For example, if the cut in pressure is set at
½ psi and the differential is set to 1 psi, the
pressuretrol will send power to the burner
when the pressure is ½ psi and cut power to the
burner at 1.5 psi.
Typically, gravity return systems should cutin
at about ½ psi and pumped return systems
should cut in at about 1 psi.
A subtractive pressuretrol (AKA “Cut-out”)
also has 2 adjustments, “cut-out” and “differential.”
The cut-out setting is the pressure at
which the control’s contacts will open, removing
power to the burner.
The differential setting is adjusted to the
amount of pressure UNDER the cut-out at
which power will be sent to the burner.
For example, if the cut out pressure is set at
1.5 psi and the differential is set to 1 psi, the
pressuretrol will send power to the burner
when the pressure is ½ psi and cut power to the
burner at 1.5 psi.
Typically, gravity return systems should cutout
at about 1 psi and pumped return systems
should cut out at about 2 psi.
Steam Pressure Gauge - A steam pressure
gauge is used to display the amount of
steam pressure in the boiler or system.
6 National Oilheat Research Alliance
Supply Riser - Vertical piping that carries
steam under pressure from the boiler up to the
supply header.
Supply Header – the distribution piping
from the boiler to the supply main(s)
Boiler-Header Drip Connection
(Equalizer) - Piping from the supply header
to the return that equalizes pressure between
the two.
Supply Main - Piping that carries steam
from the supply header to heat emitters and, in
a one-pipe system, condensate from heat emitters
to the return main. For this type of system
the supply main piping is pitched away from
the boiler at a minimum of ¼" per foot.
Riser – piping that carries steam up from supply
main to heat emitters.
Main Steam Vents (AKA “Quick
Vents”) – Allow air to quickly escape from
one pipe systems piping during the heating
cycle, and enter back into the system after the
call for heat ends. Figure 4.
To operate properly the vent should be installed
as shown in figure on opposite page, Figure 5.
Main vents are rated by system size – small,
medium and large. Small vents are designed
for systems under 70,000 Btus.
However, in general it’s best to install the largest
main vent available because the quicker
the air is vented from the main the better the
system will perform.
In most situations, it’s best to
operate residential steam systems at
the lowest possible pressure,
2 psi or less.