Sizing a Drying System for a Fellowship Hall
A 4,000 sq ft church fellowship hall has a 12-ft ceiling, painted drywall to 4 ft, and a sealed concrete floor. A burst domestic line released about 500 gallons; extraction recovered 350 gallons, and the remaining water is in the drywall, baseboards, and slab surface. The carpet and pad are removed. The space is open to a kitchen through a 6-ft opening; the kitchen is unaffected. Target conditions are 80°F and 45% RH. You have 12 air movers rated 1,000 cfm each and LGR dehumidifiers rated 180 pints/day at AHAM conditions (80°F/60% RH). At 80°F/45% RH, each unit is expected to remove about 110 pints/day. Initial moisture load is estimated at 250 pints/day from wet materials for the first 48 hours, falling to 80 pints/day afterward. The church needs the hall in five days.
1. How many dehumidifiers would you specify for the first 48 hours? Show the load calculation and the effect of the 45% RH derating.
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The load is 250 pints/day. At 80°F/45% RH, each unit removes about 110 pints/day, not 180, because the AHAM rating is at 60% RH. Divide 250 by 110 = 2.27, so three units provide 330 pints/day of capacity, a 32% margin over the initial load. Two units provide 220 pints/day, which is below the 250-pint load and would allow RH to drift above 45%. After 48 hours the load falls to 80 pints/day; one unit at 110 pints/day would then be marginal, so I would keep two units for control and redundancy until readings stabilize. This is a load-based answer, not a square-foot rule. I would verify with daily condensate measurement and RH trends.
2. The church offers only two dehumidifiers. What is your revised plan, and what risks do you communicate?
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Two units give 220 pints/day, which is 30 pints/day short of the 250-pint initial load. That deficit will show up as RH above 45% and slower drywall drying unless I reduce the load or add capacity. Options: rent one more LGR or a desiccant unit; reduce the wet area by removing wet baseboard and drywall below the 4-ft cut; contain the hall from the kitchen; and use the 12 air movers to increase evaporation only if dehumidification can keep up. I would not simply run two units and promise five days. I would communicate that the first 48 hours are the critical period, that the schedule has a capacity shortfall, and that the church should authorize a third unit or accept a longer drying time and possible secondary damage. Document the load calculation and the client's decision.