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Boiler & Hydronic Parts

Boiler & Hydronic Parts
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Show: 12 24 48
Pages
Show: 12 24 48

Hot water and steam heating systems have more working parts than most people realize. Beyond the boiler itself, a circulator pump moves water through the loops, a low water cutoff shuts the burner down if water drops too low, a relief valve protects against overpressure, air vents and separators keep air out of the piping, an expansion tank absorbs the water's growth as it heats, and an aquastat or boiler control decides when to fire. This category brings those parts together for residential and light commercial boilers and hydronic systems.

We carry hydronic parts from Taco, Bell & Gossett, Grundfos, Armstrong, McDonnell & Miller, Honeywell, Caleffi, Watts, Spirovent, Hoffman, Amtrol, Weil-McLain and Burnham, all with live stock counts. This guide covers what each part does, how to choose a replacement and what to check when a boiler system misbehaves.

Parts in this category

Circulator pumps and pump parts

Circulators push heated water from the boiler through radiators, baseboard, radiant tubing or fan coils. They are chosen by flow (gallons per minute), head (feet), horsepower, voltage, flange size and body material. Cast iron circulators are used in closed heating systems; bronze or stainless steel circulators are required for open systems and domestic hot water recirculation, where fresh water would rust cast iron. Replacement cartridges, bearing assemblies, couplers, volute gaskets and flange gaskets let you rebuild many pumps without replacing the whole circulator.

Low water cutoffs

A low water cutoff (LWCO) turns off the burner if the boiler's water level drops below a safe point, preventing a dry fire that can crack the heat exchanger. Probe-type cutoffs sense water electrically; float-type cutoffs use a float chamber. Steam boilers and many hot water boilers require one. Match the voltage, mounting (tapping size and probe length) and boiler type (steam or hot water), and use the listed probe or float parts for service.

Water feeders

Automatic water feeders add makeup water to a steam boiler or hot water system when the level drops. Some combine a backflow preventer and fill valve in one assembly.

Relief valves

Boiler relief valves open if system pressure exceeds their rating, typically 30 PSI on residential hot water boilers. They must match the boiler's maximum allowable working pressure and BTU rating and are never plugged or capped.

Air vents, purgers and separators

Air in a hydronic loop causes noise, poor heat and corrosion. Automatic air vents, coin vents and microbubble air separators remove it. Steam systems use main vents and radiator vents to let air out as steam fills the piping.

Expansion tanks

Water expands as it heats. A diaphragm expansion tank gives that expansion somewhere to go so pressure stays below the relief setting. Tanks are sized by system volume and fill pressure.

Aquastats, aquastat relays and boiler controls

Aquastats sense boiler water temperature and turn the burner on and off. Aquastat relays add a low-voltage thermostat circuit and circulator relay in one control. Outdoor reset controls adjust water temperature based on outdoor temperature for comfort and efficiency. Immersion wells hold the sensing bulb in the boiler water and must match the control and the boiler tapping.

Valves and gauges

Mixing valves blend hot and cold water for radiant floors or domestic hot water safety. Pressure reducing (fill) valves keep the system at its set cold-fill pressure. Flow check valves stop gravity circulation in zones. Backflow preventers protect the drinking water supply. Radiator valves and thermostatic radiator valves control heat at each radiator. Pressure and temperature gauges let you see what the system is doing.

Choosing the right replacement

  1. Start with the part number on the old part and search it here. Listings show cross-references when the manufacturer publishes them.
  2. Match the system type: steam or hot water, closed or open (potable) system.
  3. Match connection size and type with the Pipe Size and Connection Type filters (NPT, sweat or union).
  4. Match the electrical rating with the Voltage filter: 120V, 24V or millivolt.
  5. Match pressure and temperature ratings, especially for relief valves, mixing valves and tanks.
  6. For circulators, match flow, head and flange-to-flange length, and choose bronze or stainless for open systems.
PartKey spec to matchCommon mistake
CirculatorFlow, head, flange size, body materialUsing cast iron on domestic hot water
Low water cutoffSteam or hot water, voltage, probe lengthWrong probe length for the tapping
Relief valvePressure setting and BTU capacityUndersized BTU rating
Expansion tankSystem volume and prechargeNot setting precharge to fill pressure
AquastatControl type, range, well sizeReusing a corroded or mismatched well

Common hydronic problems

  • Relief valve dripping: often a waterlogged expansion tank or a fill valve set too high, not a bad relief valve.
  • Gurgling or cold radiators: air in the system. Check vents and the air separator, and purge the zones.
  • Circulator hums but no heat: seized impeller or failed cartridge. Many pumps can be rebuilt with a replacement cartridge.
  • Boiler short cycling: aquastat differential set too tight, oversized boiler or a failing control.
  • Burner lockout on low water: a dirty LWCO probe or float chamber, or a real water loss from a leak.

Installation and safety

Boiler safety controls, relief valves and low water cutoffs protect people and property. They should be installed and tested by a qualified technician following the boiler manufacturer's instructions and the codes that apply where the equipment is installed. Pipe relief valve discharge to a safe location, never install a valve between the boiler and its relief valve, and test low water cutoffs according to the manufacturer's schedule. Use lead-free parts where the code requires them for potable water.

Annual boiler checklist

A yearly service visit keeps a hydronic system safe and efficient. Typical checks include testing the low water cutoff by the manufacturer's method, flushing float-type cutoffs on steam boilers, verifying that the relief valve is not weeping and is piped to a safe drain, checking expansion tank precharge with the system drained to zero pressure on the tank side, confirming cold-fill pressure at the gauge, bleeding air from high points and verifying the circulator runs quietly with no leaks at the flanges. The technician should also inspect the aquastat setting and differential, verify the immersion well is not corroded and confirm the burner shuts down when the high limit is reached. On steam systems, cleaning the gauge glass and checking main vents and radiator vents restores even heat.

Open versus closed systems

A closed heating loop is filled once and treated so oxygen stays low, which lets cast iron circulators and steel components last. Any system that brings in fresh water continuously, such as domestic hot water recirculation or a combination heating and potable water system, is open and needs bronze, stainless or lead-free components wherever water touches them.

Shopping this category

Filter by brand, part type, voltage, pipe size, connection type and horsepower. Each listing shows the manufacturer part number, cross-references when published and the quantity in stock now. Need help? Call 1.888.509.3391 with your boiler model or the numbers on the old part.

Frequently asked questions

How do I know what size circulator I need?

Match the original pump's model, or have a technician calculate the flow and head the system needs. Pump curves on the manufacturer's spec sheet show performance at each flow rate.

Why does my boiler relief valve keep leaking?

The most common causes are a failed expansion tank, a fill valve set too high or debris on the relief seat. Have the system pressure checked before replacing the valve.

Do I need a low water cutoff on a hot water boiler?

Requirements depend on the boiler, its location and local code. Steam boilers always need one, and many hot water boilers are required to have one as well.

Can I rebuild my circulator instead of replacing it?

Many circulators accept a replacement cartridge or bearing assembly. If the volute is cracked or badly corroded, replace the whole pump.

What is the difference between an aquastat and an aquastat relay?

An aquastat senses temperature and switches the burner. An aquastat relay adds a transformer, thermostat terminals and a circulator relay so one control can run the boiler.

How often should a boiler expansion tank be checked?

Check it at least once a year during routine service by testing the air precharge and tapping for waterlogging.