I Hot Water Hydronic Systems

Boiler:

appliance that heats water using oil, gas, or electricity as a heat source:
• Some boilers can use more than one source
• Conventional boilers supply water at 180°F; newer technology provides a wide range of temperatures

Cast Iron boilers

Most commonly found in residential applications
• Holds 15 to 30 gallons of water
• Takes a long time to heat up
• Holds onto heat for a long time
• Air in the system can corrode the boiler

Steel boiler

Water being heated surrounds steel tubes
carrying combustion gases
• Combustion gases flow through the tubes to
heat water
• Baffles in the steel tubes slow the flue gases to
transfer more heat to the wate

Copper Water-Tube Boilers:

• Characteristics of copper water-tube boilers:
1. Much lighter than cast-iron boilers
2. Only a few gallons of water

Low-Mass Boiler

Heat up and cool down quickly
• Shorter run and off times
• Used on closed loop systems
• Less susceptible to corrosion

The Basic Hydronic System: Expansion Tank

Water expands when heated, so expansion tanks accommodate added volume to prevent excess
pressure
• Standard expansion tank is a large tank located above the boiler

Force hot water from heat source through piping to heat transfer units and back to the boiler using
centrifugal force
• Made up of a motor, a linkage, and an impeller

Diaphragm-type expansion tanks

have two sections separated by a semi-permeable rubber membrane
• One side contains pressurized air; other is open to water circuit

Air Separator and Air Scoop

• Air is one of the biggest enemies of a hot water
hydronic heating system
• Air separator separates air from water using
collision and adhesion
• Directional air scoops separate and remove air on
horizontal runs of straight pipe

Air Vent and Aquastat

Air vent removes air from system
• Can be manually operated or automatic
• Temperature-limiting control (aquastat)
maintains water temperature in system

High-Limit Control and Pressure Valves:

High-limit control shuts own the heat source if
boiler water gets too hot
• Water-regulating valve (pressure-reducing
valve) reduces pressure of water entering the
boiler to desired level
• Pressure relief valve discharges excess water
when expansion creates pressure:
• Set to relieve at or below maximum
working pressure of the low-pressure boiler
(30 psig)

Low-Water Cut-Off and Zone Valves

Low-water cut-off deenergizes system if
level of water in system drops below
desired level

Zone valves

are thermostatically
controlled valves that control water flow to
the various zones in the system
• May be gear-motor or heat-motor
operated, and can often be opened
manually
• Available in two-port and three-port
varieties

Balancing Valves:

Balancing valves ensure resistance to water flow is
the same in all flow paths
• Resistance of water flowing in system causes friction
• Valves are installed in each circuit branch
• Pressure differential bypass valve relieves noise and
pressure from closing multiple zone valves in two-
port systems
• Opens incrementally as valves close

Control Valves and Sensors

Flow control valve prevents water from flowing through wrong heating loop (ghost flow)
• Outdoor reset control senses outdoor ambient temperature and adjusts the water temperature
in the boiler
• Thermostatic radiator valves are a common method for controlling temperature in each of
multiple zones