Hello. Im in the process of replacing my 57 year old oil boiler. I just purchased a used Weil Mclain Gold WTGO-3 Boiler with a Riello 40 burner for $500. The old boiler is on its way out and I need to replace this thing before the winter. The used WM is about 6 years old and in excellent shape. The original owner switched to gas. It came with two 007 Taco Circulators installed on the feed, a L4081B Aquastat, a DHW coil, and a safeguard 170SV low water cut-off installed on the feed. I asked the origianl owner if he was running relays for low voltage thermorstats or just direct line voltage, but he didnt know, as he was not to familiar with burners at all. All the wires, including the circulators, come to a junction box with a service switch. All the neutrals come together in this box. There are 4 wires coming out of the junction box. 2 Line wires,( 1 each for the circulators) 1 Line wire for the service switch, (which leads to the water cut off H terminal), and 1 red wire, ( which leads to the W terminal (LOW SIDE) of the aquastat). The other wires going through the junction box are as follows. 1 Black wire coming from the burner going to the B terminal (HIGH SIDE) of the aquastat. 1 green ground wire coming from the burner, grounded at the junction box. Last, 1 red wire coming from the water cutoff (terminal P2), going to the R terminal at the aquastat. Again, all the neutrals are tied together in the junction box. If im not mistken, its kind of simple how its wired. Line voltage is intorduced to the service switch, in turn, going to the low water cut-off. As long as theres water in the feed, the low water cut-off supplys power to the P2 terminal going to the R terminal (HIGHT SIDE) at the aquastat. (low water-cutoff acting like a simple switch). From there, the aquastat has 120v introduced, firing the B terminal (HIGH SIDE), A nomaly closed switch, in turn firing the burner, up until its set temp (180F +/- Diff), then the switch opens, shutting the burner. On the low side of the aquastat has got me stumped. Again I have a red wire coming from the W terminal from the aquastat, coming out of the junction box. Not sure where this went. The aquastat has 2 terminals up top. 1 B (spade Terminal) on the HIGH SIDE. 1 W (push in terminal) on the LOW SIDE. On the bottom of the aquastat there are 2 R terminals. 1 on the HIGH side and 1 on the LOW side. They also have a jumper installed, jumping the two R terminals. Im adding an additional zone to the unit (total of 3) with low voltage thermostats. I know to move all the ciruclator wires to the SR-503 Box, and to bring in Line Voltage to the SR-503 from the low water cut-off/service switch) to power the board. My question is how do I wire a Taco SR503 to this aquastat? The aquastat has NO T-T or ZR ZC connections. Im guessing it gets wired to the low side of the aquastat, but do I remove the jumper at the R-R terminals? Im not too sure which is which on the SR-503. Does the ZR send 120V or senses 120v on the SR-503? or vice versa with ZC? Do I keep the burner black wire on the B terminal and the 120v red wire (from the water cut-off) on the R terminal (HIGH SIDE). Also, the burner will provide DHW with the built in coil, so it will need to fire for how water, even when theres no call from a thermostat. If I wire the SR-503, will it still fire at the LOW setting (160F +/- DIFF) if the jumper has to be removed at the R-R terminals? I searched all over the internet for an answer and also contacted TACO. The answer I received from taco is to connect the SR-503 to the thermostat I was using originaly. That didn't really help. Im still stumped. The rest of the install is very simple for me, but this has got me stumped and I just dont want to cross wire anything. A simple solution would be to change the aquastat, but If i dont have to right now, I rather not. Thank you all for any solution.