Pressure Switch on Luxaire Furnarch

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My Mother-in-law has a luxaire furnace. The flashing LED indicates that the pressure switch is stuck open (2 flashes) - this is probably because the fan with the pressure tube does not start. If I bypass the control and run the motor straight from power, the switch now reports stuck closed (which makes sense). If i pull the tube off and put it on quickly the furnace will start no problem.
When I first looked at it, the switch wouldn't switch at all - I had to clear out the tube going into the fan housing. Logic is telling me that the issue is the control unit which isn't starting the fan, which makes the sensor report stuck open - correct?
PS I did take the pressure switch off and test it - it works fine. Before I go out and but the control unit I was just looking for a second opinion.
Thanks!
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On Oct 30, 10:37 am, snipped-for-privacy@gmail.com wrote:

Pressure switches operate at very low pressures. If your test was a brute force one like blowing into the small tube and checking continuity, the test could be meaningless. Before buying a pricey control board, it would be interesting to see if a new pressure switch and related tubing would solve the problem. Good luck.
Joe
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Thanks Joe, while it is true that the test was 'brute' force, shouldn't the motor switch on first? otherwise there would never be a vacuum ????
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snipped-for-privacy@gmail.com wrote:

Pressure switch is of differenbtial type. Never bow into it. If incuder motor turns normal and there is no obstruction in the tube, maybe the swtch is a toast.
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snipped-for-privacy@gmail.com wrote:

air to clear them, sometimes a piece of wire is needed too. If you removed the switch, make sure you reconnect the correct terminals. If it's like many of them, it has a common (C) a normally open (O) and a normally closed (N). This is the type that has a microswitch on the back and looks like this example which has only two terminals:
http://www.johnstonesupply.com/catimages/4557.jpg
There could be some debris in the switch also. Sometimes whacking with a screwdriver handle can knock it loose and get it back into operation, often permanently. I would check all the safeties because control boards can be quite expensive.
TDD
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The switch is just like that. I did actually have to clean out the connection and tube to even make it work, so I'm pretty sure that the switch is working as the furnace does at least run now after I 'faked' it out. Should the motor run before turn on before it does a status check on the switch?
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snipped-for-privacy@gmail.com wrote:

If you can fake it out, it's not the control board. There could still be debris or a partial blockage in the metal line. You can take a piece of stranded wire and strip about inch of insulation off, fray the wires into a sort of brush and run it in and out of the metal tube. Those pressure switches are very sensitive and any restriction in the draft inducer blower or exhaust from the furnace could keep things from running. The way most furnaces of your type work is:
1: The thermostat calls for heat by sending 24 volts AC down the white wire to terminal "W" on the control board. 2: The draft inducer blower starts running and the motor has a switch that tells the control board that the motor is turning. 3: The control board expects a signal from the pressure switch within a few seconds. 4: If the control board gets a proper signal from the pressure switch, the igniter comes on, I assume it's a hot surface igniter. You can tell by the bright orange glow in amongst the burners. 5: The electric gas valve opens feeding gas to the burners. 6: If the burners ignite, a flame sensor probe signals the control board that there is a flame. 7: After a few minutes, the heat exchanger gets hot enough to trip a sensor that tells the control board to turn on the air handler blower.
Nothing will work if things are out of sequence or no signal is received when expected by the control board's program. There are several over temperature sensors and one or more "anti roll out sensors" which detect if the flame rolls out backward into the furnace. The safety sensors and anti roll out switches usually have a little manual reset button between the electrical terminals. Your furnace may vary slightly from my description, a spark igniter instead of a hot surface type for instance but most standard modern 80-90% efficiency gas furnaces work this way. This is not a complete explanation of the inner workings of the furnace but it may help you understand it better.
TDD
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*** So, based I'm deducing that the control is history as the thermostat does close the circuit (triggers), but the draft inducer does not start. I'll check the wires as a final option, but I feel now that I'm in denial about the unit. Thanks everyone for the input.
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snipped-for-privacy@gmail.com wrote:

involved in the various functions. Since I've never seen your furnace, I can only generalize. I repair equipment all the time that I've never seen before but I know who to ask. I helped a friend work on a new HVAC unit that I had no experience with and we thought something was wrong. After a quick call to the factory rep our questions were cleared up and what we thought was a malfunction was, to my surprise, normal operation.
TDD
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*** So, based I'm deducing that the control is history as the thermostat does close the circuit (triggers), but the draft inducer does not start. I'll check the wires as a final option, but I feel now that I'm in denial about the unit. Thanks everyone for the input.
You have not verified that the control is bad unless you have verified that the pressure switches are in the correct state for start-up (probably open) and the control has been reset. Also verify that your AC wiring is correct as some controls will not work if the ground is missing or if the hot and neutral lines are reversed.
Don Young
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The switch is just like that. I did actually have to clean out the connection and tube to even make it work, so I'm pretty sure that the switch is working as the furnace does at least run now after I 'faked' it out. Should the motor run before turn on before it does a status check on the switch? At least some of the controls check the status of the switch(es) before starting the fan. If the status is correct, the fan is started and the status of the switch(es) is monitored for a status change. This is an additional safety in case the switch is stuck or bypassed.
Don Young
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Don Young wrote:

state for the switch to be in until after the draft inducer has had time to spin up and activate the pressure switch. Anything that's out of sequence will cause a shut down and or lockout. Often a lockout can be reset by turning the power or thermostat off then back on.
TDD
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On Thu, 30 Oct 2008 08:37:58 -0700 (PDT), snipped-for-privacy@gmail.com wrote:

Replace the switch Clean out the tubes, port or condensate lines Replace the inducer motor Replace the control board Replace the Heat exchanger Prob a few other possibilities too. Depends on how much time and money you want to waste. Bubba
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And this hack advise is from a guy that knows from on job learning, no schooling of course.
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On Fri, 31 Oct 2008 05:10:18 -0700 (PDT), ransley

No, actually that is exactly what the remedies could be. You on the other hand ransley are a complete and udder fuckwit. One merely has to google search your posts to figure that out. Sucks to be stupid like you, ransley, eh? Bubba
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Is there ever a post or customer, or person you dont use profane insults with. You must be a sucessfull business person, HAAAAAAAA hAAAAAAAA Haaaaaaa.....
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On Sat, 1 Nov 2008 08:54:27 -0700 (PDT), ransley

I am sucessful. Quite, in fact. Why? Does that make you jealous? Oh to be a simpleton such as you, ransley. Bubba
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So post em moron, you are the guy that said 20 years is needed to learn to do Your job, So I say you just hacked your way through 19 yrs 11 months 3 weeks of unhappy customers. Our local tech college teaches you with Pros, not hacks. But geese im no Hvac person, but I can sure spot a loudmouth dumb ass crook.
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On Sat, 1 Nov 2008 08:58:34 -0700 (PDT), ransley

Why exactly would I waste my time reposting your stupid ass posts and cluttering up the usernet with more and more of your crap? One merely has to do there own google search and see your stupid posts. Yes, it has taken me 20 yrs. 25 in fact. I learn each and everyday. Its a shame you dont learn a thing. Unfortunately that happens to idiots like you. Dumber than a box of rocks. Do worry though ransley. There are plenty of idiots in the world like you. You arent alone. Bubba
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snipped-for-privacy@gmail.com wrote:

That switch is differential type pneumatic switch. If you blow into it, it'll be damaged, If inducer motor spins normal and there is no dirt inside the tubing, the switch assembly is a toast.
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