tion of hot water when the thermostat is
not calling for heat.
Without a flo-control valve, gravity
circulation occurs because hot water tends
to rise through a system’s piping simply
because it weighs less than the colder
water in the piping.
Flo-conrol valves open, allowing water
to move through the system when a circulator
operates and they close when the
circulator stops running.
Flo-control valves can be used in systems
with multiple zones to allow heat to
flow only into the zones that are calling
for heat. Most can be opened manually
to allow for temporary gravity flow if the
circulator malfunctions.
While they are normally installed in
the supply piping, there are situations in
which an additional flo-control valve is
required on a return line.
Flo-control valves are also used in
primary-secondary systems on both the
feed and return lines of all secondary
circuits except those that are below the
primary circuit’s piping.
11 National Oilheat Research Alliance
13Indirect water heater—Indirect
water heaters are storage
tanks with internal heat
exchangers. They are equipped with an
aquastat that controls a circulator to move
hot boiler water through the inside of
the heat exchanger, heating the domestic
water in the tank.
14Thermal expansion tank
—Domestic water expands
when it is heated and 50
gallons of cold water becomes about 52
gallons of hot water. Thermal expansion
tanks will take-up the excess water,
preventing continual discharges from the
relief valve.
These tanks are necessary when back
flow devices are installed; back flow
devices prevent the expanding water from
flowing back into the water main.
15 Temperature & Pressure
relief valve (T&P)—The
T&P valve provides automatic
temperature and pressure protection
for the water heater. Typically, PRVs open
and release water when the water temperature
reaches 210 º F and/or the water
pressure reaches 150 psi.
Figure 12