Any post Y2K gas fired boiler is almost certainly cold-start tolerant, and doesn't need to be maintained at any temperature.
As long as the boiler controls aren't configured to inhibit the pump that drives the indirect loop until the boiler is up to temp, setting the indirect to call for heat when it drops to 120-125F (or higher) ensures that even on cold start the boiler is pre-heated by the thermal mass of the indirect, and spends very little time burning in condensing mode. As long as the loop water coming back from the indirect, entering the boiler isn't below 125F for an extended period very little bad can happen to the boiler, but it needs to be above 130F for most of the burn to protect a terra cotta lined flue from some amount of condensation. (A standalone HW heater in the same flue would have the same issue, only worse... google "orphaned water heater". )
The
Bradford White indirects come pre-set to 120F, with a 5F differential (see p. 27) which means it kicks on a call for heat when the temp in the tank drops to 115F, and turns off when it reaches 120F. But the differential is (apparently) adjustable, as well as the max temp. Set the max temp on the indirect to ~140F and the differential to 15F-20F (they don't really explain how to do that in the manual, but it may be in the Honeywell documentation for the controls.)
The bigger the differential, the longer and more efficient the burn on the boiler, but the higher the storage temp on the tank, the higher the standby loss from the tank. But the standby losses of a 140F indirect will be far below that of a boiler maintained at 140F, since it's much-better insulated.
Even with a fairly massive boiler, the summertime water heating efficiency of the indirect won't be much worse than an atmospheric drafted HW heater. During the heating season the higher duty cycle the boiler has due to the additional load of the indirect only increases the net efficiency of the system as a whole.
With 4 zones and an indirect it's probably worth retrofitting an Intellicon 3250 HW+ to control the thing, unless you have an even smarter zone-controller already in place. (An
Intellicon retrofit can be a DIY project for those with electrician-skills.) It's a ~$200 piece of hardware from internet sources, but typically $500 or more to have installed by a pro. With the indirect set to 140F, the boiler's output will typically be ~ 160F at the end of a burn using dumb bang-bang aquastats, but the Intellicon anticipates the end of the call for heat (based on the temperature changes of the return water "learned" from prior burns), and will kick the burner off early, which allows heat to be purged from the boiler into the tank, parking it at a lower temp than it would have had earlier. This limits the amount of heat abandoned in the boiler on each call for heat, thereby reducing the boiler's standby losses, even during the heating season.
This type of control increases efficiency during the heating season by inhibiting the burner until the return water hits the programmed low-limit on a new call for heat, purging any remaining heat stored in the boiler into the system before firing up. This can dramatically improve the part-load efficiency of multi-zoned systems, since no one single zone is much of a load for the boiler, resulting in short burn cycles and high standby temps unless controlled more intelligently. The more oversized the boiler to the actual heating loads the bigger difference this method of control will make. But even if the boiler were perfectly sized for the whole house design condition, the average load of one zone when it's 40F out instead of -3F is likely to be less than 10% of the boiler's output- it's by-definition well-oversized for the individual zones.