Tankless Coil Cannot Keep Up With a Shower Demand
A homeowner with a 1.0 GPH oil-fired boiler and an internal tankless coil complains that hot water lasts only about 5 minutes in the shower. The boiler aquastat is a Honeywell L8124 set to LO 160°F and HI 180°F, with a 10°F differential. The circulator runs continuously during a call for heat. The tankless coil is rated 3.0 GPM continuous at 180°F boiler water. The nozzle is 1.0 GPH 80°B at 140 psig, and the burner is clean with smoke 0, CO2 12.2%, and a steady flame. A pressure gauge on the domestic line shows 50 psi; the mixing valve is set to 120°F. When the shower runs, boiler temperature falls from 180°F to 145°F within 3 minutes, and the burner cannot recover until the shower is shut off. The home has a single bathroom and the heating load is mild (outdoor 45°F).
1. Calculate the approximate domestic hot water capacity of the boiler at 80% efficiency and explain why the observed performance falls short.
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At 1.0 GPH, input is about 1.0 × 140,000 = 140,000 BTU/h. At 80% efficiency, output is about 112,000 BTU/h. For a 70°F rise (50°F inlet to 120°F mixed), 1 GPM requires 8.33 lb/min × 70°F = 583 BTU/min, or about 35,000 BTU/h. So the boiler has a theoretical capacity near 112,000 ÷ 35,000 = 3.2 GPM, and the coil is rated 3.0 GPM at 180°F.
The shortfall is not raw burner capacity; it is how the capacity is used. The circulator runs continuously, so space heating competes with the coil. The aquastat low limit is 160°F, and with a 10°F differential the boiler can fall toward 160°F before the burner responds. A tankless coil rated at 180°F loses substantial capacity at 145°F because the temperature difference between boiler water and domestic water shrinks. Without priority, the shower draws the boiler down faster than the burner can recover.
2. What corrections would you recommend to restore adequate hot water?
Read the worked answer
Restore priority and higher boiler temperature:
- Wire the circulator to stop on a domestic hot-water call (priority), so the burner’s full output goes to the coil. Use a priority zone control or aquastat with priority switching.
- Raise the low limit to about 180°F and set the high limit to 190–200°F, following the boiler/coil manufacturer’s range. This keeps the coil’s driving temperature up.
- Check the mixing valve; 120°F is reasonable, but a failed valve can limit flow or temperature.
- Inspect the coil for scale. A scaled coil transfers less heat and may need replacement.
- Verify the actual firing rate: a 1.0 GPH nozzle at 140 psig flows about 1.18 GPH, so the burner is not undersized.
After adding priority and raising the low limit, retest a 10-minute shower draw. Boiler temperature should stabilize above 160°F and the burner should keep up. If it still cannot, recommend an indirect water heater.