ADU Addition on a Shared 1-Inch Meter with Marginal Pressure
A 1,450-square-foot house in Reseda has a 1-inch city water meter and a 3/4-inch Type L copper service line 70 ft long with 8 ft of rise to the house. Static pressure at the front hose bibb is 55 psi. The house has two lavatories, two tank water closets, one bathtub, one shower, a kitchen sink, dishwasher, washing machine, and two hose bibs. The owner is adding a detached 500-square-foot ADU 55 ft behind the house with a lavatory, tank water closet, shower, kitchen sink, washing machine, and one hose bibb. The contractor proposes a single 1/2-inch PEX line from the house to the ADU, 90 ft developed length, with no pressure-reducing valve and no pressure test. The homeowner reports that shower pressure already drops when the washing machine runs. The plan reviewer asks for a water supply calculation and a test procedure before rough-in.
1. Calculate the total water-supply fixture units and evaluate whether the existing service and meter are adequate.
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Using standard water-supply fixture units: existing house = 2 lavatories (2) + 2 water closets (5) + bathtub (2) + shower (2) + kitchen sink (1.5) + dishwasher (1.5) + washing machine (2) + 2 hose bibs (5) = 21 WSFU. ADU = lavatory 1 + water closet 2.5 + shower 2 + kitchen sink 1.5 + washing machine 2 + hose bibb 2.5 = 11.5 WSFU. Total = 32.5 WSFU. Applying the Hunter curve or demand table, the estimated demand is roughly 18 to 20 gpm, not the sum of individual fixture flows. A 3/4-inch copper service at that flow has high velocity and significant friction; over 70 ft plus fittings it may lose 10 to 15 psi. Add 8 ft of rise (about 3.5 psi), meter loss, and building friction. A static pressure of 55 psi can leave only about 30 psi at the house and less at the ADU. The complaint about the shower dropping when the washer runs is consistent with a marginal service. A pressure gauge and flow test at the meter are needed before accepting the design.
2. Evaluate the proposed 1/2-inch PEX line to the ADU. What size and corrections are indicated?
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A 1/2-inch PEX line is not acceptable for a 90 ft developed run serving an ADU. At an ADU demand of roughly 8 to 10 gpm, 1/2-inch PEX has a velocity well above the usual 8 fps design limit and a friction loss of many psi per 10 ft; over 90 ft plus fittings, the loss can exceed 30 psi. The ADU also has its own elevation and fixture losses. With only about 30 psi at the house, the far shower will not maintain a 20 psi residual and will be affected by other fixtures. Use a larger line, at least 3/4 inch and often 1 inch for this length, sized by calculation. Consider a separate ADU service if the meter allows. Verify that the meter and service can deliver the total demand. A pressure-reducing valve may be needed if static pressure exceeds 80 psi, but it is not a fix for undersizing.
3. What pressure and flow tests should be required before rough-in and final?
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Before rough-in, install a calibrated pressure gauge at the front hose bibb and at the ADU. Record static pressure and flowing pressure with known flows, such as opening two hose bibs and measuring gpm, and request a municipal meter flow test if available. For the new PEX, pressure-test at the pressure and duration required by the Los Angeles Plumbing Code, using a gauge at the low point, and record the results. Check for leaks, proper support, and purging. After fixtures are set, recheck flowing pressure at the farthest shower and at the ADU while the washing machine and a hose bibb are running. The design should maintain the code minimum residual at the farthest fixture. Document all readings for the inspector.