Burner Ignores Thermostat, Stays On

[SometimesRight]

This is a Weil McLain 268V (P-268-W) boiler with a Beckett AFG burner, Honeywell Round thermostat, Honeywell aquastat (L8124A, C) and primary (R8184G4009) . Thermostat was set at 68 degrees and burner was still going at 78 degrees when I shut off the emergency switch. I turned on the switch later to test things, the burner started. I hoped it might be a bad thermostat, so I disconected the thermostat wires from the aquastat and the burner shut off within a half minute or so. I went out and bought a new thermostat, but the burner had come on again in the mean time, with the thermostat wires still disconnected.
Any suggestions on what could be wrong? Thanks!

[Phelps]

Did this just happen out of the blue, by itself, with nobody tampering with the boiler or changing out any associated part?

Have you inspected where the thermostat wires are connected at the burner making sure some watery lime scale isn't bridging the terminals?

I also was contemplating a stuck contactor of some sort, but I can't understand how something could be energizing it without getting it's power source through the thermostat's white return wire, unless there is some dead short or possible backfeed coming from somehere. I would really look at the thermostat terminal board area in the boiler though, first.

I can't wait to hear what the cause of this one is.

[suprheat]

If you disconnected the t-stat wires from the controller and it still runs then you can eliminate the t-stat and associated wiring. Now you stated that the burner would go off on it's own and start on it's own - that eliminates that the burner control has welded contacts. So it seems the focus should go to the aquastat. I would guess that the circulator/burner relay contacts are welded. Tap on the relay with a screwdriver and see if it opens up. If this is the problem, it may be due to the circ pump or burner motor pulling too much amperage or together they pull too many amps or maybe the relay is just worn out. You should be able to run 110v separate to the burner motor which is actually the preferred way - I've just run across people cutting corners and not running 110v separate to the oil burner control. If the contacts were welded and you were able to free them up you should notice operation returning to normal. BUT - it may work for a while but I would plan to change it...once contacts become welded they're never the same even if you clean the pitting and carbon off. The good news, however, is that it looks like your high limit is working... I would guess that is what the burner is cycling on.

[SometimesRight]

Thanks for your responses. The line voltage is connected to the aquastat, the burner primary's thermostat terminals are bridged with a jumper (this is the setup indicated in Weil-McLain's manual for the boiler, when there is a circulator in the system). I did replace the wiring from the service panel to the aquastat previously, because it was NMB running on the surface of the basement wall (I replaced it with EMT and added a GFCI (no recepticle) and emergency switch, and equiment ground). Is there a down side to connecting the thermostat to the oil burner primary instead of the aquastat? I'll take another look to see if I can move the line voltage from the aquastat to the burner.

[Phelps]

This is what can happen when some sort of ad lib work transpires. I am not going to think this thing thru, but I'd venture to guess that since it's wired up differently, that this could be your problem. And I would get 'er wired back the correct way...in a way that creates a situation where if the stat shuts off, the heat must shut off.

Do you know that on the news some years back they had a story where an elderly couple were found dead because their furnace system got stuck and it was in the 90's in their house?

[SometimesRight]

Maybe this will work?

I found a Beckett troubleshooting link that led me to believe that I could reduce the low boiler temperture cut in setting, since this causes the burner to come on if there is no call for heat. The hot water for the house is from a separate electric water heater. I double checked the wiring connections, and I made no previous changes that could result in this problem. I connected the hot wire and ground wire to the same terminals on the aquastat as before, and those were correct according to the Aquastat manual. The new equipment grounding connection was merely by connecting the EMT conduit to the aquastat's box. I don't recall if there is a boiler water temperature guage to check if the water was below the low limit temperature when the burner was running, but is seems wrong that house would get as warm as it did (78 degrees, 10 degrees above the thermostat setting) just for the water in the boiler to come up to the minimum temperature. By the way: I couldn't tell where on the aquastat I should tap to see if the burner contacts opened.

[Phelps]

You should be able to adjust the cut-in setting. But, *I* have never heard of or realized that a boiler can get it's power to the burner without going first through the thermostat. This just doesn't sound right.

Just because the set point of the aquastat calls for heat, doesn't seem like it should actually fire up the boiler unless the thermostat upstairs is calling for heat.

If anyone else here has any ideas, feel free to jump in.