Had a super call me last spring convinced his building was losing water pressure. Tenant on the top floor complained the sprinkler heads "looked low" (not a real thing you can see, but people say it anyway). I pulled the last three years of main drain results off his riser and the numbers hadn't moved more than 2 psi in either direction the whole time. Nothing was wrong. He just wanted someone to look at a piece of paper and tell him so. That's most of this job honestly, reading numbers other people are scared of.
If you already know the schedule and the 13.2.5 basics, you're past that. This one's about what the gauge is actually telling you once you've got it open.
What you're reading off the gauge
Two numbers matter. Static pressure is what you get with the drain valve closed, system full, just sitting there. Residual is what you get once you crack the drain open and let it flow. The drop between them is the number everybody actually cares about, because it's your rough proxy for how much water this supply can actually deliver under demand.
A big drop from static to residual isn't automatically bad. Some supplies are just weaker and drop 15 to 20 psi normally. A small drop isn't automatically good either, if static was already marginal to begin with. What you're really doing is comparing this test's drop to last year's drop on the same riser, same drain, same conditions as close as you can get them.
The comparison rule, and why it's the whole point
NFPA 25 wants you comparing this reading to the original acceptance test or the last several tests, not judging it in isolation. A single number means almost nothing without history. 62 psi static might be perfectly normal on one riser and a red flag on another three buildings over, because the street main feeding it is a different size, different age, different demand from other users nearby.
Here's an anonymized set from three risers I've actually pulled data on, numbers rounded and buildings not identifiable:
| Riser | Static (psi) | Residual (psi) | Drop (psi) | Prior year drop | Call |
|---|---|---|---|---|---|
| A (retail strip) | 68 | 51 | 17 | 16 | normal, within a few psi |
| B (mixed use, 4 story) | 74 | 39 | 35 | 19 | flag, drop nearly doubled |
| C (warehouse) | 55 | 48 | 7 | 9 | normal, if anything improved |
Riser B is the one that gets a phone call. Static barely moved, residual fell off a cliff compared to last year. That shape, static holding steady while residual craters, points you toward something restricting flow when water actually has to move, not a general pressure problem.
Closed valve or a real supply problem
This is the part techs get wrong under time pressure, because both show up as "bad numbers" if you're not paying attention to which number moved.
A partially closed control valve usually drops both static AND residual together, often hard, and the drop-to-drop delta versus last year can look almost normal even though both absolute numbers are trash. Static that's suddenly way lower than every prior test, with a proportionally similar residual, smells like a valve. Go check it before you write anything down. I've walked back to a PIV that was three turns off full open more than once. Cheap fix, embarrassing if you report it as a supply issue instead.
A real supply problem, like a failing PRV, a partially closed check on a backflow, or the municipal main itself losing capacity, tends to hit residual harder than static, because the restriction only matters once flow demand goes up. That's basically riser B above. Static near normal, residual gone, drop way up. If you've eliminated a closed valve as the cause (and you should physically confirm the valve position, not assume), that pattern gets escalated, not just noted.
Quick gut check I use: if closing and slowly reopening the same valve doesn't change the numbers, it's not the valve. If it does, you just found your answer and saved everyone a service call.
What actually goes on the report
Write both numbers, not just the drop. An inspector or the AHJ reading your form six months from now has no way to sanity check "17 psi drop" against anything, but two absolute numbers next to last year's two absolute numbers tell the whole story at a glance.
Note what you compared against, specifically. "Compared to 2025 annual test" beats a bare number sitting alone on the form. If you found a valve that wasn't fully open, note the position you found it in and that you corrected it, that's a deficiency you found and fixed, which is a different animal than a deficiency you're reporting and walking away from.
If the drop looks like a real supply issue and not a valve, that's an impairment-adjacent finding depending on how bad it is, not just a checkbox. Don't self-diagnose the water utility's problem on the form, just document the reading, what you ruled out, and that it needs follow up with the owner or the water purveyor. I've seen techs write "low pressure, may need booster pump" on a report when they hadn't even checked the PIV yet. Slow down, rule out the cheap thing first.
Quarterly frequency applies in some configurations, we covered that and the rest of the drain test schedule over on the sprinkler inspection schedule breakdown if you need the due date logic.
Anyway. I've spent more hours than I'd like retyping last year's gauge readings from a scanned PDF just to put them next to this year's on the same form, full disclosure I'm building FireDeck partly because that comparison shouldn't require digging through last year's paperwork by hand.