pressure switch on gas furnace

[jonoh]

vent blower (working) replaced last year limit switches (working) vacuum tube is clear Pressure switch (working outside of unit) problem: not enough vacuum to trip pressure switch

[Steve Scott]

[Tekkie®]

Steve Scott posted for all of us...

NO WAY!

[Bubba]

You need a new vacuum. Try Kirby, Oreck, Red Devil or Dustbuster Bubba

[jonoh]

[jonoh]

[Bob_Loblaw]

"jonoh" wrote in news:1161192490.124932.186170 @h48g2000cwc.googlegroups.com:

Unless you have a slope guage,and know how to diagnose and repair mids and highs, you're in over your head. There could be a lot more going on here than you know, or are capable of identifying. Call someone who does.

[Steve Scott]

You need to determine the pressure/vacuum exactly. A gauge to do so will cost around $100. That and the pressure/vacuum the switch is designed to operate on and you know where the problem lies.

BTW, they rate the switches in inches wc, not what size paper towel the tube will hold.

You can't seriously expect to come >It's virtually zero. I don't have meter/guage but it will barely hold

[Bob_Loblaw]

Steve Scott wrote

LOL!! New keyboard please!

[jonoh]

I may be out of my league (I'm a mechanical engineer) but it does seem pretty cut and dry (towel reference). Furnace is about 15 years old. 1. Vent blower comes on (check) 2. creates suction to pressure switch (no check) (Plenty of draft for burners though) 3. pressure switch gets enough negative pressure (suction) to trip 24V switch. I can bypass pressure switch and jump to next step to pinpoint problem

1. gas valve opens and burners ignite 5. limit switch trips when hot enough and blower starts

Do I have it wr> You need to determine the pressure/vacuum exactly. A gauge to do so

[Zephyr]

Close. The "limit switch" your referring too is likely [or could be] the "fan switch." The "limit switch" is to keep the furnace from overheating.

If the F.AU. is 15 years old, it is likely not a "high - low fire" unit currently being marketed. I'd check the flu, be sure there is not an obstruction [any obstruction would cause the draw from the inducer to change - one of the reasons the mfg. put the little pressure switch there]. If there is not an obstruction, and the inducer is up to speed [the second reason for the pressure switch], then it is possible the switch is bad. But I'll bet the inducer motor isn't up to speed. They are generally a shaded pole motor and can run "under speed" when the bearings / windings begin to fail.

You'd be ahead to call someone who knows and is familiar with these heaters and would be able to tell you what your problem is. I realize you are an engineer from your posting. I don't know what type of engineering you do, but it you are paid enough, then why monkey around. Have it fixed correctly by the person with the right training. Unless you're broke and can't afford a trained repair person?

[jonoh]

Thank You for your opinion and input. You are the reason I tried this site. Thanks again. I'll take it off and inspect for obstructions first then call for help if that is not the problem.

Thanks aga> Close. The "limit switch" your referring too is likely [or could be] the

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[Steve Scott]

Yes, you do. If you're really an engineer than you know data is important. Having the unit run after jumping the pressure switch tells you either the switch is bad or it's not seeing enough pressure/vacuum. How you determine if the switch is bad is by checking just how much pressure/vacuum the switch is exposed to. I simply don't know any other way to tell you but you have to know what the pressure/vacuum the switch is exposed to and what the value is supposed to be.

BTW, the burners firing doesn't tell you the draft is adequate. The pressure switch tells you that.

And what do you mean the limit switch trips? That shouldn't do that under normal >I may be out of my league (I'm a mechanical engineer) but it does seem

[Bubba]

[jonoh]

[Noon-Air]

Damn ignorant yankees

[Bubba]

[shinesapp]

[Bubba]

[jonoh]

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