Sizing a Replacement Boiler With Connected Radiation and Indirect Domestic Hot Water
An inspector is called to a 1920s two-story house in a 6,000 heating-degree-day climate. The owners are replacing a 150,000 Btu/h input, 80% AFUE atmospheric boiler with a 120,000 Btu/h input, 95% AFUE condensing boiler. The house has a calculated design heat loss of 68,000 Btu/h at 0°F outdoor and 180°F supply water. Connected radiation is 18 cast-iron radiators with a total equivalent direct radiation (EDR) of 420 sq ft. The new boiler's net I=B=R rating is 108,000 Btu/h. The owners also want a 40-gallon indirect water heater with a 1.0 gpm recovery coil rated 40,000 Btu/h at an 80°F temperature rise. Existing near-boiler piping is 1¼-inch steel with a single 1/12 hp circulator, no outdoor reset, and a fixed high limit of 180°F. The contractor proposes to pipe the indirect as a priority zone and set the boiler high limit to 190°F. The homeowner complains that the old boiler short-cycled in mild weather.
1. Determine whether the proposed boiler output is adequate for the connected radiation and the indirect water heater. Show calculations, state assumptions, and identify any sizing concern.
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At 180°F average water temperature, cast-iron radiation is roughly 150 Btu/h per sq ft EDR: 420 × 150 = 63,000 Btu/h. The design loss is 68,000 Btu/h, so the radiation is slightly short at 180°F. Raising supply to 190°F adds output, but it also reduces condensing efficiency. With a 1.33 pickup factor, the radiation load is 63,000 × 1.33 ≈ 83,800 Btu/h, below the boiler net rating of 108,000 Btu/h. The indirect coil adds 40,000 Btu/h. If the indirect is priority, the boiler only sees 40,000 Btu/h during recovery, which is well below its 108,000 Btu/h net output. Capacity is therefore adequate, but the boiler is oversized for both the 68,000 Btu/h design load and the 40,000 Btu/h indirect load. A fixed-input boiler will short-cycle unless it modulates, has a buffer, or uses a larger indirect coil and priority control. The 190°F high limit should be justified by radiation output, not used as a substitute for proper sizing.
2. What control sequence and near-boiler piping details would you require to prevent short cycling and protect the condensing boiler during indirect priority?
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Use outdoor reset with warm-weather shutoff and a boiler supply sensor. Wire the indirect aquastat as a priority demand: on a DHW call, close the heating zone valves or stop the heating circulator, run the boiler to the indirect setpoint, and use a mixing valve at the tank outlet. Provide primary/secondary piping or a hydraulic separator so the boiler loop can maintain minimum flow while the indirect coil sees its design flow. Set the boiler high limit only as high as needed for design-day radiation and DHW, and verify the boiler can modulate low enough to match the 40,000 Btu/h coil without cycling. Add a boiler bypass if return water can drop below the manufacturer's minimum. Use post-purge and a condensate trap per the manual. Do not use the 190°F limit to mask short cycling.
3. What field measurements would confirm the system is delivering design heat rather than merely cycling?
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Measure supply and return temperature at each zone and at the boiler, and measure or calculate flow. Use Q = 500 × gpm × ΔT. For example, 5.5 gpm at 20°F ΔT is 55,000 Btu/h; 5.5 gpm at 35°F is 96,250 Btu/h. Compare measured heat delivery with the room-by-room heat loss. Record cycle length and cycles per hour in mild weather, flue gas temperature, O2, CO2, CO, and condensate volume. Verify indirect recovery by timing the tank temperature rise. Check radiation surface temperature and room temperature in the farthest rooms. If the boiler cycles faster than the manufacturer allows, correct piping or controls before assuming the boiler is defective.