Intermittent Burner ignition

OMEC

I have an oil fired, hot air heating system.
My Nest Thermostat reported that there were 11 instances the last month where the temperature fell while the system was in the heating cycle, indicating that the burner did not activate.
Burner ignition appears to be intermittent when thermostat calls for heat, Can hear a click at the furnace when increasing the thermostat setting but system does not always start until giving the control card box a light tap. Once it's started it runs fine and shuts down based on the temperature setting but may or may not activate on the next heating cycle.

Checked the wiring connections at and around the control card and everything looks OK.
Can the control card cause intermittent failures?

JUGHNE

Someone here will suggest trying a different T-stat other than the Nest.

EdTheHeaterMan

Nest is a funny thermostat. There are no actual switched contacts inside the thermostat like the other thermostat you had before. The switching action actua;;ly happens electronically with a triac and other electronic devices that changes resistance from high resstance (off) to low resistance (on). with some electronic control boards that come with furnaces these days, that type of switching action has some flaws. When the thermostat calls for heat the Low resistance switching circuit may not actually get low enough every time to pull in the heat relay on the furnace control card.

@JUGHNE may have the Best idea. Try the Resideo/Honeywell T9, or the Ecobee WiFi thermostat, or some other brand of smart thermostat

If you are stuck on using the NEST for other reasons, you can try an isolation relay. Ask me how if you are interested in the setup

OMEC

I started having the problem this year with an old LUX PSP511A thermostat before switching to the Nest.
The Nest only highlighted how frequently the problem occurs.

omec

HydronicMike

If you have to tap the 'control card box' (whatever that is) you have to trouble shoot that.

mattmia2

if it is clicking at the furnace the t-stat is probably calling for heat. probably either a control that isn't responding or it is already locked out from a failed ignition attempt so either an igniter or flame proving problem.

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1870128#Comment_1870128

So banging on the side of the furnace will get the burner to start.   This sounds like insufficient voltage to pull in the relay but once the relay is pulled in, it operates properly. And after that it may pull in just fine for several attempts until one time when the relay fails to pull in.  This does not sound like a primary control ignition failure because you do not mention that any reset button needs to be pressed.  The relay I ‘m referringto is the one on the “Control Card Box”   

I might say that the Control Card Box is more than likely an electronic fan timer for oil heat like the ST9103A made by Resideo/Honeywell. Can you verify that?  Sometimes those fan timer boards have a problem with solder joints on the back of the board and can be easily repaired if you are comfortable with soldering on a PCB.  If not then the only remedy is to replace the ST9103A 

To verify the problem is the PCB, you need to carefully remove the furnace cover (usually the blower compartment cover) and using a delicate hand connect the thermostat wire from R to W on the board.  If the burner starts as a result of a direct jump connection then the control is not the problem.  If the jumper connection fails to start the burner then the control board is the problem and needs to be repaired or replaced.  

The instruction in Italics is easier said than done

Mr. Ed

PS, I do not recall that the NEST thermostat was ever a problem on the furnaces that use the ST9103A Electronic Fan Timer

HVACNUT

Does the control look something like this?

C,Y,G,R,W are the thermostat connections. Then there's R and W that also go to TT on the primary. That is controlled by the heat relay under the cover at the top of the terminal strip. That relay is bad. If that's your setup, you need a new Beckett AC Ready Kit. You can order it with or without the transformer. You dont need the transformer.

OMEC

https://forum.heatinghelp.com/discussion/comment/1870137#Comment_1870137

EdTheHeaterMan ---- Thanks for your advice!!.

As you surmised , my control card is the ST9103A type. Inspecting the card I noticed a slight discoloration around one resistor/capacitor.
Jumpering the R/W contacts on the card results immediate initiation of the burner ignition cycle. I then ran four system starts using the thermostat allowing about 5 minutes between each restart. On two attempts the ignition cycle started immediately after I heard the relay click in, on one attempt there was a noticeable 2 to 3 sec delay between the relay click and start of the ignition cycle and one attempt there was no relay click when the thermostat called for heat until I tapped the card to start the cycle,

The discoloration on the control board suggests it may be the culprit for the intermittent failures. Is there a way to test the 24 volt circuit to and from the thermostat rule it out as the problem

		Thanks again,
		omec

EdTheHeaterMan

yes. When you had the one time out of the 5 where there was no click, that is when you would use a very light touch with a jumper wire to connect R and W on the ST9103A to see if the thermostat is the problem (not a tap on the box). If the jumper wire starts the burner, the thermostat is the problem. If the very light touch with the jumper wire does not start the burner, then the problem is the ST9301A control board.

109A_5

Same problem with two different thermostats. My guess from here is your ST9103A Electronic Fan Timer has a bad solder joint. Common solder joint failure locations are the relays and the connector for the wiring harness.