Alot of oil fired boilers use a honeywell burner control that mounts on a 4 square box right on the burner gun itself. On that controller there is a Jumper that jumps the t-stat contacts. If you break that wire with a contact , the gun won't fire on a call for heat. But your circ pump(s) and(or) zone valves will all work anyway, if your boiler water is up to temp. You said that your setup was in series so I'm picturing the hot water from the boiler flowing into the oil fired then into the loops. A strap on aquastat is usually a single contact.Use it to break either low or 120volts. Best way to hook up auxillary heat is two pair low voltage wires. 1pair to break the t-t contact on the oil gun, 2nd pair to make the t-t contact on the auxillary controller switched by a relay in parallel with 120v feed to oil gun. Hope that got the point across. Sometimes it's easier to picture it than explain it.