I've got an older "Day and Night" natural gas furnace in my home, model
#800-19a. Setting the mercury thermostat turns the blower on, the
standing pilot is lit, but the burners do not ignite.
After doing a little online research, I suspect it might be a bad
thermocouple, which is a relatively easy fix. But I thought I'd run
this by the good, knowledgeable people of this newsgroup first.
Is there anything else I should look at?
Here\'s some of my work:
That model may actually be: 80U-19A
Typically, a bad T/C will keep the pilot from lighting (though not always).
Any failure in the 24V control to the gas valve will keep the
Main burner from igniting. An AC voltmeter will help tracking
Your Day/Night should have a Fan/Limit switch like this:
Failure of the Limit contact to close will keep the gas valve
Right, if thermoouple is bad pilot won't stay lit. Thermocouple's
function is supplying small amount of gas to keep the pilot lit.
When it is calling for heat, measure across gas valve terminal to see
24V AC. If you do, gently tap it to see what happens. Either you're not
getting power to valve or the valve is sticking.
They are probably rare now but some older natural gas systems did have a
pilot light without a safety cutoff, sometimes fed from a separate small
line and valve tapped into the feed lilne ahead of the main gas valve. They
heated a thermocouple or mercury bulb which in turn allowed the main gas
valve to open. I guess the thinking was that the vent would safely dissipate
the small amount of gas if the pilot went out.
They are not rare systems.
I've still got four older Bryant furnaces in my rental apartments that
don't have a safety pilot system.
They have a bimetallic switch heated by the pilot. If the pilot flame
goes out, the switch opens and shuts down the main gas valve. However,
the pilot gas keeps flowing. One of these furnaces had the pilot blow
out over a summer seaon. the tenant never noticed the gas smell.
Probably most went up the chimney.
Recently, a local gas company tech told me he still see lots of them.
They are pretty common in my area and are not considered unsafe.
GE had some units like that also that had a temp switch in the pilot
flame that broke the circuit to the gas valve if it wasn't red hot.
Carrier and it's other companies-- Payne, Bryant and Day&Night-- had
one that did the same thing, but was all one assembly, whereas the GE
had the same pilot burner as the 100% cutoff, and the switch screwed
into it with the same threads as a thermocouple. They were referred to
as 90% cutoff. Both were made until around the early 70's. A few other
brands used variations of the same setup eons ago. All were for NG only,
no LP. Why they used such a thing I have no idea. It really doesn't look
like it would have been any cheaper at all, and if it indeed was, it
couldn't have been enough to justify the safety issue Larry.
Help!!! I am having a similar problem with an old hanging furnace. I
woke this morning with the blower fann running but no heat! I had to
throw the curcuit to get the fan to stop. I have cleaned the
thermocouple, it was slightly corroded, but i cant seem to get the
pilot to ignite. Does electricity need to be on to have this ignite?
Please help!!! Plus i might not even be in the right spot . Any info
would be most helpful!!
If the thermocouple goes to the gas valve a bad thermocouple will kill
the pilot. Some real old systems had the thermocouple go to a
solenoid/relay which interrupted the thermostat circuit to the main
burner gas valve. There was a pushbutton to reset the solenoid. If the
thermocouple goes to the gas valve Tony's suggestions sound good.
Have you lit the pilot before (do you know how it is supposed to be lit)?
The valves with thermocouple I have seen you turn a knob on the gas
valve to a "pilot" position, push down on the knob and light the pilot
while holding the knob down. The pilot should light with a bad
thermocouple and no power. You need to hold the knob down until the
thermocouple gets hot. If you waited long enough and the pilot goes out
when you release the knob the thermocouple is probably bad. If the pilot
lights turn the knob back to the original position.
It was bugs! BUGS!
I took the thermocouple out hoping to polish it down a little. The
pilot assembly came out with it. As I checked the parts, little pieces
of insects started falling out of the pilot nozzle. Then bigger pieces.
Then a whole housefly. Mummified. Probably in there since the furnace
was fired up last in February or something. It seems they had flown up
there, perhaps attracted by the rotten egg smell, during the long
dormant season here on the Central California Coast, and died,
partially plugging the pilot.
I cleaned out all the parts, put everything back together, and the
burners fired right up!
I still can't believe this. Been in this house twenty years and never
had anything like that happen before.
Lurkers with similar symptoms take note.
Here\'s some of my work:
I've cleaned out entire ant colonies from HVAC
equipment. Insects are one of the first things
I look for when HVAC systems quit working. I've
also removed the dessicated carcases of mice from
control circuits because some silly human failed
to pop a plug into a 1/2" conduit knock out. Mice
will crawl right through those 1/2" holes and get
electrocuted which will often short out the power.
Hole plugs are readily available and the phone
companies and other utilities use insecticide strips
inside control boxes to prevent insect infestation.
I don't know where I could get the same kind of
insecticide strips, I'll have to search.
BTW, those are some incredible knives Frank. I wish
I had that sort of artistic skill.
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