The idea of constant circulation is becoming more popular today with the use of radiant in-floor heat, large water volume systems and modulating/condensing (mod/con) boilers. Many of you may ask why I said mod/con boilers. With the proper operation of mod/con boilers operating with longer boiler cycles will mean longer or constant circulator run times. 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 very early 1900's were constant circulation systems which were gravity circulation. They consisted of large cast iron radiators large pipes installed at strong angles so the hot water would rise up and down due to thermal changes. These systems were installed long before the invention of circulator pumps.

Constant circulation was claimed 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 can 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 have to add that the OD reset controls have come a long way with solid state technology.

Constant circulation ran until the thermostat satisfied. The system water temperatures were normally 30º to 40º less than a system as on/off circulation.

The larger water content systems matched to a cast iron boiler and a single thermostat did a wonderful job with this application. It would not be effective with multiple zone baseboard systems today. With single zone modern systems and a cast iron boiler today would still work well but we would have to be concerned with boiler flue gas condensation and would have to considerboiler protection.

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.

You cannot do constant circulation with a zoned system. You would need circulators running all the time or zone valves that would need to be open all the time and a system pump running continuously. So zoning would not make sense.  If using multiple zones look at two stage thermostats for some insight. Due to zoning the boiler can be 100% - 200% oversized if properly sized to start with when smaller zones are running. This condition worsens when the boiler is oversized and zoned.

A few tips when applying constant run pumps. The pump is just wired into an on/off switch. The pump could be operated with an OD sensor to turn it off at a given OD air temperature. If using ODR it may be operated with warm weather shut down. All the radiation should be the same style. Don't mix cast iron with copper tube. With area such as Florida rooms they may become more uncomfortable.