2nd zone thermostat/pump wired incorrectly. Is this setup properly to prevent boiler condensation?

You can see ZR,ZC aren't being used in the Hydrostat, so the boiler has no way of knowing the second zone wants heat. Running cold water through the boiler isn't going to help anything, that's for sure. I would install a 2 zone control panel and use the isolated end switch to TT on the Hydrostat. It can be done using the 845A, but it's almost harder.

That's a good boiler when it runs clean. When its dirty, it's a PITA to clean, with those tension plates on top.

You need to add 2 wires from the R845A to the Hydrostat control and your problem will be resolved.

Connect 5 and 6 from the R845A to ZC and ZR on the Hydrostat. They are under the removable sticker.

No additional controls needed. Just a piece of wire with 2 conductors from the R845A to the HydroStat control. EASY PEZY!

Why would you want to set the Low limit to maintain any temperature? There is no tankless coil, so no need for a minimum temperature. Just leave it a cold start boiler with Lo Limit set to OFF.

When you connect ZC to ZR on the Hydrostat (or any control that has the ZC ZR terminals), you are taking power from the L1 terminal on the Aquastat Relay and connecting it to the B side of the High Limit on the control. From the B on the High Limit the power then goes thru the limit contacts to the R on the limit and then to the B1 on the Aquastat relay. This way an end switch (like the 5 and 6 on the R845A) can make the burner operate thru the High Limit without making the circulator that is connected to the Aquastat relay operate. The Zone 2 circulator will operate and the burner will operate independent of the Zone 1 Circulator with just one wire that has 2 conductors.

I understand what you are saying now…. @HVACNUT. That would make sense on the comfort side of things. Wasteful on the operation side, but on the comfort side I guess I might say that's OK. Not liking 120 VAC wiring…. I have known Plumbers that did not want to touch wires and I have known electricians that did not want to touch water pipes. Understanding and respecting both, I sometimes think that is kind of crazy. But you do have a point.

A cold start boiler will experience some condensation of flue gasses on start up every time. "Eventually" the exhaust temperature and the boiler water temperature rises enough that the condensation evaporates away back into the flue gas and is eliminate thru the chimney. The longer that "Eventually" is the more damage is done to the boiler. in most cases that time is less than 2 minutes and no real damage is done. If however you allow the boiler to operate with the return water below 130° for an extended period of time, the condensation never goes away, and acids form in that water. Those acids eat away at the cast iron and cause premature boiler failure.

By establishing and maintaining 130° return water quickly you will allow your boiler to last much longer. As far as the boiler returning to the cold start temperature after the call for heat is completed, that will not be a problem. You see, as the call for heat is over, there will be no flame and therefore no flue gasses to condense. So letting the boiler temperature to drop below 130° over the time between heating cycles will not have flue gas condensation issues.

I hope this helps in understanding the flue gas condensation issue.

That should have been addressed by the near boiler piping when it was installed. If your boiler is connected to a system that has a large water volume then there were instructions for piping a bypass from the supply to the return in order to have the boiler heat up to 140° in pretty short order. The bypass would mix hot supply water with the cold return water in order to make the return temperature get above 130° within a few minutes. The rest of the system would take heat from the supply slowly by gravity, as it was originally designed. There are even mixing valves that are designed to protect the boiler if your system has a cold return problem. https://www.supplyhouse.com/Caleffi-280175A-1-1-4-Female-NPT-ThermoProtec-Thermostatic-Mixing-Valve-130-Degrees-F-Tset Since you boiler was installed 20 years ago, I think it is a little late to worry about this problem for this boiler. This would be something to think about for you next boiler.

The main reason you have the cold zone is because the R845A only operates the circulator pump and will not operate the burner. Since your system operates at such a low temperature for Zone 1, your are never going to get heat in zone 2. that is because zone 2 requires a higher operating temperature. By adding the wire from 5 and 6 on the R845A and connecting it to the ZC and ZR on the HydroStat, you will actually raise the boiler temperature whenever zone 2 calls for heat. This may actually make your existing flue gas condensation problem happen less.

Have you been able to add that wire yet?