Hydronics (Gas and Oil): Pressure, Circulation and Air Control
Water-side heat: pressure, circulation and air control
Hydronic questions test the relationship between pressure, temperature and air. Know the fill pressure: about 1 psi per 2.31 ft of height above the fill point plus 4-5 psi, so a two-story house typically fills cold at around 12 psi. The relief valve is normally 30 psi, the expansion tank precharge matches cold fill pressure, and the pressure-reducing valve maintains it.
Know the components: boiler, aquastat (operating control), high limit, low-water cutoff, circulator, zone valves or zone pumps, air eliminator or scoop, flow-control valve, and primary/secondary piping with closely spaced tees for hydraulic separation. Know that the circulator should pump away from the point of no pressure change, which is the expansion tank connection.
Traps: setting an expansion tank precharge without isolating and draining it, diagnosing a no-heat zone as a bad zone valve when the system is air-bound, ignoring a weeping relief valve caused by a failed tank or overfill, and confusing boiler water temperature with supply water temperature at the zone.
Worked example: a zone must deliver 30,000 Btu/hr at a 20°F design drop. gpm = Btu/hr ÷ (500 x ΔT) = 30,000 ÷ (500 x 20) = 3 gpm. Select the circulator and pipe size for 3 gpm at the calculated head rather than for the boiler's total output.