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Posted by jonoh on October 19, 2006, 3:45 pm
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 again
> 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?
>
> --
> Zyp
>
>
> > 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
> > 4. gas valve opens and burners ignite 5. limit switch trips when
> > hot enough and blower starts
>
> > Do I have it wrong???
>
> > > 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 into a newsgroup of professionals
> > > and get any sort of serious response without providing the information
> > > needed to correctly diagnose the problem. alt.home.repair is the
> > > proper place for group guessing.
>
>
> > > >It's virtually zero. I don't have meter/guage but it will barely hold
> > > >a dime size piece of paper towel. Tonight I plan on taking of vent
> > > >blower to see if trash has fell near opening. Does it have a baffle
> > > >to seperate vent and hose fitting?
>
> > > >> What's the pressure/vacuum at the switch and what's the switch rated
> > > >> for?
>
>
> > > >> >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--
> > > >> If a tree falls in the woods and kills
> > > >> a mime, does anyone care?--
> > > If a tree falls in the woods and kills
> > > a mime, does anyone care?
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