Furnace sometimes, but not always, fails to start


I have an oil-burning furnace, with an intermittent problem. Most of the time when the temperature drops below the thermostat level, it kicks in just fine. But every so often, with no particular regularity, it fails to start. I have to go down to the basement and press the manual restart button (which always works with just one push). I have found no factor that seems to explain when it fails to automatically start. (For example, it seems no worse on particularly cold days, or damp day, etc.) It is just that once every day or so, it fails to come on.
I have a service contract, and the furnace is old but has been maintained. A couple service guys have been out to look at it, and each has done a little something, but nothing has definitively solved the problem. (I am embarrassed to admit here that I am NOT the handyman type. I do not watch over their shoulders, and I do not know precisely what they have tried.)
I have a guy coming out again in a couple days, and I am hoping to be a little bit better armed with knowledge, and be more involved in the diagnosis. I was wondering if anyone has ideas on what sorts of things could cause this intermittent problem.
Thanks.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I had a similiar problem, the high voltage transformer might be going especially if it is older. You can test it by yourself if you have any electrical knowledge. Usually they swing out, make sure the furnace, is off at the main switch. I like to take the transformer out , and put is on the bench. after disconnecting the two wires.
Get a lamp cord and connect it to to the wires of the transformer. Get a large screwdriver, with a good plastic insulated handle, put the screwdriver on one of the high voltage electrodes and move it slowly toward the second electrode. The spark should jump at least 3/4 of an inch. Nice and blue. If not your xformer is weak, easy to buy and replace, about $80.00
Check your electrodes on the gun, they should be about 1/8" apart,and about 1/8" in front of the nozzle. While you got it apart, get another good quality nozzle, and replace your old one, with the same specs. Usually .75 to 1.50 @ 80 degrees.
Also, check your Cadmium cell and make sure its clean. If it doesn't see a good flame, it will shut the the gun down. Did you change the oil filter? If the air filter is plugged, proper air flow will not flow through the furnace, and shut it down on high limit temp. The oil pressure to the nozzle should be about 100 lbs.
Have a pro set up the draft and air over the flame!
My 2 cents worth..

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In my case, the electrodes needed adjustment.
Twice it wouldn't come on with the thermostat. I hit the reset and it fired right up with no problems. Third time it happened, I hit the reset, the motor came on for a minute, and still no fire. I switched it off and called the oil company.
Tech comes out, tries to bleed the oil lines, then adjusts the electrodes. He switches it on, and BOOM!! It blew smoke and fire all over the place. Tech picks himself up off the floor and asks how many times I hit the reset. "Just once, like you people told me to do!".
I guess the old Chrysler Airtemp has an attitude... He admitted he'd never worked on one that old before.
Anyways, there are many different things that could cause that. Electrodes, bad/dirty photocell, air in the lines, etc..
You've got the right idea with the service contract. The oil furnace is the ONLY thing I will not work on myself.
-rev
snipped-for-privacy@gmail.com wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@gmail.com wrote:

When you do reset it, does it rumble, or is there a smell of oil or smoke?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
shebaaa wrote:

Well, the sound of the burner kicking in is always a sort of mild rumble, but it does not sound any different from usual when I am doing it via reset. No smell of oil or smoke that I have ever noticed.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@gmail.com wrote:

Probably not the transformer then(though it should be checked). When was your last cleaning? I assume the Techs checked pump pressure and flow. If not it could be partially clogged filter, line or strainer. Then I would check amp draw on the burner motor. Then if nothing found depending on the Primary control, check the relay contacts if possible. These are just the steps I usually take when trouble shooting at a customers.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Right on with checking that motor assuming it isn't a problem with the fuel line.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Doesn't "sometimes" imply "not always"?
snipped-for-privacy@gmail.com wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.