I have a furnace that keeps locking out for an hour at a time when it
returns to normal until the next lock-out occcurs. My furnace
contractor has replaced the control board twice, the gas-valve once,
and put in three new flame sensors. They now are wondering if my LP gas
could be at the root of the problem because the flame detectors have a
green tinge to them.
I have two questions:
1) The ignition failures that lead to the lock-outs do not involve any
flame. That is, they occur before ignition; the gas just doesn't seem
to flow and the ignitor is definitely on. If the flame sensor was the
problem, wouldn't the failure occur after the flame was present, that
is it would not detect the flame even though one was actually present?
Isn't the flame detector's only function to detect for absence of flame
when gas is flowing into the burners and to shut off the gas at that
2) Assuming it is the LP gas, what could be wrong with it? What sort
of adulteration or impurities should be looked for? If it's a problem
with the gas, why would it work most of the time and not others? The
first failure followed the last fill date by three months. The second
failure followed the next fill date by three weeks. Now, on the same
tankful, lockouts are increasing in frequency to four or more a day.
I'd greatly appreciate any suggestions on what could be wrong. I'm at a
Most all gas furnaces cycle in this manor, the stat calls for heat, the
inducer motor starts and satisfies a pressure switch. After a short time, 15
seconds perhaps the igniter starts. Now on a hot surface igniter, it needs
to heat for a few seconds before the gas valve opens, where a spark ignition
will fire at the same time the gas valve opens. The gas valve will open and
stay open for a couple of seconds before the control board even looks for
flame. After a couple seconds for an ignition trial, the board will "look"
for flame sense and keep the valve open, or if no flame is sensed go into
lockout, or retry.
If your furnace locks out on flame sense failure and no flame has been
present, you have other problems other than the flame sensor itself. It
could be many, board, gas pressure, valve, even wiring could be a problem.
Thanks for your informative and quick response, Greg.
Your description of my furnace cycle sounds exactly like what is
happening. I don't know, however, that I have never noticed a spark
ignition at the point the gas valve opens--even when it is functioning
normally. I do have a distinct orange glow, though, that seems to
describe a "hot surface ignition." That glow subsides at ignition or
Gas pressure has been measured both at the point it enters the furnace
cabinet and inside the valve. Both pressures have read normally.
Is their a way to isolate whether or not the intermittent failure
occurs from the gas side or the spark side?
Other than to throw parts at it, no.
This is one of those times when patience is required. Somebody may need to
just baby sit the darned thing for a while. Usually by jumping the stat
connections at the furnace, and watching it go through it's ignition
sequence with a voltmeter or two connected at the appropriate places a tech
will find the problem. Sometimes it may take some time though!
I had one I spent the better part of the day watching! A couple other techs
had looked it over and gave up. Those intermittent problems can really be a
Greg, again thanks for your input. They have had meters on several of
the circuits, but never had anything but normal readings. At the time,
though, the furnace was functioning normally. It did fail twice while
they were here, but they didn't have a meter hooked up at the time.
One tech looked at the other at the first of the failures and said, "no
gas." That's what caused them to put in a new valve. But within a
day, I was having lock-outs again.
I think they're now grasping at sraws, however. They suspect a problem
with the composition of the LP gas, and pointed to the green residue
on the flame sensor. I don't see how that has anything to do with the
ignition failures, particularly after you described the cycle to me.
There certainly is no problem with the gas water heater sitting right
next to the furnace.
Bubba, I have not paid anything for any of the parts or labor except
for the first service call six weeks ago, or so. The furnace is only
28 months old and I have a ten year-warranty. I am concerned, though,
that with their expressed concern about the quality of the LP gas, that
they may be very frustrated and starting to "bail out" on me.
There is no spark sent anywhere on a Hot Surface System, there is not even a
device that creates a spark on the furnace. The HSI may need to burn up to
45 seconds before voltage is sent to the gas valve. Is there voltage to the
gas valve or does the sequence of ops end before that?
There is correct voltage to the gas valve at least 80% of the time and
everything performs correctly; it is an intermittent problem. To the
best of my knowledge, though,a reading has ever been checked during an
ignition failure, though.
when this problem started, I strongly suspect that there is a signal
problem between the board and the valve; the board, itself, has been
replaced twice, the valve once. Are there connectors or switches
between the valve and the board?
Thanks for your response. If you think of anything more, I'd really
appreciate hearing about it. It locked out eight times over the
weekend. I'm guessing that number represents almost a third of the
total lockouts since early January.
Are you sure you are telling us the whole story bill?
You've let this supposed "furnace contractor" do all these parts
replacements and still letting him back for more? Are you paying for
all this? Bill, do yourself a favor and find a real company because
the one you found isnt.
Model# Serial# may be of help.
that keeps locking out for an hour at a time when it
The flame sensor does exactly what you would think it does. It senses
current rectification through the flame to ground. The control module looks
for that current after it opens the main valve. If it is not there then the
system goes into lock out. Each unit is a little different but the basics
are the same for all.
Could have other inert gases mixed in with it. None, have the LP guy
come out and check it. Pockets of air in the line (wag). It sounds like
permanent failure is imminent.
Is the guy working on it the one that installed it? You might want to
call the Mfg and explain the situation to them and see if they can remedy
the problem. It's hard to tell what other possible solutions there may be
w/o being able to put my hands on it, I can only make a WAG. This may be a
bad Molex plug, wire connector slowly deteriorating, piece of debris in gas
You have a train furnace it it sounds like a pressure problem in the inducer
or the pressur switch. Tell the company to come out with their manometerand
t fittings if they come out again. This will have to be fixed by a
qualified Trane dealer, not someone who is not versed in service. So far it
seems you have had parts changers out there
The Trane dealer's technicians came out again today. This time,
luckily, they had a meter hooked up to the new valve when the furnace
failed to ignite. They listened to the sequence of clicks and
discovered that the solenoid didn't "click" when it should have when
the voltage to the solenoid was normal during this period . They
replaced the five day-old valve with another new valve.
I'm keeping my fingers crossed that they have finally solved the
Thanks to everyone who helped me with this.
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