8 National Oilheat Research Alliance
Figure 9
Most residential PRVs are set at 12 psi,
which is enough pressure for a typical
two-story house with the heating system
in the basement.
The amount of pressure needed is calculated
as follows:
1 psi will lift water 2.31 feet.
If the total distance from the PRV to the
top of a radiator on the second floor is 18
feet, then:
18÷2.31 = 7.8 psi
Just less than 8 psi will lift water to
the top of the system, but it won’t be
at enough pressure to circulate water
throughout the system, therefore 3-4 psi
are added to give positive pressure at the
system’s highest point. This PRV should
be adjusted to provide 12 psi.
If the building is higher, use this formula:
Height from PRV to top of radiation on
highest floor ÷ 2.31 + 4 = recommended
pressure setting.
For example, a home where there is 30
feet from the PRV to the top of the radiator
on the highest floor should be set to
operate at 17 psi:
(30 ÷ 2.31 = 13 ) + 4 = 17 psi
7 Backflow preventer—These
devices are installed on boiler
feed lines to
prevent contaminants
in the heating
system water from
flowing back into,
and contaminating,
the potable water
supply when supply
pressure is lower
than system pressure.
Figure 9.
8 Expansion tank—Since the water
in a closed heating system expands
by about 5% when it is heated, the
system needs a place for the “expanded”
water to go as well as provide a way for
that water to return to the system after
it cools. Without that added space, the
system pressure would rise and the relief
valve would discharge, lowering the
water level in the system just about every
time the burner cycled. Then, when the
Figure 10
Diaphragm Expansion Tank
Air Scoop
Automatic
Vent
Empty
Air Scoop
Automatic
Vent
Partially
Filled
with Water