Gravity Boiler Replacement

Since this is hot water… and since you have easy access in the basement… you are going to be a whole lot happier down the road if you repipe the whole thing with individual supply and return lines for each radiator, feeding off and returning to a manifold, and pipe the boiler to the manifold with as primary/secondary. Put balancing valves on the individual radiator feeds. You'll need two pumps — one for the boiler and one for the radiators.

But you certainly don't need the individual lines to be 2 inch! Half inch or three quarter inch is going to be ample. Better yet, if you can keep the boiler temperature below 200 F (and you should) you can use oxygen barrier PEX…

I maintain a 2-story, 4-unit condo building built in 1924 with two gravity-conversion heating systems. One system heats one side of the building; the other heats the other side. Both systems have all original large diameter piping and radiators. Each system has one boiler and one circulator pump, supplying about 10 radiators. And I am amazed at how evenly the heat is distributed through all radiators, on two floors, from just one Taco 007 circulator, through the original large-diameter piping.

So if your system was piped correctly by the old-timers back in the day, and I would guess it was, you could be surprised at how well the distribution can work with the original piping. If it was my house, I'd be inclined to start with the minimum and just reduce each supply near the boiler down to a 1-1/4" copper manifold with one Taco 007 circ on the manifold and a balancing valve on each of the two supplies. This is basically your "Figure 1" with a 1-1/4" manifold. A Taco 007 will give you 15+ gpm in large diameter gravity conversion piping, and that should be plenty of flow for one floor of an average old house. (You didn't say what size boiler you'll be installing, so the 1-1/4" pipe size assumes you would have something similar to our WGO-5 boilers or smaller).

If this is a customer's house and not yours, I can understand not wanting to get a callback. But I don't see how you could go wrong with a Taco 007 on a 1-1/4" manifold (assuming a not-too-big boiler) with balancing valves feeding into the original large diameter pipes. That's going to be a lot less costly than ripping everything out and repiping, and if that building is like ours, it will work fine.

Also beware that because of the large water volume, you'll need to pipe a bypass into a new cast iron boiler.

how do you abate a cat….

I would plan on coming back a couple times to make sure everything is balanced, it will require some tweaking no matter what you do to get everything even unless it is a homeowner that understands how to adjust it themselves.