My home's air heater (installed in '96) runs on natural gas and is
located in a crawl space underneath the house. The blower is working,
but I'm only getting cold air from the heater vents in the floor. My
gas stove operates just fine, so I assume my gas has not been shut off.
My digital thermometer appears to be working, at least in part - it
tells me the inside temp. is 46 degrees, but I can't tell it to make
the temp 70 degrees. I'm a pretty new homeowner, so I haven't studied
this system and related problems yet, but now I'm feeling motivated.
I'm also feeling low on funds until next year, and I can't easily
afford a technician right now. Any advice as to how to proceed?
What's most likely to be wrong and how do I test to find out? Thank
you for offering your best advice.
I am not an expert but have used gas appliances for years and these
are the things I would check. We are assuming you have gas but no
ignition. Most furnaces have some type of access panel or door .
Get your flashlight, go in the crawl space and see if you can find this
panel. Pull it off and check for a pilot light. Inside that access
panel or inside the door to the panel there will be detailed
instructions telling you how to re-light the pilot and possibly other
procedures. Read these instructions. This will be your cheapest and
easiest fix. Sometimes you can light the furnace with a match. This
is a sure sign that it is the pilot or ignitor.
If yours doesn't have a standing pilot then there will be some sort of
electronic ignitor which makes a spark. You should be able to replace
It's also possible that your thermostat has failed somehow. Replacing
it is a cheap and easy job as well.
Tony, thanks for addending your support to Lawrence's advice. This
improves my confidence in the matter by a smaller degree than that by
which Abe reduced it. (I don't mean to sound ungrateful, I'm just
trying to provide informative feedback so as to act in a manner
conducive to raising standards for contributions. My personal web
ethics, made easier to uphold by our relative anonymity.-)
Tony Hwang wrote:
I am not trying to contradict you, just trying to find the answer to
the following question: Do they not put standing pilot light furnaces
in Crawl spaces?? Until I recently replaced my units, I had both in the
attic and both had standing pilot lights.
I can't imagine that it's safe to have an open flame and the valve
closest to it in a hidden spot like a crawlspace or attic. I would
think (or hope rather) that such things are required to be easily
accessible, such as in a garage or laundry room.
> I can't imagine:
> I would think (or hope rather)
Abe, I can't say that your hopes and imaginings justified your emphatic
pronouncement that I have "BIG problems". My body temperature must
have risen, or I'd not have broken into a sweat momentarily upon
reading your words. But now the sweat is evaporating and I'm
experiencing a chill worse than that which I normally experience
indoors in a 46 degree F atmosphere. All in all, I wish I had not read
any of your words on this matter. Nonetheless, I wish you greater
success in your future postings.
Actually, I did learn something from you that I respect, Abe. I didn't
know one could request that a comment not be archived. This seems like
a conscientious decision one can make to keep on-topic information
readily accessible in a forum. In the future, I'll imitate you in this
for certain of my own comments, if you tell me how to do it. And now I
understand that you were using the forum in a more conversational,
relaxed sort of way b/c you didn't intend to archive all your comments.
I was just cold with you because I'm cold right now! ..And web
anonymity makes me a sardonic dick sometimes. (Now I want this comment
to be gone in six days!)
Where'd you come up with the above info?
Until the last 15 years or so, most furnaces including horizontal ones
typically used in crawl spaces had pilot lights.
There nothing wrong or unsafe about that, unless you like to pour
gasoline down into your crawlspace, in which you're likely to have
greater problems than a pilot light...
I've got several places with horizontal furnaces installed in the
early 1980's that are horizontal flow and in crawl spaces.
As another poster pointed out, the only reason for elimination of a
pilot light was to achieve SLIGHTLY increased energy efficiency.
I installed a furnace in 1994 that still used a pilot light. It had an
AFUE of 78% versus the identical furnace with spark ignition which was
rated at 80%. I didn't sweat the 2% difference. I got the pilot
ignition furnace at a $200 discount...cheapsake that I am...
As for the original poster, I suspect that since his furnace is more
recent, it doesn't have a pilot light.
Also, the fact that his fan comes on is a clue. Many pilotless
furnaces have a blower control PCB that uses a simple time delay to
turn on the fan rather than a heat sensor. Thus, for example, if the
thermostat calls for heat, the main blower turns on 2 minutes later,
whether or not the main burner has achieved ignition.
Most later furnaces also have a draft inducer or small blower that
starts the air flow up the chmney. If that blower doesn't come on the
main burner will not ignite since there is a draft flow sensor switch
that must turn on to allow ignition.
To trouble shoot the above type of furnace, I'd turn the thermostat
all the way up, check to see if the draft inducer blower was
operating, check for a glow at the hot surface igniter or a snapping
spark at the spark ignitor, if it has one and also check to see if
there is a main control PCB with indicator lights. Many recent
furnaces have indicator lights that light in a sequence showing
certain fault conditions. If he knows how to use a volt/ohmmeter, more
tests can then be done...
Abe, I'm afraid I find your answer to be useless and somewhat
disconcerting. I encourage you to expound upon it with information I
can act upon, or at least find applicable meaning in.
> If the OP has a furnace pilot assembly in a crawl space under
> house, then he/she has BIG problems!
First, you might want to become familiar with the "logic" of your
For example, the gas furnaces I'm familiar with have a pilot light that
heats a thermocouple which enables the thermostat to actuate a
solenoid valve to send gas to the main burner.
There's a "fan-switch" with simple mechanical sensor inside the
heat-exchanger (not fire-side) with two temp settings- one for fan-on
and one for high-temp-cutout. With furnace cold, fan-on is open,
cutout is closed. On rise to fan-on set-point, contacts close, and
fan THEN goes on.
When house temp get to t-stat set-point, burner valve is de-energized
and burner stops. When heat-exchanger temp drops to somewhere
around 10-15 deg F below fan-on set-point, fan-switch opens and
If furnace temp gets up to high-temp-cutout setting (ex: blower
tossed belt), burner solenoid valve is de-energized. (Dunno about
the rest, since I've fortunately never been there.)
Anyhow, get a picture of what should be happening first; then you
can find out what's missing. It's really bizarre that the fan should
come on with the furnace cold-iron, for one. Mfg. should be able
to give you some help on design of control-logic, too.
Please don't blow things up.
99.9 % of furnaces built today don't have pilot lights
If you have a furnace that the control voltage is supplied by power
generated at the pilot you have a millivolt system. It would not use a
thermocouple it would use a powerpile generator. If you have a millivolt
system which is not likely it dates back pre 60s. If however you have a
system that uses a pilot safety control, it doesn't work as you describe.
The control voltage is supplied by a transformer.
The thermocouple generates a small amount of power (millivolts) that
energizes a electro magnet to hold the pilot safety in place. When you
depress the knob to relight the pilot you are bringing the electro magnet in
contact with a metal surface, temperature differential produces power from
the thermocouple energizes the magnet and holds the safety in place.
Ask the guy at the parts counter for a fan-switch and if he brings you
something back it won't be what you want. I think you are referring to a
combination control or fan limit control.
With a simple fan limit it will shut down the burner until the fan cools the
furnace below the limit point and the burner comes on again. That's why some
furnaces added a ECO or TCO added to the circuit to stop the furnace from
continuing to cycle on limit. An TCO (thermal cut off) is a one time use
His furnace made some time around 1996 has little in common with your
furnace. The biggest difference will be that it has solid state controls,
the fan can be controlled by time, temperature or a combination of both, has
a purge blower and lacks a standing pilot.
A furnace installed in '96 will almost certainly use a Hot Surface Ignition
system. Your most common failure would be the Hot Surface Igniter. The HSI
normally last for several seasons and if it has not be replaced since '96 it
is "very likely" that the carbon steel that it is made of has cracked and
will not heat to ignite the burners.
Standing pilots and spark ignition furnaces have given way to HSI systems,
spark ignitions because of reliability concerns and standing pilots because
they waste energy.
Jolt, you wrote an outstanding and admirable reply to my question! I
find much value in your answer! It looks like you even went so far as
to research the matter on my behalf, for which I am extremely grateful.
I could not have researched this matter anywhere near as conclusively
or efficiently as you did, with your greater knowledge of the subject.
Even if I can't fix the problem myself and I have to hire a pro, at
least I'll be able to define the most likely problem using correct
terminology, with specific reference to system operations. Your
information will help me avoid paying too much for an expensive
overkill scam-fix. I don't doubt that you've already saved me money,
in addition to time and frustration! I'm going to find out whether HSI
systems tend to be generic parts or system-specific proprietary
technology, and whether they require trained repairmen to replace them.
I'll report my findings back here in case they should serve to
supplement for others' benefit the very helpful information that you
supplied. Thanks again, Jolt! And please continue to add worth to this
forum by visiting often in the future!
Ok! I found a picture of what an ignitor looks like at
Same page has what appears to be useful info regarding same. The
information concurs with or supports everything Jolt said, so these
sources lend e/o mutual credibility. Good! Paraphrasing some
statements from the above site: Don't confuse terminology - There are
ignitors and there are electronic ignitor controls - either can break,
although the ignitors have the shortest lifespan. Ignitor controls
cost the consumer more than mere ignitors and sell for around $100,
apparently. (But these controls are relatively cheap to manufacture, so
they're used in a lot of furnaces these days.) Ignitors are
proprietrary parts, so you have to buy one specifically made for your
brand of furnace. Better yet, buy several at a time, and keep them
handy by your furnace for when they break and then it snows all of a
sudden, and your internal house temp drops to 46 deg F. No ignitor
equals no heat. Interestingly, ignitors can also serve as flame
sensors in certain new systems. Keep an ear out for either term if you
hire a repairman: "ignitor" "flame sensor" may both refer to the same
HSI is often used when re-lighting a pilot light would prove to be a
pain in the butt, so that's very likely what I have in my crawl space,
since I kind of dread having to go down there. You can test an ignitor
to see if it's functioning well. Use a multi-meter to test its
connection point - you should read 120 volts. If the ignitor "doesn't
glow" then it's bad. When replacing ignitors, don't touch them -
finger oils can shorten their lifespan. Check out the site if you have
a problem with your furnace that sounds like mine, and take Jolt at his
word in this thread (& pay attn to him in other threads;-)
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