Matt Wehland wrote:
> Hello,
> I have just moved into a new house (rental) and am having some problems
> with the furnace.
> I need to get the house in order before I can start on the carport (my
> shop, see I almost got this post on content)
> I would just as soon fix the furnace myself (in exchange for rent) or be
> able to diagnose some problems and perhaps push for replacement, really
> would like a replacement.
Speaking as a professional furnace repair technician and avid
do-it-yourselfer I am advising you to get a qualified furnace tech on
this thing NOW. There are way too many horror stories in this
business. Get a pro to check it over. If he tells you to get rid of
it, get rid of it. It may not be nessesary though.
> I've searched on the web but all I can find are furnace repairing services,
> not much DIY info. If anyone can point me in the right direction I would
> be grateful.
There isn't much info on gas furnace repair on the web for obvious
reasons.
> While the house is only 5-7 years old, the furnace was probably (Ok
> definately) used. The house is a text book case of how Not to build, and
> everything was scavenged and quality is a joke.
That's why replacing it is a good idea.
> I can't read the data plate on the furnace to get make/model, this will
> probably make it difficult to get parts. I just tried cleaning it and I
> think I managed to remove as much of the writing as I did the grime. Oh well
Operation and sequence is fairly straightforward and in most cases parts
are universal (to a point). If this is a standing (always on) pilot,
not spark or hot surface ignition, then a competent furnace tech ought
to be able to get it working correctly if it's no too fouled up.
> The first problem is that the fan doesn't run correctly when the thermostat
> is on automatic fan. It doesn't seem to start when it should and shuts off
> even while the burner is running. I suspect a bad sensor.
> There only seems to 3 sensors in the furnace-
> One for the pilot light (connected to the gas regulator) That's the
>thermocouple pilot safety device.
> Another placed above the burners in the heat exchanger area (also
> connected to the gas regulator) This is probaly a three or four wire device.
> You didn't say. Typically two switches in one package, with a 3 to 12 inch
>probe protruding into the heated air chamber. One is the High Limit
>(overtemp)switch - it opens on temp rise. The other is the Fan Cycle switch -
>closes on temp rise. The fan switch ought to close when the heat exchanger
>temp is in the 120-135 degree range although higher or lower temps are used
>depending on design. Hi Limit should kick the burner off at 195 to 210
>degrees. Some are manual reset and some are auto reset. Sometimes a bias
>heater is built into the fan circut. Then you will have five or six wires to
>the device.
If the device is three wire, then the 120 volt hot line feeds both the
fan and gas valve circuts. If it's four wire then the valve and fan are
seperate circuts. The fan is 120 and the gas valve is 24 volts, fed from
the conrtol transformer. The 24 volts control circut will be switched by
the thermostat and control the fan in the cool mode by swtching the fan
relay. You may have a fan relay for heat also.
> And the third is placed even higher on the furnace, still in the heat
> exchanger/exhaust area. This is the one I suspect. Could be an auxillary hi
>limit or vent stack switch. The heat exchanger is the entire box over the
>burners, the flue is the vent draft box the exhaust pipe hooks to. The exact
>location of the switch will give you a better idea of its function.
> I don't have a meter to check this sensor and just pulling the 2 connections
>and trying them
> both open and shorted seems to make no difference. Would this be a
> thermistor type sensor that would create a voltage with sufficient heat, or
> is it just a switch that would short or open? It's a switch.
> The other main problem is that the furnace seems to run extremely hot, the
> metal towards the front top by the hot air output is extremely hot to the
> touch. I mean it will burn you. This doesn't seem right and since this
> portion of the furnace is only a couple of inches from the walls can't be
> safe. Could this be a cracked heat exchanger? Getting weird here. The
>flue/draft assembly will be very hot, but it should be in front and at the
>top. It lets the combustion gas vent to the atmosphere. Dependinfg on
>efficiency of the furnace, this gas is 200 to 800 degrees. The newer super
>high efficincy units exhaust flue gas at 30 degrees above the heated air temp.
> They extract that much heat from the combustion gas. I am assuming this is a
>fairly conventional upflow furnace. Gas valve/burner assembly above the fan
>box. Cold air in the bottom, hot air out the top. If you have a counter flow
>furnace, the fan box is generally on top and the flue pipe runs through the
>fan box in front of the fan. All sorts of variations are possible of course.
>
If the fan is not cycling on at the proper time the flue box can get
very hot. You haven't sealed this garage up tight as a drum have you?
A lack of combustion air can cause serious problems including death.
> Thanks for any help, once I get the house and office in order I will be
> starting on other fun projects (I am going to research wood burning
> furnaces right now, did I mention the house it located on 100 acres, much
> of it wooded? Propane is expensive and I haven't gotten to run my chain
> saw much lately. I'd rather save my money for a 4 wheeler to move wood
> than pay for propane.)
You know that wood heats you three times, right?
Once when you cut it, once when you split it and once when you burn it.
Be safe.
Matt
--
Matt Mahoney
Vancouver WA
With over 150 pieces of paper from several manufacturers, schools,
trade associations, three states, two branches of The Government of The
United States of America, and a union, all attesting to the fact that I
am a heck of a smart guy and would never blow smoke up your kilt.
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