furnace blows cold air

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Hello.
I have an Amana Air Command Hi Efficiency 90 Gas Furnace that just stopped working this afternoon. I noticed that cold air was blowing out the heat registers and the temperature in the house was 8 degrees below the thermostat setting.
This furnace is a pilotless unit with a hair-dryer-shaped exhaust fan to vent the exhaust gases.
Here is how the furnace normally operates (to the best of my memory):
- the thermostat "cool-off-heat" switch is on "heat"
- the thermostat fan "on-auto" switch is on "auto"
- I turn up the thermostat setting, requesting heat
- there is a short delay (for the igniter to heat up?)
- the gas kicks in and ignites and the exhaust fan turns on
- there is another short delay (for the heat exchanger to come up to temperature?)
- the blower fan kicks in and voila, there's heat
Here is the present abnormal operation:
- the thermostat "cool-off-heat" switch is on "heat"
- the thermostat fan "on-auto" switch is on "auto"
- I turn up the thermostat setting
- the blower fan *immediately* comes on and starts blowing unheated air throughout the house
- the exhaust fan never comes on
- there is no gas smell, and the burners never fire up
- I cannot see any indication of an igniter heating up (not sure where to look, but I think if there was something glowing in there I would see it)
Here is my working hypothesis: There is a thermal safety sensor/switch somewhere in there that thinks the heat exchanger is overheated. So even though the heat exchanger is dead cold, it wants to keep the blower motor on to cool it off. And it inhibits the ignition and the release of gas to the burners. So when I turn the thermostat up, requesting heat, all I get is the blower motor coming on.
Does my reasoning make sense?
And, is it pretty straightforward to replace the thermal safety switch?
And, where is said switch/sensor?
And finally, is there a quick and simple way to momentarily bypass the thermal safety switch to test my hypothesis... so I don't waste time and money buying the wrong part?
Thanks.
EJ
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Your reasoning makes sense, but I doubt that it's that simple. I have a similar furnace (different brand). The start-up sequence is the same. However, when I asked about DIY service should something go wrong, the installer said that I should read the book first and then look at the "error codes" that flash when the furnace doesn't operate. He pointed out a little bank of LEDs that blink the error codes. That tells me there's a computer in there someplace and just about anything could be going on.
Before getting into the hardware, my suggestion is to read the installation/service manual to see if you can get any clues from the furnace itself. If you don't have the book, check on the web. Many can now be downloaded. I always check for the obvious -- tripped circuit breaker, loose wires, etc., and then try shutting off the power for a minute or so. That resets the computer.
TKM
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snipped-for-privacy@etherjones.us wrote:

...
Rather than guessing, I suggest you check the manual. Most newer furnaces have a built in system to monitor the thing and like cars tell you want it thinks is bad.
--
Joseph Meehan

Dia duit
  Click to see the full signature.
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snipped-for-privacy@etherjones.us wrote:

Hi, Read the manual? Error code? If I must guess, flame sensor is not sensing the flame if you visually confirmed flame ignited. Good luck, Tony
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I believe he said "the burners never fire up".
wrote:

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I would think you are on track. First, verify the gas is making it to the furnace. Make sure you hear the 'click' that opens the gas valve for the ignitors to fire. It's an electrical activated solonoid that you can hear or check with an ohm meter. Next, find that thermal protection fuse which should be located near the burners. Look on a site that sells them so you'll know what you're looking for. They have two leads and should read zero ohms if it's good. I would certainally hire a professional if the problem is beyond this.
J
snipped-for-privacy@etherjones.us wrote:

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My money is on a sticking relay, but I agree that he should hire a professional.
wrote:

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Bob, that is a possibility, but I'm sitting here looking at a bag of 50 thermal fuses that I purchased just for this problem. My last job was a Trane unit with a bad thermal fuse. I cleaned around the burner entrance and replaced the fuse and problem solved.
J
Bob wrote:

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The reason I said a relay, and not a thermal link, is because the OP said the blower motor was coming on. Now he said it's the inducer motor.
wrote:

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On Tue, 24 Jan 2006 21:00:19 -0500, Joey

Golly. If you can't afford a computer monitor, at least get a television.

Remove NOPSAM to email me. Please let me know if you have posted also.
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I apologize profusely. In my haste, I gave wrong info in the previous post.
I just spent the last hour with the panel off the furnace, looking for obvious problems (burnt wires etc) and re-checking what's happening.
The blower motor is NOT coming on. The vent (exhaust) motor IS coming on.
When I dial up the thermostat to request heat, all that happens is that the vent motor turns on. It stays that way for as long as I leave the thermostat turned up. The igniter never glows, the gas does not turn on (no gas smell), and there is no fire.
When I dial the thermostat back down, the vent motor shuts off after about 30 seconds.
This unit has a White-Rodgers model 50A50-206 control with the diagnostic LED. The LED does not indicate any problem (no flashes).
Just to confirm that the control was not dead, I removed one wire from the vacuum sensor that monitors the vent motor's suction. Then I got 3 flashes on the control's diagnostic LED, indicating that the sensor was stuck open. So the control was healthy enough to detect that.
What are the most likely culprits here? Does a failed igniter fit these symptoms? Is the igniter easy to test? How? I'm assuming that the White-Rodgers control does not monitor the igniter, because there is no error code listed for it. The error codes from the White-Rodgers are pretty limited. It's not like a modern auto computer which monitors just about everything.
I'm not smelling any gas *at all*, so the control is apparently not opening the valve. Is it supposed to, even if the igniter is faulty? In other words, does the control monitor the health of the igniter, and refuse to open the gas valve if it's not glowing? Or does it just wait a short time, then open the gas valve, and if the temperature doesn't rise rapidly it turns the gas back off?
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Do you think it's time to call a professional?

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wrote:

Quite simple problem really. You need to replace the thermostat.
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wrote:

Have you read the whole thread? Did you notice that some things did go on when he turned the thermostat to a higher temperature than that room was? If some things do happen, how do you reach the conclusion it is the thermostat?
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Just put him in your kill file like everyone else has done.

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Fuck off boob, you don't know shit, which you have proven time and time again.
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wrote:

Because it's always the thermostat. at least according to all the moronic homeowners/do it yorselfers I see every fucking day. And besides, because I said so, that's why. Dumbass.
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wrote:

Gosh, Bob, you're right!
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wrote:

I missed this. You repeat to others what people you don't respect say. Is this so others won't respect you too?

Very angry!
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On 24 Jan 2006 14:57:13 -0800, snipped-for-privacy@etherjones.us wrote:

There is no way to troubleshoot a system over the internet. it would be best to call a local company to check it on site. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ spam protection measure, Please remove the 33 to send e-mail
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