Burner shuts off prematurely...

[wolfmomof3]

Hi, I'm new here and sorry if this sounds overly amateur, but I could use some troubleshooting help.
We have a new yorker boiler (fuel oil) with a bladder type expansion tank and a L8148A aquastat. The circulating pump is running almost constantly as the thermostat is calling for heat (therm set at 70, room temp is 65). We have had to hit the reset manually (water temp started out at 60 degrees, PSI is at about 12). The burner ignites and burns for roughly 105 seconds and shuts off - leaving the circulating pump running. After the burner is going for about 15-20 seconds I can hear hissing coming from the expansion tank. When knocked upon, the expansion tank does NOT sound hollow - but when opened at the bottom, no water comes out. Hubby manually reset the burner approx 5x and brought the water temp up to 100 Degrees. He thinks electrical - I don't know what to think but after some research am thinking it has something to do with the aquastat???? Does anybody have any advice, tips, or suggestions????? Thanks in advance!!!

[OldGrouchy]

You should call whoever you have service the oil burner. It sounds like the flame sensor, although there are many other things which could cause a loss of flame. That really isn't a DIY project if you have no experience with burner service. If you have to keep pushing the reset button (on the burner assembly, I assume) then it is either losing flame or thinking it has lost flame and cutting out on safety.

[NJ Trooper]

Absolutely agree with OldGrouchy!

You are literally playing with fire.