Here are a few points to keep in mind if you're thinking modular:

First, steam is dynamic. It can move at 60 miles per hour in a steam heating system and still be within the normal design range. As it moves, steam loses pressure though friction and condensation. How much pressure it loses depends on the pipe size and configuration, whether or not you've insulated the pipes, and the boiler's Gross load. Because steam is dynamic (always moving, and always dropping in pressure), the pressure at any two points in the system will never be the same. This is true even after the system has heated from one end to the other.

I mention this because it's easy to confuse steam with compressed air. You might think that once you get some pressure at the boiler, that same pressure will be everywhere in the system, as it would with compressed air. But that's not the way steam works. Steam can condense; it's always losing pressure.

As it whips its way through the common header, the steam can affect the water lines in the individual boilers. A mere one-ounce difference in steam pressure equals a 1-3/4-inch difference in water column, and depending on how you pipe the system, this can present you with a real challenge when you're trying to balance your boiler water lines. And that's why you should never interconnect modular boilers on their condensate-return side.

Here, forget modular systems for a minute and imagine you just specified two big boilers to provide heat for an old elementary school. One boiler is a stand-by to the other. You plan to have the superintendent alternate the boilers once each week during the heating season.

But worse than that, after a number of feed cycles, both boilers will probably flood. And as the water level gets closer to the exit nozzle, the steam will carry boiler water into the header and possibly into the system piping. That leads to wet steam, unbalanced distribution, water hammer, and high fuel bills.

Check any good steam-heating text, and you'll see that to make the two big boilers work together you have to either provide each boiler with either an independent feed pump or a motorized valve. Each pump or motorized valve takes its orders from the individual boilers' pump controllers. Piped this way, feed water can flow only into the boiler that needs it, and never into a boiler that's off.

Now change things around a bit. Instead of using two big boilers let's use, say, five little boilers. We'll connect them all together on the condensate-return side, and we'll feed them with a single boiler feed pump that takes its direction from a single pump controller. We'll mount that controller somewhere in the middle of the group of boilers.

I think you're making a big mistake if you interconnect those returns on your modular systems. Each boiler should have a pump controller and a motorized valve with an end switch that will start and stop the feed pump. Without those segregated condensate-return lines your individual boiler water lines will be bouncing like the cylinders in a V-8 engine.