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Constant Circulation....What's up with that?????
The idea of constant circulation is becoming more popular today with the use of radiant in-floor heat, large water volume systems and modulating boilers. 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. These systems were installed long before the invention of circulator pumps.
Pumps were installed on the gravity hot water systems and were turned on and off. Constant circulation was claimed to save people money on their fuel bills. The most extensive testing was doe by the University of Illinois. Their test proved that constant circulation can save the owner 20% to 30% on their fuel bill. The large gravity hot water systems being the largest savings. The idea of constant circulation was also tested against early OD reset and actually saved more fuel than the early OD reset controls. I have to add that the OD reset controls have come a long way with current technology but I bet it is still fairly close. The reason I say that is OD reset controls sets a target temperature for a given OD temperature. Constant circulation ran until the thermostat satisfied. The system water temperatures were normally 30º to 40º less than a system without constant circulation as opposed to on/off circulation. That would put the larger water content systems around 140º.
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 too effective with multiple zone baseboard systems today. With single zone modern systems and a cast iron boiler would not be a terrible application we need to be concerned with boiler protection.
Let's fast forward to today. Looking at the application of constant circulation works best on large water content systems of 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 I would do it on primary/secondary systems so the constant flow does not go through the boiler and mod/con boilers or cast iron boilers with the water flowing through the boiler. The latter will save more money due to the the thermal transfer of cast iron and thermal storage of cast iron.
It would be hard to do constant circulation with a zoned system. You would need pumps and zone valves and I feel it would be cost prohibitive unless it is a commercial system which will help balance the heating. If using multiple zones look at two stage thermostats for some insight. When zoning the boiler can be 100% - 200% oversized if properly sized to start with when smaller zones are calling. This condition worsens when the boiler is oversized and zoned.