Hydronic Boiler Fill Pressure, Expansion Tank, and Relief Discharge
A 500 MBH cast-iron boiler serves a 60-ft-tall building with 1,200 ft of 1-1/4 in fin-tube baseboard. Design supply is 180 F and return 160 F. The boiler has a 50-psi maximum allowable working pressure and a 30-psi relief valve. The expansion tank is connected to the boiler supply between the boiler and the circulator, and the circulator discharges into the system. The relief valve discharge is piped 20 ft to a trapped floor drain. The system is filled to 12 psi cold and contains 30 percent propylene glycol. The owner reports the relief valve weeping at startup and air noise at the top floor. The contractor proposes to raise the fill pressure to 18 psi and add an automatic air vent at the top of the riser.
1. Evaluate the fill pressure and relief valve setting for the 60-ft building. Show static pressure.
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Static pressure = 60 ft x 0.433 psi/ft = 26 psi. To maintain about 4 psi at the top, the fill pressure at the boiler must be about 30 psi. A 12-psi cold fill is insufficient, which explains air noise and poor top-floor performance. But a 30-psi relief valve cannot coexist with a 30-psi fill: the valve will weep at startup as the system heats and expands. The relief setting must be above the maximum system pressure but not above the boiler MAWP. With a 50-psi MAWP, a 50-psi relief valve and a fill of about 30-32 psi are appropriate, provided all components are rated. The contractor's proposal to raise fill to 18 psi is still too low for 60 ft.
2. Evaluate the expansion tank location relative to the circulator and the point of no pressure change.
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With the circulator discharging into the system, its suction side is the boiler supply where the tank is connected. That makes the tank connection the point of no pressure change, which is correct; the circulator head should not be imposed on the tank. The tank location is acceptable if it is truly on the suction side and not on the discharge. The problem is likely tank sizing/precharge and the low fill pressure, not location. For 30% propylene glycol, the expansion volume is larger and the tank must be sized for the actual system volume and maximum temperature. Check the tank precharge against the fill pressure and the acceptance volume.
3. Evaluate the relief valve discharge piping and the glycol/air issues.
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The relief discharge must not be trapped. A trapped floor drain connection is prohibited; the discharge must terminate within 6 in of the floor or a receptor, with no trap, no valve, and no reduction in size, and it must not be connected to a drainage system. A 20-ft run is acceptable only if it is properly supported, pitched, and not trapped. The glycol increases expansion and can affect relief capacity; verify the valve capacity and pipe size. The automatic air vent at the high point is reasonable, but it does not correct low fill pressure or an undersized expansion tank. Fix the fill/relief/tank design first.