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/condensing (mod/con) boilers. Many of you may ask why I said mod/con boilers. With the proper sizing and operation of mod/con boilers, 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 massive. The first hot water systems in the early 1900s were gravity circulation systems. They consisted of large cast-iron radiators and 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 University of Illinois did the most extensive testing. Their tests proved that constant circulation could save the owner up to 30%. The large gravity hot water systems with pumps added are the largest savings. The idea of constant circulation was also tested against early OD reset and 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 is satisfied. The system water temperatures were normally 30F to 40F, with less than a system's 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 consider
boiler protection .Let's fast forward to today. The application of constant circulation is most effective in large water content systems or in-slab radiant systems. The larger the heat sink, the greater the savings. Constant circulation will also improve the home's comfort. If I were 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 were 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 the home were zoned with zone valves.
If using multiple zones, look at two-stage thermostats for some insight. Due to zoning, the boiler can be 100% - 200% oversized even if properly sized to start with, when all 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 boiler 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 a warm weather shutdown. All the radiation should be the same style. Avoid mixing cast iron with copper tubing. With areas such as Florida rooms, they may become more uncomfortable.
Disclaimer:
The information found on this website is for informational purposes only. All preventive maintenance, service, and installations should be reviewed on a per-job basis. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property, or bodily injury from applied information found on this website.