Examining Flame Rectification
By: Adolfo Wurts
Once summer
is behind us, and customers begin to use their heaters again, you'll probably
be getting calls for service on the heaters. "I think my heater's working
because I can hear it turn on, but then it turns off again" is a common
complaint. If customers asked for a pre-season tune-up on their heater, much
like they do for their air conditioner before the hot weather starts, this
complaint can be prevented. In most cases there is no preseason check-up and
you'll be getting the calls.
Let's take a look at what happens. The weather turns cool and your customer
activates the inside thermostat. The thermostat sends a signal to the heater
that it should turn on in order to increase the temperature so that the room
temperature matches the thermostat. The heater then opens the gas valve. This
signals the igniter which, when it hits the gas, will turn into a flame. The
flame will continually burn the gas until the thermostat tells it to turn
off. There is a sensor that indicates to the controller that the flame has
lit. However, if, for some reason the flame doesn't light, the heater will
automatically close the gas valve so that the gas will not continue to flow
without a flame to burn it off.
When you get the service call the first thing you should test for is if the
flame is present or not. There are a few ways a direct ignition system can
confirm the presence of a flame, including a thermocouple, a pressure sensing
bulb, a fire eye, and flame rectification. This article is about flame rectification.
The job of the flame sensing rod is to tell the controller that the main gas
burners have ignited. If no flame is present after a certain amount of time,
the controller needs to take the appropriate actions, first of which is to
close the gas valves to the main burners.
Here’s the way it works. The controller applies alternating voltage
between the flame sensing rod and the base of the flame (ground). The ions
in the flame provide a high resistance current path between the two. Because
the surface of the base flame is larger than the sensing flame rod, more electrons
flow in one direction than the other. This results in a very small DC offset
current. If there is a flame present, the DC offset is detected by the controller,
which tells the gas valve to remain open. If there is no current flow, the
controller will close the gas valve and the system will purge itself of any
remnant gas before trying to re-ignite or lock-out. The DC offset is small,
only in micro amps.
It’s very important that the flame sensing rod works properly. Dirt,
corrosion, or bad connections in the flame sensing circuit can cause the controller
to think the flame didn’t ignite. The gas valve will be shut down prematurely.
Controller manufacturers publish specifications for the flame diode DC offset
current.
You can measure the current in the flame sensing rod by putting an instrument
in series with the flame sensing rod. The instrument must be capable of measuring
1 to 10 micro amps DC and have a resolution of .1 micro amps.
Before you connect any meter to the flame sensing circuit you must determine
if the system uses a separate flame sensing rod or uses the hot surface igniter
as both the igniter and sensing rod. Typically in hot surface ignition systems,
the flame sensing rod is separate from the hot surface igniter, in which case
it’s easy to connect to the circuit.
For the more common type of direct ignition systems, where the flame rod and
igniter are separate, the Fieldpiece Instruments’ AUA2 milliamp head
or any other Fieldpiece meter with micro amps can be connected in series between
the controller and the flame sensing rod. The AUA2 comes standard with quick
connects for easy hook up to already existing male plugs. Some controllers
require a mini-plug connection. The Fieldpiece AUA2 milliamp head and the
AQK3 Flame Diode Test Adapter Kit includes both a pair of adapters to enable
easy connection of the AUA2 or a meter to quick connects plus an adapter for
controllers that require a mini-plug connection. When the flame is on, there
should be a measurable mADC signal, typically under 10mADC (mA = micro amps).
Compare this to the manufacturer’s spec.
If the micro amp DC signal is too low, the furnace will perceive this as a
“no-flame situation” and the gas valve will close. Here’s
what to check:
1) Make sure there is AC voltage between the flame diode and the base of the
flame. Measure AC voltage from the flame sensing rod to the base of the flame.
The value varies by model (in neighborhood of 90V), but the important thing
is to ensure that there is voltage present. If there is no voltage, check
to see that the wires from the module to the flame sensing rod are correctly
connected or if the flame sensing rod or the wire connected to it is grounded.
2) If voltage is present, make sure the flame rod and burner are clean and
free of dust, dirt or debris. If they are not, take them out clean them and
reinstall. Cleaning the flame rod and burner will remove impedance from the
flame current circuit and thus should increase flame current.
3) If the current is still too low, check to make sure the connections from
the controller to the flame diode and the connection from the controller ground
to the flame rod ground is low (it should be well under 100 ohms). Another
way to check this is to install two temporary connections directly. One from
the controller ground to the flame base and another from the flame rod to
the flame rod connection on the controller.
4) If the current is still too low, replace the flame sensing rod. Now you
should be able to measure the correct DC offset current.
5) If the heater is still not operating correctly, the problem is probably
the controller.
However, when these systems use
the same hot surface igniter for sensing and ignition you may put yourself
and your equipment in danger. During the ignition part of the start-up cycle
there are many amps going through the hot surface igniter. Only after the
ignition cycle is finished will the hot surface igniter perform the function
of a flame sensing rod.. In practice, one instrument cannot conduct both very
high and very low currents without mechanical switches. Robertshaw makes an
adapter (model 900-041) which has a switch that is normally open and routes
the flame sensing circuit through your multimeter when the pressed. This should
be done only after the ignition cycle has been completed.
The information written in this article is of a generic nature and may or
may not apply to any particular situation. Always follow manufacturer’s
specifications, requests and suggestions. Never do anything while working
on a furnace that could potentially put you or anyone else in danger. Never
leave the equipment unattended unless you have properly disabled the system
or completed and tested the job to ensure proper safety requirements have
been met.
Adolfo Wurts is a
Senior Research Specialist at Fieldpiece Instruments and is a certified HVAC/R
technician.
Fieldpiece
Instruments, Inc.
580 W. Central, Suite A
Brea, CA 92821
(714) 257-9060 FAX (714) 257-9069
fporders@fieldpiece.com
