze

I maintain a mix of hydronic systems: an Aqua-Hot unit in a motorhome, a small closed-loop radiant floor at home, and a glycol-backed solar thermal array on a detached garage. Those systems all need a reliable heat-transfer fluid that won’t corrode components or turn to slush at the first hard freeze. After a full season using the Star brite boiler antifreeze across those setups, I came away impressed with how predictably it mixes, circulates, and protects, with a few cautions worth noting.

What it is and why that matters

This is a propylene glycol–based heat-transfer fluid formulated specifically for boilers, closed hydronic loops, and solar thermal. It’s ethanol-free and contains a corrosion inhibitor package designed to protect common metals (copper, brass, steel, cast iron, aluminum) and typical seals found in hydronic equipment. At full strength it advertises burst protection to -100°F; that’s not the same as freeze protection, but it signals a very concentrated base you can dilute to a target freeze point.

I prefer propylene glycol for occupied spaces and RVs because it’s considered less toxic than ethylene glycol. You still need to dispose of it responsibly, but the safety margin around service work and small spills is better.

Mixing and commissioning

I used the fluid three ways:
- Aqua-Hot: top-up and partial exchange after a minor service.
- Radiant floor: complete flush and refill of a 1-1/4” PEX loop in a slab zone.
- Solar thermal drainback loop: partial replacement after a stagnation event.

Being a concentrate, it’s designed to be diluted with demineralized or distilled water. I followed the label chart to set different targets for each system. A few practical notes from the process:

• Use a refractometer with a propylene glycol scale. It’s the quickest way to verify your final concentration in the loop, especially when residual water in the system can otherwise throw off your math.
• If you’re flushing a neglected loop, plan on at least two rinses with demineralized water to clear scale and old additives before filling. Inhibitors don’t play nicely with unknown residues.
• Don’t chase the lowest possible temperature “just because.” Higher glycol percentages increase viscosity and reduce heat capacity. For most temperate regions, a 30–40% mix is appropriate; colder climates might push higher, but there are diminishing returns.

In my radiant loop, I settled around the mid-30% range. With the Aqua-Hot, I matched the manufacturer’s recommended concentration and confirmed with a refractometer at the service port. Both systems bled out cleanly. I didn’t observe foaming in the fill bucket, and the fluid seemed to purge air as expected through the microbubble separator.

Performance in real use

Heat transfer
- At reasonable concentrations, heat transfer was in line with what I expect from propylene glycol. In the radiant floor, delta-T across the manifold matched pre-change baselines once I increased the circulator speed one step to account for the slightly higher viscosity. That’s normal behavior with glycol and not unique to this product.

Pumps and seals
- The ethanol-free formulation is a plus. Some “RV-style” antifreezes rely on alcohols and can be harsh on seals or cause vapor issues. Here, the fluid behaved predictably under both low and moderate head. The low-power pumps in the solar loop had no trouble moving it after I set the concentration properly.

Corrosion protection
- Inhibitors are the hidden value in a boiler fluid. After a season, my pH readings remained in the expected alkaline range for a PG inhibitor package, and a peek inside a brass air separator showed no visible tarnish beyond normal patina. That’s a good early indicator that the additive package is doing its job.

Stability under heat
- Solar thermal loops are the hardest test because stagnation can cook glycol and form acids. I intentionally replaced a portion of the solar loop after a hot summer; the fluid kept its clarity and didn’t show the sharp acidic odor that signals degradation. Still, any glycol in a solar loop should be monitored closely, and I treat 3–5 years as a realistic service interval or sooner after an overheat.

Practical tips to get the most from it

• Always mix with demineralized or distilled water. Hard water will eat your inhibitor reserve and can precipitate scale.
• Verify with a refractometer. Trust but verify; charts get you close, instruments make you accurate.
• Target the freeze point you actually need. Full-strength is for storage and shipping, not for best thermal performance. Most hydronic systems are happiest below 50% glycol unless you truly need very low protection.
• Check pH and inhibitor reserve yearly. Simple test strips will tell you if it’s time to add a booster or replace the fluid.
• Keep oxygen out. Closed systems should have a working expansion tank and tight connections. Oxygen ingress accelerates corrosion and burns through inhibitors.
• Label the system. Note the fill date and concentration on the near-boiler piping. You’ll thank yourself the next time you service it.

Compatibility and caveats

• It’s designed for hydronic boilers, closed loops, RV hydronic heaters (Aqua-Hot/Hydro-Hot), and solar thermal. It’s not a potable water antifreeze and not for automotive engines.
• Avoid mixing with other unknown glycols. If you’re switching brands, a full flush is smart to avoid inhibitor clashes.
• Viscosity rises quickly at high concentrations and low temperatures. If you’re operating near the edge of a pump’s curve, plan accordingly with proper dilution and, if available, a higher pump setting.
• The jug is a standard one-gallon container. As with any fluid shipment, make sure caps are tight before staging indoors. My jugs arrived intact; I still store them in a tray during transport to keep things tidy.

Cost and value

You can buy cheaper glycols, but they often lack a robust inhibitor package or include alcohols that I’d rather not put in hydronic components. Considering the concentrate status—one gallon can yield multiple gallons of working solution—the per-gallon-in-system cost is reasonable. More importantly, it behaved like a purpose-made boiler fluid, which is really what you want: predictable freeze protection, quiet pumps, clean metals, and no surprises.

What I’d change

• A more prominent on-label reminder separating “burst” from “freeze” protection would help newer users set expectations. This is a general industry confusion point, but clarity matters.
• I’d love to see a built-in tamper seal beneath the cap to reassure users who are storing multiple jugs season to season.
• A small included chart card or QR link to a mixing calculator would be handy on the job.

None of these are dealbreakers. The fundamentals—the base chemistry, inhibitors, and field behavior—are well executed.

Who it’s for

• RV owners with Aqua-Hot or Hydro-Hot systems who need a compatible, ethanol-free boiler fluid.
• Homeowners and pros maintaining closed-loop hydronic boilers and radiant floors who want reliable corrosion protection.
• Solar thermal users who understand the maintenance that glycol loops require and want a fluid that tolerates intermittent high temps when properly maintained.

Bottom line recommendation

I recommend this boiler antifreeze. It’s a concentrated, ethanol-free propylene glycol with a solid inhibitor package, it mixes cleanly, protects as expected, and stays friendly to pumps and metals when you set the concentration correctly. If you need a purpose-built hydronic fluid for an Aqua-Hot, a traditional boiler loop, or a solar thermal circuit, it hits the mark. The price reflects the specialized formulation, but the real value shows up over time: stable pH, quiet circulation, and components that still look good at the next service interval.