Although the popularity of constant circulation is growing today, it is far from a new idea. The idea of constant circulation is probably 60+ years old. The idea was promoted as a way to save fuel and improve comfort by doing away with hot and cold spots in the home when fuel was cheap. Boilers were steel or cast iron and radiation was large. In fact, the first hot water systems in the early 1900's were constant circulation systems which were gravity circulation. They consisted of large cast iron radiators large pipes installed uphill so that hot water would rise and drop in the system due to thermal changes. These systems were installed long before the invention of circulator pumps.

Constant circulation was proven to save people money on their fuel bills. The most extensive testing was done by the University of Illinois. Their tests proved that constant circulation could save the owner up to 30%. The large gravity hot water systems with pumps added being the largest savings. The idea of constant circulation was also tested against early OD reset and actually saved more fuel than the just OD reset controls. I must also add that the OD reset controls have come a long way with solid state technology.

With constant circulation the burners will only run until the thermostat satisfied. The system water temperatures were normally 30º to 40º less than a system as on/off circulation.

Let's fast forward to today. Looking at the application of constant circulation works best on large water content systems or in slab radiant. The larger the heat sink the more money saved. Constant circulation will also improve on the home's comfort. If I was doing constant circulation today on a mod/con boiler I would use primary/secondary piping systems so the constant flow does not go through the boiler. If I was doing constant circulation on a cast iron boiler, I would not do p/s piping to take advantage of the thermal storage of cast iron and water volume.