Glycol Concentration and Flush-Water Disposal at an Occupied School
A 6,000 ft² school addition has an eight-borehole vertical loop, each 350 ft deep, 2 in HDPE, with a calculated system volume of 320 gal. The design loop minimum temperature is 20°F, and the manufacturer requires freeze protection at least 10°F below that minimum. The contractor proposes a 20% propylene glycol mixture by volume, mixed in a 55 gal drum and checked by a floating-ball hydrometer. Before adding glycol, the crew plans to flush 500 gal of chlorinated municipal water through the loop and discharge it to a storm drain because it is just water. The school is occupied during construction, and the loop vault is near a playground.
1. Evaluate the proposed 20% propylene glycol concentration and the method of verification. What should be required?
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The manufacturer requires freeze protection at least 10°F below the 20°F minimum, so the target freeze point is 10°F or lower. A nominal 20% propylene glycol mix is unlikely to provide that margin; verify against the manufacturer's freeze-point chart and likely need about 30% by volume. Do not rely on a floating-ball hydrometer alone without temperature correction; use a refractometer or the glycol supplier's method. Calculate the mix for the actual 320 gal system volume, circulate thoroughly, and check concentration at the farthest point. Record the freeze point and add biocide/inhibitor as required.
2. Is discharging the 500 gal of chlorinated flush water to a storm drain acceptable? What environmental and safety controls should apply?
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Discharging 500 gal of chlorinated flush water to a storm drain is not acceptable without checking local pretreatment requirements; storm drains often lead to surface water. Contain the flush water, test chlorine and pH, dechlorinate if needed, and discharge to sanitary sewer only with approval. Never discharge glycol to a storm drain or ground. Because the school is occupied and the vault is near a playground, use barriers, lock the vault, keep hoses out of walkways, and follow confined-space and electrical safety rules.
3. What pressure-testing and fluid-fill sequence should be followed before the system is turned over to the school?
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Pressure-test the loop with water before adding glycol, because glycol is costly and a leak is harder to clean up. Protect the test water from freezing with temporary heat or a safe antifreeze procedure approved for the test. After the test passes, purge air, add the correct glycol mix, circulate until uniform, and recheck concentration and pH. Verify that the expansion tank, relief valve, and pump seals are compatible with propylene glycol, and label the system with the fluid type and concentration.