Hot Water Circulator:

When servicing old hot-water heating systems that originally circulated by gravity, but now use circulating pumps, I began to notice a lot of oversized circulators.
In some cases, the circulator was so severely oversized that the boiler could hardly get any heat out to the radiators. The water couldn't pick up much heat in the boiler or shed it in the radiators because it was moving so fast.
There is very little resistance in the large pipes of a gravity system, so the consequences of over sizing the circulator are more severe than in a newer system with smaller pipes.
Looking for some way to determine the proper circulator size, Both tell you to use the next size larger circulator on a gravity conversion. Doing the math, I determined that both manufacturers were essentially saying to use a circulator about 50% larger on a gravity conversion than on a newer system designed for forced circulation.