Heater not working - 46deg inside!

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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.
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snipped-for-privacy@hotmail.com wrote:

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.
It's also possible that your thermostat has failed somehow. Replacing it is a cheap and easy job as well.
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If the OP has a furnace pilot assembly in a crawl space under the house, then he/she has BIG problems!
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Abe wrote:

Hi, I'd check the flame sensor/ignitor first when at it.
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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:

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Abe 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.
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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.
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> 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.
Abe wrote:

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I'm very sorry about the way I worded my first reply. It was a knee jerk reaction.
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No worries, Abe! Everybody has done it. I wasn't criticizing your character, of course - not my place & insufficient data! Peace, man.
Abe wrote:

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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!)
snipped-for-privacy@hotmail.com wrote:

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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...
Doug
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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 the > house, then he/she has BIG problems!
Abe wrote:

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Thank you, Lawrence. I really appreciate your taking the time to answer at such length - very generous of you!
Lawrence wrote:

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I do nearly all of my home repair myself. The big exception is HVAC. (and anything on the roof...) You kinda have to know what you are doing.
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snipped-for-privacy@hotmail.com wrote:

First, you might want to become familiar with the "logic" of your system.
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 fan stops.
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.
J
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wrote:

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 thermal cut-off.

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.
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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.
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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!
jolt wrote:

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Ok! I found a picture of what an ignitor looks like at
http://www.toad.net/~jsmeenen/gasvalve.html#hsi
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 part. 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;-)
snipped-for-privacy@hotmail.com wrote:

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