Product Overview

INTRODUCTION
To evaluate which valve will satisfy your needs, it is necessary to
determinetheapplication.Therst,andmostbasicbreakdown,is
directversusindirectredappliances.
Direct red units do not have a heat exchanger and all products
of combustion generated by the gas burning device are released
directly into the airstream being heated. They are commonly used
in space heating or make-up air applications, process drying, and
baking ovens.
Directredburnershaverawgasinjectedintotheburnerandthe
burner relies entirely on the air being pulled across the burner for
combustion air. It may take advantage of the mixing effects of the
blower by using a pull through system which locates the burner
on the suction side of the blower. This means the air being pulled
across the burner is at a negative pressure (usually not greater than
-1.5”w.c.).Otherdirectredapplicationsmayuseapush through
system, which locates the burner downstream from the blower.
This means the air being pushed across the burner is at a positive
pressure(usuallynotgreaterthan3”w.c.).
Directredburnerscanbeofextremelyhighturndownratios,in
some instances, 30:1. The high turn down ratio allows the minimum
temperature rise to be low enough that the unit does not have to be
cycled on and off to maintain temperature.
Indirect red appliances use a heat exchanger and all products
of combustion generated by the gas burning device are vented
outdoors. They are usually supplied with an atmospheric burner or
a power burner.
With an atmospheric burner, the air being supplied to the burner
is at atmospheric pressure. They have limited turndown ratios
(maximum input: minimum input) of usually 3:1 or 4:1. Due to the
limited turndown ratio, the minimum temperature rise cannot be
held low enough, and as a result, the main gas valve must usually
be cycled to maintain temperature.
A power burner is a burner in which either the gas, air, or both
are supplied at pressures exceeding line pressure for gas and
atmospheric pressure for air. Maintaining the proper gas-air mixture
for modulation usually requires pressure control of both the air and
gas. Burners of this type are usually not capable of modulation with
the Maxitrol Selectra
®
system. (See Exa Star modulating valves or
ratio (zero) regulators for these applications.)
DIRECT FIRED
Valves designed for direct red applications (negative
pressure/pull through) M411, M511, M611, MR212D, MR212E,
MR212G, and MR212J.
These valves are designed exclusively for negative pressure (pull
through) applications. They use a counter spring to keep the valve in
the closed position despite the downward pull of negative pressure.
M411,M511, and M611valves are setfor applications upto 1.5”
w.c.andarenotadjustable.MR212valvesarefactorysetforap-
plicationsupto-1.5”w.c.andareeldadjustableforupto-3.5”w.c.
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M/MR Series Literature
With the main valve closed, an adjustable bypass provides a
minimumringratetotheburnerwheneverthesafetyshut-off
valve(s)isopen.Thisislowretotheheater.Sincethepressure
regulator maintains a constant supply pressure, the minimum
ring rate is also held constant. The MR212’s regulator is
an integral component while a separate upstream pressure
regulator is required for the M411, M511, and M611.
As voltage is applied to the control’s solenoid, a magnetic force
is applied to the plunger. The plunger in turn pushes down on
the main diaphragm of the M411, M511, and M611 valves or
allows pressure to develop in the upper chamber of the MR212
valve. These forces on the main diaphragm are very similar to
the spring loading of a gas pressure regulator. When the force
is sufcient to overcome the counter spring, the main valve
will open and gas will ow through the main valve in addition
toowingthroughthebypass.Withsufcientvoltage,themain
valve (modulator valve for the MR212) will fully open and the
pressure regulator will limit the burner pressure to the desired
amount, thus establishing a controlled maximum high re
condition.
Wenowhavetwoextremes.Therstislowrewithnoforce
ontheplungerandowthroughthebypassonly.Thesecond
ishighrewithsufcientforceontheplungertofullyopenthe
valve (modulator valve for the MR212), with the gas pressure
regulator controlling the maximum ring pressure. Force on
the plunger between these extremes results in modulated gas
ow.Thevoltageversusoutletpressurecurve,throughoutthe
modulating range, is fairly linear.
There are also applications where it is desirable to have two
separatehighreburnersettings.Typicalapplicationswouldbe
a two speed fan operation or LP natural gas change over. This is
accomplishedbyaddinganadjustableresistancetothecircuit,
thereby limiting voltage to the valve. Changing from one setting
to another is done by connecting a single pole single throw
switch (customer supplied) that when in the open position allows
theoutlet pressureto beadjusted upto 2”w.c.below normal
maximumhighre.Negativepressurevalvesdesignedfordual
pressure settings are designated with a “- 2” sufx (example:
MR212D-2). Field conversion kits (KT/10542) to modify single
pressure valves are also available.
The working voltage to operate the modulating controls can
be supplied by the A1014, A1024, A1044, A1494 and AD1094
Ampliers or the SC11 Signal Conditioner. (Refer to Bulletin
SEL1444_CC_EN, SEL94_CC_EN, and SC11_MS_EN.)
Valves designed for direct red applications (positive
pressure/push through) MR212D-1, MR212E-1, MR212G-1,
and MR212J-1.
The valve’s operating principles are identical to the negative
pressure MR212. The only difference is the valve has been
designed to operate on positive pressure (push through)
applications. Positive pressure valves designed for dual
pressure settings are designated with a “- 3” sufx (example:
MR212D-3).