pressure switch on gas furnace

A little help here.. I have a gas furnace about 13 yrs old, and it runs most of the time. the times it doesnt it seems that the pressure switch is not working. i would take the little hose off it for a second and then it turns on. i replaced the switch with another one, not exactly the same it was .30 vs .35 plumbing store said it was close enough, but that didnt make it go on. what else could be the problem? is it bad to keep the hose off so it works everytime? the furnace guy tried to charge me over 1000 total, 100 for diagnostic, 470 for ignitor, (which works) and pressure switch another 470, which i purchased for 40. some garbage huh? thanks
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My guess is that the blower that creates the pressure/ vacuum for that switch to function is at fault. It's never advisable to disable safety devices, but maybe it's time to try a new furnace tech
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You've got a high efficiency furnace so the most of the heat has been extracted from the exhaust. So it won't rise up the chimney--you have to push it up.
The blower that does that is called an "inducer blower." When the furnace gets a call for heat, it starts up the inducer to start blowing air (either sucks it into the burners ahead of the flame or blows it out after the flame--different mfgrs do it different ways). Since the whole point is to create a "draft" up the chimeny (just like using newspaper in your fireplace to combat the cold air rushing down the chimeney, the furnace needs to know when it's accomplished that. The pressure switch tells the controller board that there's sufficient air movement to go ahead and fire up the ignitor and then open the gas valve.
If your furnace has a few years under its belt, chances are the inducer blower is weak. Technicans check that out by running an amperage draw test on the motor. If it's using too much juice, they replace 'em.
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On Tue, 13 Jan 2009 17:54:06 -0600, Rick-Meister

I dont think Ive ever found a weak inducer yet. It either works or it doesnt.
Just so you dont get confused here......... The draft inducer motor does NOT push flue gases up the chimney. The job of the inducer is to remove flue gases from the furnace. After that, it is up to the draft/draw of the chimeny to take over. In 99.9% of the cases, that doesnt happen either. The chimneys were sized for much bigger appliances. Thus, the need to line the chimney with a much smaller, appropriately sized flue liner. Bubba
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Just went through a furnace problem. Mine was the result of an igniter failure. The igniter used in my furnace (silicon nitride) turned out to be the most costly of all the ones that I looked up. The average igniter cost anywhere between $30 to $50. Mine came along at $110 online and $134 at the local HVAC company. those component prices seem way out of line. MLD
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snipped-for-privacy@gmail.com wrote:

What I have seen happen is the metal tube where the hose slips on the draft inducer will get clogged. A stiff wire like #12 copper or sometimes a piece of coat hanger can be used to clear the metal tube. You can pull the tube from the switch and listen for air flow when the draft inducer turns on. Some pressure switches work on a drop in pressure rather than an increase in pressure and you will have to get the correct switch or one that has a double throw micro switch. I'm assuming that your furnace is similar to the one on this site:
http://tinyurl.com/7mzmap
The prices given to you by the repairman are insane. Even the most crooked companies around here aren't that brazen. Me and my friends would charge under $200.00 for a repair like that.
TDD
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On Tue, 13 Jan 2009 13:05:44 -0800 (PST), " snipped-for-privacy@gmail.com"

So now you have two problems. You've wasted money on a pressure switch that wasnt defective and now have the wrong one in its place. Keeping the hose off is not the way the furnace was designed to operate. If it wasnt needed, the manufacturer wouldnt have wasted the money on it. Not saying that the price the furnace guy was going to charge you was fair or not but after you start up your own hvac business you'll start to see why we charge the prices we do. Its not 1950 anymore incase you havent noticed. Bubba
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On Tue, 13 Jan 2009 13:05:44 -0800 (PST), " snipped-for-privacy@gmail.com"

Our 5-yo high efficiency gas furnace quit working recently, and the cause was a slight sag in the exhaust PVC piping run (horizontal out the side of the house) that apparently was not exhausting the fumes and kept the pressure switch from activating the fuel (or something like that). All is well now, no parts were replaced.
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On Jan 13, 3:05pm, " snipped-for-privacy@gmail.com"

This is condensing? Blockage or ice on air intake- exhaust is common, an animal, bird can get in them if there is no screen. Get a different pro to check the whole thing out, when is the last time it was cleaned completly and gone over by a pro. You dont take off hoses or anything, safteys are put in for a reason.
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its just strange that it works fine during the day, but at night they periodically have to turn it off and on. sucks even more since the northeast is getting a cold spell at this time. no chimney its a condo the vent goes outside and doesnt seem to be blocked cuz it comes out just fine.
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wrote:

This is condensing? Blockage or ice on air intake- exhaust is common, an animal, bird can get in them if there is no screen. Get a different pro to check the whole thing out, when is the last time it was cleaned completly and gone over by a pro. You dont take off hoses or anything, safteys are put in for a reason.
If it's a condensing furnace then it might be important to know where the condensate drain goes. If it's dumped outside and the water freezes at the end of the drain line that could be a major reason for a furnace shutdown. During the day it might be warm enough to prevent freezing. Just a thought. MLD
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