Gas/Propane Basics...
Timmie McElwain
President, Gas Appliance Service
gastc@cox.net
Hydronic Controls
On Gas Systems
There are not a lot of differences between gas and
oil hydronics except that the burners and fuels
are different. In this series of articles, I will
address these differences and how to work with them
as well as diagnosis and problem-solving. Some steam
related facts will be provided, but the emphasis will be
on Forced Hot Water systems use on gas, both Natural
and Propane.
The series will focus on the older systems and
approach Modulating/Condensing equipment. We
want to address some controls which are unique to
gas. Figure 6 on page 22 is an example of a typical
gas package boiler used since the 1950s.
Hydronic systems use the circulation of hot water
through pipes to heat residential spaces. The water
or steam in the distribution system heats devices
such as baseboard convectors or radiators. In these
systems, the thermostat usually controls the operation
of the circulator pump and the burner, either directly
or through a zone valve. Burner operation also
is controlled by an Aquastat controller, which is
sometimes used to maintain boiler water temperature
at a level suitable for heating (and for domestic hot
water if it is required). Both gas and oil systems
use the same controls. The oil burner fired hydronic
boiler in Figure 1 is an illustration of a typical boiler
arrangement.
In hydronic heating systems, boiler water is heated
by an oil or gas burner. The combustion chamber
supplies heat, through a heat exchanger, to the boiler
water and vents combustion products to the flue. The
expansion tank accepts the overflow of water caused
by the expansion of boiler water being heated. The
pressure-relief valve provides a safety release if the
water in the system is too hot or its pressure is too
high. The feed water valve controls the flow of water
into the closed hydronic system either to replace water
loss or to refill the system after it has been drained.
The sight glass provides a handy visual reference
indicating the water level in a steam boiler. The
pressure gauge registers the pressure inside the boiler.
The circulator pump circulates hot water through the
distribution system. Zone valves allow different zones
in the heated space to have their own thermostats and
comfort control.
Hydronic Controls
Figure 2 shows the control functions (including
domestic hot water) used in hydronic systems.
The thermostat senses temperature, calling for
heat when it is below the set point and turning off the
system when the temperature reaches set point.
The high limit, electrically connected to the burner
circuit, stops the burner if water temperature or
pressure is too high. The high limit may reset
automatically or it may require manual reset.
The circulator relay switches the circulator pump on
and off. This relay enables a low voltage thermostat
circuit to switch the line voltage circulator circuit.
The low limit switches the burner on in response to
a fall in boiler water temperature and switches it off
when the boiler water reaches the desired minimum
temperature. The low limit is required in heating
systems that also provide domestic hot water. With the
low limit, the boiler water is kept hot all the time even
if the thermostat is not calling for heat.
The circulator control is also required in systems
that provide both heating and domestic hot water. The
circulator control prevents the circulator pump from
coming on if the burner is operating in response to
the low limit. The circulator control grants priority to
domestic hot water because the circulator cannot run
until the low limit opens.
In some systems, a pressure-regulating valve is
required to reduce line pressure to system pressure.
Steam systems and some hot water systems require
a low-water cutoff. This device shuts off the burner
if the boiler water level is too low. It is an important
safety feature that prevents firing the boiler with
Figure 1: Boiler Components insufficient water.
20 ICM/September/October 2017