Dental Office Tankless Heater and Hand Sink
A 1,200-square-foot dental office has an assigned occupant load of 10 and one existing single-user restroom containing a water closet and a lavatory. The tenant adds a sterilization room with a hand sink and replaces the old 40-gallon tank water heater with a 199,000 Btu/h condensing tankless heater in a 30-inch by 30-inch closet with a louvered door. The new heater serves two lavatories and the hand sink and has no storage tank. The contractor proposes to run the T&P relief discharge through 1/2-inch copper into a trapped floor drain in the closet and to reuse the existing 4-inch galvanized vent connector. The owner wants to know if the restroom must be expanded.
1. Does the single-user restroom need to be expanded for an occupant load of 10, and what code issues arise from the new hand sink?
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For a business occupancy with an occupant load of 10, the minimum fixture count is one water closet and one lavatory. The existing single-user restroom satisfies that count, so it does not need to be expanded. The single-user arrangement is also acceptable as the only toilet facility where it is accessible.
The hand sink is a new fixture. Even if it is not a fixture-count requirement, once installed it must comply: it needs a code-approved trap, a vent, hot and cold water, and a fixture fitting. It cannot be run to a floor drain or an unvented drain. In a sterilization area, a hand sink is good practice and may be required by the health authority, but the plumbing code issue is that the new sink must be trapped and vented.
2. Evaluate the proposed T&P relief valve discharge through 1/2-inch copper into a trapped floor drain.
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The proposal has two defects. First, the relief valve discharge must not be reduced. The discharge pipe must be the full size of the valve outlet, never smaller, and must be rated for the temperature. Reducing to 1/2-inch is not permitted. Second, the discharge must not be trapped. A trap can hold water and interfere with relief, and the discharge must terminate with an air gap. The acceptable arrangement is full-size pipe terminating 6 inches above the floor or above a receptor with an air gap, with no valves and no trapped connection. A floor drain may serve as a receptor only through an air gap, not as a direct trapped connection.
3. Evaluate the proposal to reuse the 4-inch galvanized vent connector and to rely on the louvered closet door for combustion air.
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A condensing tankless heater is a Category IV appliance, and its vent must be listed for the appliance and installed per the manufacturer's instructions. Galvanized vent connector is not acceptable for a condensing appliance because the flue gas is corrosive and the vent must be the listed material, typically PVC, CPVC, or polypropylene. The 4-inch size also may not match the manufacturer's required vent diameter.
Combustion air cannot be assumed from a louvered door. A 199,000 Btu/h appliance in a 30-inch by 30-inch closet requires a calculated combustion-air supply. The louver free area may be insufficient, and the closet may need two permanent openings, a direct-vent installation, or a listed combustion-air system. The contractor must verify the appliance listing and the closet's free area before installation.