T&P Relief Discharge and Morning Lukewarm Water After a New RPZ
A 48-unit assisted living building has a central gas-fired condensing water heater rated 400,000 Btu/h with 130 gallons of storage, a thermostatic mixing valve set to deliver 120 degrees F, and a recirculation loop with a bronze pump on a timer (off 11 p.m. to 5 a.m.). Storage is maintained at 140 degrees F. Last month the water utility installed a 2-inch reduced-pressure-principle backflow assembly on the domestic service main. Since then, the heater's temperature-and-pressure relief valve has discharged twice, the relief outlet terminates 12 inches above a floor drain and shows mineral staining, and fourth-floor residents complain of lukewarm water between 6 and 8 a.m. The maintenance technician proposes to replace the T&P valve with a higher-rated one and to raise the aquastat setpoint to 160 degrees F. Domestic static pressure at the building is 85 psi, and a 4.5-gallon expansion tank is installed on the cold line downstream of the RPZ with a 40 psi air charge.
1. Diagnose the T&P discharge and evaluate the technician's two proposals.
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The RPZ turned the domestic system into a closed system. Its check valve prevents expanded water from pushing back into the main, so during every recovery cycle the pressure rises until something gives - in this case the T&P, typically set at 150 psi and 210 degrees F. The mineral staining confirms repeated weeping, not a one-time event.
The technician's proposals are both wrong and unsafe. A T&P valve is a listed safety device; substituting a higher-rated valve removes the last line of protection and can lead to tank or piping failure. Raising the setpoint to 160 degrees F increases both the expansion volume and the scald hazard at every fixture downstream - a serious problem in an assisted living occupancy.
The correct fix: verify and size a thermal expansion tank on the cold side downstream of the backflow preventer. Two problems are visible here. First, a 4.5-gallon tank may not have enough acceptance volume for 130 gallons of storage plus the distribution piping through a 60-degree rise; the sizing calculation, not the tank's nominal size, governs. Second, a 40 psi air charge against an 85 psi system is too low - the bladder will displace early and the tank will be waterlogged or ineffective. The precharge should be set to the system's minimum pressure at the tank location. Check the bladder for failure, recharge or replace the tank, and re-verify the relief drain terminates through an air gap at the required distance above the floor.
2. Why is fourth-floor water lukewarm between 6 and 8 a.m., and how should the plumber approach the complaint?
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The timing points to the recirculation strategy, not the heater. The pump is off from 11 p.m. to 5 a.m., so the loop cools overnight. At 5 a.m. the pump restarts and pushes the cooled volume ahead of it; on the fourth floor, the farthest point, residents draw that cooled slug during the morning peak. That is a design and control issue, not a failed heater.
Work through it in order:
- Control: start the pump 60 to 90 minutes earlier, or replace the timer with a thermostatic control that maintains return temperature. Continuous low-flow recirculation is another option if the energy penalty is acceptable.
- Hydraulics: verify actual pump flow and head against the loop, and confirm the balancing valve is set so the far end of the loop gets its share. A loop balanced only at the near riser will always disappoint the top floor.
- Crossover: check for a failed check valve on the recirculation return or a single-handle faucet allowing cold-to-hot crossover. Crossover produces lukewarm water that looks exactly like a recirculation problem.
- Mixing valve: confirm the thermostatic valve responds correctly at low morning flow. A sticking or oversized valve can deliver erratic temperature at low draw.
Do not raise the storage temperature to compensate; that increases expansion and scald risk without fixing the loop.
3. What temperature-control and relief-valve requirements apply in this occupancy, and what should the plumber verify before sign-off?
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In an assisted living facility, scald protection is both a plumbing code issue and a licensing issue. The plumber should verify:
- Delivery temperature at fixtures used by residents is limited to the value the adopted code and the facility's licensing rules require - commonly 120 degrees F for bathing and handwashing, with some public-lavatory applications at 110 degrees F. Confirm the local number with the AHJ rather than assuming.
- The master mixing valve is a listed thermostatic mixing valve (ASSE 1017) rated for the flow, installed with check stops and strainer screens, and labeled with the maximum setting. Point-of-use protection (ASSE 1070) may be required at individual tub/shower valves.
- Legionella control: storing at 140 degrees F and tempering to 120 degrees F at the valve is a recognized strategy, but only if the recirculation loop returns hot enough to keep the whole system above the growth range. A cooled return line defeats the purpose.
- The T&P relief valve is the correct rated device for the heater, is not capped or plugged, and its discharge pipe is full size, not threaded at the outlet, and terminates through an air gap within the required distance of the floor drain.
Sign-off should include a temperature reading at the farthest fixture at peak morning demand, not just at the mixing valve outlet.