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Posted by jolt on November 28, 2006, 6:14 pm
>
> zometool@hotmail.com wrote:
>> My home's air heater (installed in '96) runs on natural gas and is
>> located in a crawl space underneath the house. The blower is working,
>> but I'm only getting cold air from the heater vents in the floor. My
>> gas stove operates just fine, so I assume my gas has not been shut off.
>> My digital thermometer appears to be working, at least in part - it
>> tells me the inside temp. is 46 degrees, but I can't tell it to make
>> the temp 70 degrees. I'm a pretty new homeowner, so I haven't studied
>> this system and related problems yet, but now I'm feeling motivated.
>> I'm also feeling low on funds until next year, and I can't easily
>> afford a technician right now. Any advice as to how to proceed?
>> What's most likely to be wrong and how do I test to find out? Thank
>> you for offering your best advice.
>
> First, you might want to become familiar with the "logic" of your
> system.
>
> For example, the gas furnaces I'm familiar with have a pilot light that
> heats a thermocouple which enables the thermostat to actuate a
> solenoid valve to send gas to the main burner.
>
99.9 % of furnaces built today don't have pilot lights
If you have a furnace that the control voltage is supplied by power
generated at the pilot you have a millivolt system. It would not use a
thermocouple it would use a powerpile generator. If you have a millivolt
system which is not likely it dates back pre 60s. If however you have a
system that uses a pilot safety control, it doesn't work as you describe.
The control voltage is supplied by a transformer.
The thermocouple generates a small amount of power (millivolts) that
energizes a electro magnet to hold the pilot safety in place. When you
depress the knob to relight the pilot you are bringing the electro magnet in
contact with a metal surface, temperature differential produces power from
the thermocouple energizes the magnet and holds the safety in place.
> There's a "fan-switch" with simple mechanical sensor inside the
> heat-exchanger (not fire-side) with two temp settings- one for fan-on
> and one for high-temp-cutout. With furnace cold, fan-on is open,
> cutout is closed. On rise to fan-on set-point, contacts close, and
> fan THEN goes on.
>
Ask the guy at the parts counter for a fan-switch and if he brings you
something back it won't be what you want. I think you are referring to a
combination control or fan limit control.
> When house temp get to t-stat set-point, burner valve is de-energized
> and burner stops. When heat-exchanger temp drops to somewhere
> around 10-15 deg F below fan-on set-point, fan-switch opens and
> fan stops.
>
> If furnace temp gets up to high-temp-cutout setting (ex: blower
> tossed belt), burner solenoid valve is de-energized. (Dunno about
> the rest, since I've fortunately never been there.)
>
With a simple fan limit it will shut down the burner until the fan cools the
furnace below the limit point and the burner comes on again. That's why some
furnaces added a ECO or TCO added to the circuit to stop the furnace from
continuing to cycle on limit. An TCO (thermal cut off) is a one time use
thermal cut-off.
> Anyhow, get a picture of what should be happening first; then you
> can find out what's missing. It's really bizarre that the fan should
> come on with the furnace cold-iron, for one. Mfg. should be able
> to give you some help on design of control-logic, too.
>
> Please don't blow things up.
>
> J
>
His furnace made some time around 1996 has little in common with your
furnace. The biggest difference will be that it has solid state controls,
the fan can be controlled by time, temperature or a combination of both, has
a purge blower and lacks a standing pilot.
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