Apartment Boiler With Upper-Floor Air Binding and Near-Boiler Noise After a Pump Change
A four-story, 42 ft tall apartment building has two 1,500 MBH atmospheric cast-iron boilers in the basement, piped in parallel with a single variable-speed circulator located on the return. The compression tank connects to the boiler supply header, and the air separator sits downstream of the pump on the supply. Since a plate-and-frame domestic hot water heat exchanger was added and the circulator was replaced with a constant-pressure ECM model, tenants on the upper floors report cool radiators, the near-boiler piping is noisy, and the boilers kettle at what the aquastat reads as 210°F supply. The system is full of water, all zone valves operate, and the fill/pressure-reducing valve is set to 12 psi with a 30 psi relief valve. The pump suction gauge reads 4 psi and the discharge gauge 22 psi while running.
1. Using the pressure readings, show why the upper floors cannot heat, and state the fill pressure that should be set.
Read the worked answer
Static head to the top of the system: 42 ft / 2.31 = 18.2 psi. Add a 4–5 psi margin at the top for venting and to stay above the water's vapor pressure at design temperature, giving a cold fill of about 22–23 psi.
The system is filled to 12 psi, which supports only 12 × 2.31 = 27.7 ft of water column - about 14 ft short of the top floor. The upper radiators therefore sit under a partial vacuum. Air comes out of solution at that low pressure, collects at the highest points, and air-binds the upper zones; even where water reaches them, there is almost no pressure to drive flow.
The pump suction gauge at 4 psi confirms the low-pressure zone. The tank connection on the boiler supply, downstream of a pump located on the return, means the pump discharges toward the point of no pressure change, so the entire pump head is subtracted from the suction side of the loop. The pump is making 22 − 4 = 18 psi, roughly 41 ft of head, which is a big number to impose on a low-static system.
Corrective fill setting: 22–23 psi cold with the pump off, verified at the top of the system, with the relief valve and compression tank checked to confirm the higher pressure is acceptable and the tank has the capacity for the expanded volume.
2. Why are the near-boiler piping noisy and the boilers kettling at 210°F? Address pump location, air separation, and the new DHW load.
Read the worked answer
Noise and kettling here are symptoms of the same piping problem, plus a load change.
Pump location and air separation. Best practice is to pump away from the point of no pressure change: the compression tank and air separator should be at the same point, on the suction side of the pump, so that the pump lowers pressure there and raises it throughout the system. Here the tank is on the supply header downstream of a return-mounted pump, and the air separator is even further downstream at the pump discharge. Air removal is least effective at the highest-pressure, lowest-temperature point of the circuit, and the pump suction becomes the lowest-pressure point where air is drawn in through valve packing, vents, and fittings. That air is what tenants hear in the near-boiler piping.
Kettling. Atmospheric cast-iron sections with a 210°F supply and a DHW plate heat exchanger on the same loop are boiling locally. The DHW exchanger can demand high firing with low water flow through the boiler, which promotes film boiling and scale, so the aquastat sees 210°F while the sections run hotter. The kettle noise is steam collapse inside the sections.
Corrective actions: relocate the tank connection and the air separator to the pump suction at the boiler outlet, move the circulator so it pumps away from that point, pipe the DHW exchanger as a primary/secondary load with its own circulator and closely spaced tees, and set the high limit to about 190–200°F with outdoor reset. Inspect and descale the sections, and verify flow through each boiler.
3. What control sequence would you commission, and what measurements would you take to prove the system is fixed?
Read the worked answer
Sequence:
Pressure. Fill to 22–23 psi cold, then verify the pressure at the top floor with the pump running and the system at temperature; it should stay positive, ideally 4–5 psi.
Reset. Add outdoor reset with a design supply of 180°F at 0°F outdoor and roughly 120–130°F at 60°F outdoor. A 210°F supply is not needed for cast-iron radiators and is the direct cause of the kettling.
Boiler protection. Enable boiler minimum flow through primary/secondary decoupling and a boiler bypass, and add a low-water-temperature or return-temperature protection feature so cold return water does not shock the sections.
DHW priority. Give the plate exchanger a defined priority and a maximum supply temperature, and confirm its pump is interlocked so the boiler never fires against a closed or throttled loop.
Air removal. Move the air separator to the pump suction with the tank, add high-point vents, and purge each upper zone after the pressure change.
Proof measurements:
- Pressure profile: boiler, pump suction, pump discharge, and top-floor gauge.
- Supply and return temperatures across each boiler and each zone.
- Air content or at least a purge record and repeated venting checks over a week.
- Flow verification through each boiler at minimum and design conditions.
- Noise and kettling observations logged at full firing and at DHW peak.
Success looks like: stable 4–5 psi at the top floor, no air noise, no kettling, upper apartments at design temperature with the outdoor reset curve in control, and a supply temperature that tracks the curve rather than sitting at 210°F.