Cold Second Floor After Gravity-to-Forced Circulation Conversion
A 2,800 sq ft two-story house has a converted gravity hot-water system. Original 1¼-inch mains and ¾-inch branches feed cast-iron radiators. The contractor replaced the old tankless coil with a 100,000 Btu/h output cast-iron boiler, added zone valves for first and second floors, and installed a new ECM circulator in constant-pressure mode. The second-floor zone is cold; the first floor overheats. Supply water is 180°F. The boiler return is 145°F. A clamp-on thermometer shows the second-floor zone supply 178°F and return 166°F; the first-floor zone supply 178°F and return 150°F. The circulator is set to 20 feet of head. The near-boiler piping has an air scoop with an automatic vent, but the system has not been purged since fill. The expansion tank is a 12-gallon diaphragm tank precharged to 12 psi; system static pressure is 15 psi.
1. Calculate the boiler flow implied by the measured 35°F ΔT and 100,000 Btu/h output. Is the circulator likely delivering enough flow for a 20°F design ΔT? What else would you measure?
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Use Q = 500 × gpm × ΔT. Rearranged, gpm = 100,000 / (500 × 35) = 5.7 gpm. For a 20°F design ΔT, the same output needs 10 gpm. The measured 35°F ΔT therefore suggests boiler flow is low for a 20°F design, assuming the boiler is actually firing at 100,000 Btu/h and the temperature sensors are accurate. Low flow raises ΔT and can starve far zones. Measure actual flow with a flow meter or use the pump curve and measured head, and verify the boiler is at full input. Check that the circulator is in constant-pressure mode and that the system curve has not changed. Do not simply raise pump speed until you know whether the problem is flow, air, or balancing.
2. The second-floor zone is cold but has only a 12°F temperature drop, while the first floor has a 28°F drop. What does that pattern suggest?
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The second-floor water is losing only 12°F, while first-floor water loses 28°F. If the second-floor zone is cold, the low ΔT means the water is not giving up heat where it should. That can happen if the zone is air-bound, if radiator valves are closed, if the zone valve is partly open or installed backwards, or if a bypass lets water short-circuit the radiators. It can also happen if second-floor radiation is much smaller than first-floor radiation, but the complaint is cold rooms, not just a lower load. The first-floor 28°F drop and overheating suggest that zone is getting a large share of flow. Check for air, purge the zone, verify all radiator valves, and inspect the piping for a gravity conversion error or a bypass before changing the circulator.
3. What corrections would you make and how would you verify them?
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First purge air from the second-floor zone and install or repair high-point vents. Verify that the zone valve opens fully and is not reversed. Check that the second-floor return is not connected in a way that bypasses the radiators. If boiler flow is low, correct the near-boiler piping and select a circulator that meets system head at design flow; constant-pressure mode may not be appropriate for widely varying zone load. Balance the zones with valves or a bypass, then measure supply and return temperature at each zone and at the boiler. Repeat the heat output calculation and compare it with room-by-room heat loss. Verify expansion tank precharge and system pressure, and confirm the boiler does not cycle on high limit. Document final flow, ΔT, and room temperatures.