Two psi systems and line regulators. Episode twenty-four of the Gas Service Animated series.

Seven inches of water column moves plenty of gas — until the run gets long or the load gets big. Raise the pressure to two psi and the same pipe carries several times more gas, so a smaller line can feed the same load. That is the whole appeal of an elevated-pressure system: long runs and large loads shrink from heavy pipe down to slim tubing.

The code draws pressure lines. Inside most buildings the piping system's design pressure stays low — a typical residential elevated design is two psi. Go above five psi indoors and special provisions apply, like running the piping in a ventilated vertical chase with pressure reduction at the branch connections. Higher pressures belong to conditions the code spells out — and to the authority having jurisdiction.

At two psi, appliances still need their inches of water column — so a second regulator lives inside: the line regulator. Same anatomy you saw in episode one — spring, diaphragm, valve — stepping two psi down to appliance-ready delivery pressure. The code's rules follow it indoors: the regulator must be listed, accessible for service, and vented. Without a listed vent-limiting device, the vent line runs to the outdoors — and for gas heavier than air, only directly outdoors.

If the line regulator sticks open, two psi heads for appliance valves built for a fraction of that. That's why overpressure protection is required downstream of the regulator. One common form is the monitor: a two-regulator set — the working regulator does the job while the monitor rides nearly full open, sensing the same downstream pressure; if that pressure rises, the monitor takes over and limits it. Relief devices and other listed means do the same job.

Sizing a two psi system is a two-table job. Point of delivery to the line regulator: use the elevated-pressure family — schedule forty pipe at two psi inlet with a one psi drop, or the copper table built for a single house line regulator. Downstream of the line regulator, switch to the low-pressure family — inlet under two psi, drops in inches of water column. Two printed warnings: the tables exclude the regulator's own pressure drop, and capacities can exceed your regulator's rating — check the manufacturer.

In a two psi layout each regulator has an address. The service regulator at the meter set drops street pressure to two psi; the meter measures at that pressure; the line regulator sits downstream near the load, making inches of water column. An accessible shutoff valve goes upstream of each regulator — when two sit in series on one line, the second doesn't need its own valve. Label the system so the next tech knows.

On a two psi job, the walkthrough writes itself. Map the pressures — street, two psi, inches of water column. Confirm the line regulator is listed and accessible. Check its venting — vent limiter or a vent line run outdoors. Verify overpressure protection is present downstream. And confirm both table families were used on the right segments. Codes, manufacturer, company procedure — always.

Two psi in small pipe, a line regulator making inches of water column, overpressure protection on guard, and two table families doing the sizing. Next episode: overhead and shop heat.