Adding Six Units Behind a 100-Gallon Heater That Weeps
A twelve-unit apartment building has a gas-fired commercial storage water heater rated 199,000 Btu/h input at 80 percent thermal efficiency, with 100 gallons of storage held at 140 degrees F. The city main delivers 90 psi; a pressure-reducing valve set at 65 psi and a meter check valve isolate the domestic system. Winter incoming water is 40 degrees F. The owner reports the temperature-and-pressure relief valve has been weeping for months, and two plumbers have replaced the valve twice with no change. The building is adding six more units by converting the basement. The engineer's peak-hour hot water demand for the expanded building is 320 gallons at 120 degrees F delivery, and the longest fixture run from the heater is 85 feet. The owner wants to keep the existing heater and simply raise storage temperature to 160 degrees F to get more capacity out of it.
1. Evaluate the owner's proposal numerically. Does raising storage from 140 to 160 degrees F close the capacity gap?
Read the worked answer
No. Raising storage temperature is a net loss, not a gain.
Usable recovery first. Output = 199,000 x 0.80 = 159,200 Btu/h. Recovery gallons per hour = output / (8.33 x temperature rise).
- At 40 to 140 F, rise = 100 F: 159,200 / 833 = 191 gph.
- At 40 to 160 F, rise = 120 F: 159,200 / 1,000 = 159 gph. Recovery drops 32 gph.
Draw-down gained by storing hotter, when mixed down to 120 F: hot fraction = (120 - 40) / (storage - 40). At 140 F that is 80/100 = 0.80, so 100 gallons yields 125 gallons at 120 F. At 160 F it is 80/120 = 0.67, so 100 gallons yields 150 gallons. The gain is about 25 gallons, against a recovery loss of roughly 30 gallons in the first hour alone, on a demand that is already short. Using the manufacturer's first-hour rating at the actual 40 F inlet and 120 F delivery, the existing unit lands near 260 to 275 gallons against 320 required.
Recommendation: keep storage at 140 F with a master mixing valve, and add storage or a second staged condensing heater. Also re-run the demand calculation, since 85 feet of run means real recirculation loss.
2. Diagnose the weeping relief valve. What is actually wrong, and what will you install?
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The valve is not the problem; the system is closed and has nowhere to expand.
The pressure-reducing valve and the meter check valve create a closed system. As the heater recovers, water expands roughly 1.5 to 1.6 percent over a 40 to 140 F swing, about 1.5 gallons on 100 gallons of storage, against a system that cannot push back through the check valve. Pressure climbs until the relief valve dribbles, and it will keep dribbling no matter how many valves are swapped.
What to do:
- Install a listed thermal expansion tank on the cold side, pre-charged to the 65 psi reduced pressure and sized for the total system volume, per the manufacturer's sizing chart.
- Verify the relief valve's Btu/h and pressure ratings are at least equal to the heater's input and the system's pressure; an undersized valve is a second, independent failure.
- Inspect the discharge piping: full size of the valve outlet, no reduction, no plug or cap, no threads at the open end, gravity flow with no trapped section, terminating 6 inches above the floor or to the exterior where it cannot freeze or scald.
- Confirm the reducing valve and check valve are functioning and that no bypass has been piped around them.
3. If the owner insists on 160 degree storage for the added load, what temperature-control devices and settings do you specify, and why?
Read the worked answer
Hot storage is acceptable only behind a tempering device, with settings justified in writing.
- Store at 140 F or above to control Legionella growth in the tank; a thermostatic master mixing valve listed to ASSE 1017 at the heater outlet blends stored hot water down to a delivery temperature of about 120 F.
- Deliver no more than 120 F to public lavatories; where showers are served, use a pressure-balancing or thermostatic valve listed to ASSE 1016 to limit scald risk.
- Maintain the recirculation loop with a thermostatic balancing valve and a check valve so the farthest fixture at 85 feet stays hot; without it, the added units at the end of the run will run cold and the occupants will open taps wider, raising demand.
- 160 F storage shrinks the relief valve margin, increases standby loss, and accelerates scale, so if it is chosen, document the mixing valve setting, label the system, and set the storage aquastat with a high-limit cutout.
On a twelve- to eighteen-unit building the honest recommendation is 140 F storage plus a mixing valve and more capacity, not 160 F storage as a substitute for Btu/h.