Fixture Retrofit Payback in a 96-Unit Affordable Housing Block
A 96-unit, six-story affordable housing property (236 occupants, built 1994) has a single master meter. Last year's bills show 14.9 million gallons of potable water; sewer is billed on 100% of metered volume, and the combined rate is $7.50 per 1,000 gallons.
Fixture inventory: 3.5 gpf gravity toilets (96), 2.5 gpm showerheads (96), 2.2 gpm lavatory aerators (192), and 2.2 gpm kitchen aerators (96). The domestic service enters at 92 psi static with no pressure-reducing valve, and the maintenance log records three hose-bib or irrigation repairs in two years.
The owner's green refinancing covenant requires every conservation measure to show a simple payback of 4.0 years or less, measured at the meter. A contractor quotes $58,400 installed for 1.28 gpf toilets, 1.5 gpm showerheads, and 0.5 gpm lavatory aerators. Separately, the plumbing engineer flags that the building's 3-inch and 4-inch horizontal drains were sized for 3.5 gpf fixtures, and the local inspector has asked for a drainline transport review before permits are issued.
1. Using 5 flushes, 8 shower minutes and 2 lavatory minutes per person per day, calculate the annual savings of the $58,400 fixture package and determine whether it satisfies the 4.0-year covenant. What do you recommend?
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Savings at the meter (236 occupants):
- Toilets: (3.5 - 1.28) = 2.22 gpf x 5 uses x 236 = 2,620 gpd
- Showers: (2.5 - 1.5) = 1.0 gpm x 8 min x 236 = 1,888 gpd
- Lavatories: (2.2 - 0.5) = 1.7 gpm x 2 min x 236 = 802 gpd
Total = 5,310 gpd, or about 1,938,000 gallons per year.
Value: 1,938 thousand gallons x $7.50 = roughly $14,540 per year. Payback = $58,400 / $14,540 = 4.02 years - it misses the covenant by about a week.
Judgement decides this, not arithmetic.
- A 4.02-year prediction has no margin. Post-retrofit billing analysis routinely shows 10-20% less savings than the estimate, so it will land nearer 4.5-5 years.
- The cheapest gallons were left out. The 96 kitchen aerators at 2.2 gpm are not in the package; dropping them to 1.5 gpm adds about 165 gpd (roughly $450/yr) for under $2,000 and pulls payback back under the threshold.
- The 92 psi service is a code item (see next question) and must not be charged against the conservation covenant, but its leak reduction still improves the real result.
Recommendation: add the kitchen aerators, repair the leaking bibs, and certify the covenant only after three months of post-install meter data confirm the savings.
2. The service enters at 92 psi with no PRV and a history of hose-bib repairs. What does the plumbing code require, and how does the pressure change your savings estimate?
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Code requirement: IPC 604.8 requires a pressure-reducing valve where static pressure exceeds 80 psi. At 92 psi a PRV is mandatory whether or not it is part of the conservation package. It should be installed with a strainer upstream, and because the PRV creates a closed system, thermal expansion control is required downstream - the new 1.28 gpf fill valves will otherwise see pressure spikes on every heating cycle.
Effect on the numbers:
- Fixture flow is pressure-dependent. A showerhead rated 1.5 gpm at 60-80 psi flows above rating at 92 psi, so the retrofit will not deliver nameplate savings until pressure is controlled.
- Leak and drip losses scale roughly with the square root of pressure. Setting the PRV to 55 psi gives sqrt(55/92) = 0.77, about a 23% cut in leakage through the same orifices - which is also why the bibs keep failing.
- A 50-60 psi setpoint keeps the top floor comfortably above the minimum for tank-type fixtures while restoring rated flows.
Judgement: budget the PRV as code compliance, count only its measured water savings toward the covenant, and record the setpoint so the claim survives audit.
3. The inspector asks for a drainline transport review before permitting 1.28 gpf toilets on the existing 3-inch and 4-inch horizontals. How do you respond?
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The concern is real but manageable, and it is a performance question, not a code-compliance question - both IPC and UPC permit 1.28 gpf replacements. Lower flush volume means less water carrying solids through long, flat runs, and 1994 cast iron is often tuberculated, which raises roughness.
Response, in order:
- Verify slope and geometry: 1/4 in/ft on 3-inch, 1/8 in/ft on 4-inch, no sags, and flag any run longer than roughly 15 ft from a stack. Slope is the strongest predictor of trouble; short, well-sloped branches are low risk.
- CCTV the two worst branches to establish actual condition. Tuberculated or offset pipe should be replaced in the same project, because reopening ceilings later costs more than the toilets.
- Specify fixtures with a MaP score of 1,000 g or higher. MaP testing, not gpf, is the accepted measure of solids transport.
- If a branch still looks marginal, keep 1.6 gpf fixtures on that branch and document it. The compliance calculation is per building, so the penalty is a few hundred gallons a year.
Document slope, CCTV and MaP data in the submittal; do not let the review become a reason to abandon a 4,000-gallon-per-day retrofit.