Mystery Leak

April 1, 2005
We had been enlisted to perform a 1,000-sq-ft tenant fit-up within an 8-year-old medical office building. A large meeting room was to be converted to

We had been enlisted to perform a 1,000-sq-ft tenant fit-up within an 8-year-old medical office building. A large meeting room was to be converted to a doctor's suite with offices and exam rooms. The suite was to be located next to an indoor therapeutic pool. On strict orders from the owner, pool odors were not to enter the suite.

The room that housed the indoor therapeutic pool was extremely positively pressurized, as evidenced by doors that would just about stand open from the torrent of exiting air. The corrosive pool vapors had previously laid waste a control board in the DDC panel, and periodic failures of electric strip heat within terminal units were occurring.

On the roof, a natatorium-dehumidification unit (NDU) was present. It was a split-style system. The air-handling piece was comprised of a deep DX cooling coil, a downstream refrigerant hot-gas-reheat coil, and a draw-through supply fan. Included in the air stream, just upstream of the supply fan, were the compressors and a refrigerant to pool water heat exchanger. Strip heat was present also. An outside-air-inlet hood was situated to introduce outside air just downstream of the DX cooling coil.

The NDU provides dehumidification before rejecting heat back to the pool water (free pool heating). If pool heating is not needed, then heat is rejected to the air-cooled condenser.

The outside-air hood was vigorously sucking outside air. Bingo — this was a single-fan system. With no return fan to encourage a relief path, the introduced outside air had no place to go. It sought a way out of the building, spreading pool vapors en route. We recommended adding an exhaust fan to serve the pool room to provide the needed relief path.

But this is not the real story. The real story is the mystery leak that had plagued the pool. A garden hose was required to be left running to add water to the pool on a continuous basis. A leak-detection agency had found no evidence of underground leaks.

In the pool-equipment room, the condensate drain line that served the roof-mounted NDU was observed discharging its ever-present load to a hub drain. This stream of water was in plain view, but drew no more attention than the hum of the pool pump. It was an expected result of proper dehumidification.

On the roof, I removed an access panel and observed the compressors not operating. I removed another access panel and observed a wet condensate drain pan.


The compressors were off. A ⅜-in. flexible tube was discharging a swift stream of water into the drain pan. I traced this tube back to the fan section. Eureka! An automatic air vent was situated adjacent to the pool-water heat exchanger. It was stuck open, which explained the continuous flow of pool water discharging to the condensate drain pan. What had appeared to be cooling-coil condensate discharging from the unit actually was the pool-water leak. The leak was estimated to be half a gallon per minute, or a quarter of a million gallons per year.
Gary Gabriel, PE
CDH Partners Inc.
Marietta, Ga.

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