Pressure Drop When Irrigation Starts
A two-story 2,400 sq ft house is served by a 1 in municipal meter and a 90 ft long 3/4 in Type L copper service. Static pressure at the hose bibb is 62 psi. With no water running, the second-floor shower delivers about 2.5 gpm. When the 12 gpm lawn irrigation zone starts, the shower falls to a thin stream and the tankless water heater occasionally shuts off with a flow error. The water heater is a 199,000 Btu/h condensing unit with a minimum activation flow of 0.5 gpm. The homeowner has added a 1/2 in branch to a new outdoor kitchen 70 ft from the meter. There is no pressure-reducing valve, no expansion tank, and no water hammer arrestors. The irrigation is teed off the service downstream of the meter but upstream of the house. The owner wants to know whether increasing the service to 1 in will fix the shower and whether a booster pump is needed.
1. Diagnose the shower pressure loss. Is the service size the only problem? Explain using pressure and flow reasoning.
Read the worked answer
The service is likely undersized for the simultaneous demand, but it is not the only problem. Static pressure of 62 psi is not excessive; the failure appears as dynamic pressure loss. A 3/4 in Type L copper service at roughly 15 gpm (12 gpm irrigation plus shower and other fixtures) has high velocity and friction loss over 90 ft plus fittings and the meter. The irrigation tee upstream of the house competes directly with the house supply, so the shower sees whatever pressure remains. The 70 ft, 1/2 in branch to the outdoor kitchen is also restrictive. Increasing to 1 in will reduce friction, but it will not cure a long 1/2 in branch, a restrictive meter, or an irrigation zone that is allowed to run at the same time. Measure static and flowing pressure at the meter, at the house inlet, and at the shower, and measure flow. If residual pressure at the tankless falls below its minimum, the heater can shut off. A booster pump is a last resort; first separate or prioritize irrigation, correct pipe sizing, and verify the meter and service.
2. What measurements and changes would you recommend before replacing the service? Include irrigation separation and water heater protection.
Read the worked answer
Recommended diagnostic sequence: install gauges at the meter outlet and at the water heater inlet; record static pressure, flowing pressure with the shower only, and flowing pressure with shower plus irrigation. Compare to the adopted code's pressure and fixture-unit sizing requirements. If the service is undersized, replace with 1 in or larger based on developed length and peak demand, not guesswork. Give irrigation its own tee with a pressure-regulating valve or separate meter, and use a controller that locks out irrigation when the house is occupied. Upsize the 1/2 in outdoor-kitchen branch. Because the check valve on the cold inlet creates a closed system, add a listed expansion tank sized for the heater and system volume; add water hammer arrestors at quick-closing valves. Confirm gas input and venting for the tankless. A booster pump should be considered only if the municipal residual pressure is inadequate after pipe sizing.
3. If the local code requires a pressure-reducing valve at 80 psi, is one required here? What does the check valve change?
Read the worked answer
If the adopted code requires a pressure-reducing valve at 80 psi, 62 psi does not trigger it; adding one would only reduce pressure further. The check valve is the key change: it makes the system closed, so thermal expansion has nowhere to go. The T&P valve may weep or the pressure may rise on every heating cycle. Install an expansion tank on the cold side between the check valve and the heater, sized to the system volume and the relief setting. Also verify that the T&P discharge is unobstructed and that no other check valve or backflow preventer was added without expansion protection. Do not solve the shower problem by raising pressure blindly; first fix the flow restrictions.