Fluids and chemicals for HVAC and geothermal

Commercial chillers, hydronic heating loops and geothermal heat pumps all depend on a closed loop of water or glycol that has to move heat for years without freezing, corroding or fouling. Choosing the fluid means balancing heat transfer against freeze protection and long-term protection of the metals in the system.

Ethylene glycol carries heat better and stays less viscous in the cold, so it is common in closed industrial and commercial chiller loops. Propylene glycol is much less toxic and is the usual fluid for geothermal ground loops and buildings where a leak could reach people or food. Both should be inhibited.

Premixes such as a 30/70 or 40/60 inhibited glycol arrive at a known ratio. Concentrates let you mix to the loop's ratio with deionized water, which keeps minerals out of plate heat exchangers and narrow ground-loop piping. A neglected fluid shows up as corrosion, scale, lower heat pump COP and early pump failure.

Grade decides where a glycol can go. Propylene glycol USP grade meets the U.S. Pharmacopeia monograph, and propylene glycol is generally recognized as safe for food use under 21 CFR 184.1666. Food plants usually specify an inhibited propylene glycol whose inhibitors are acceptable for incidental food contact for loops where a leak could reach product. Technical-grade propylene glycol is made for industrial closed loops and should not stand in for it.

Grade and inhibition are separate questions. A USP or technical glycol with no inhibitor turns acidic as it oxidizes, strips the protective oxide film from copper and steel, and can force a flush and refill within months. For most building and geothermal loops the right product is an inhibited glycol, such as an inhibited propylene glycol premix or 100% propylene glycol, inhibited, mixed with deionized water.

Rules for HVAC fluids come mostly from local codes and environmental protection. Many states regulate which antifreeze may go into vertical geothermal bores to protect groundwater, and propylene glycol is the most widely accepted. Check state well rules and your local mechanical code before you choose.

OSHA's Hazard Communication Standard (29 CFR 1910.1200) requires that SDS for every heat transfer fluid on site be available to maintenance staff. Ethylene glycol is an OSHA hazardous chemical, so a site storing 10,000 lb or more files EPCRA Tier II reports. Spent glycol should be recycled or disposed of under local rules, not drained to a storm sewer. Food plants follow FDA rules for incidental food contact and usually set their own fluid specification. Find each product's documents in the SDS library.

• Diluting concentrate with tap water. Calcium and magnesium can drop the inhibitor out as sludge, which leaves metals unprotected and clogs strainers and plate heat exchangers. Mix with deionized water.
• Ordering for the average winter. A 30% ethylene glycol mix protects only to about +4°F (-16°C). A northern site that orders it to save money risks burst coils in a cold snap.
• Buying uninhibited glycol. Without inhibitor the fluid becomes corrosive, and aluminum fins and brazed joints go first.
• Mixing glycols or inhibitor packages. Ethylene and propylene glycol together make refractometer readings unreliable, and mismatched inhibitors protect less.
• Over-concentrating. More glycol than the climate needs costs heat transfer and pump energy.