An outbreak of the Plague in 1665 forced Cambridge University to close, and Isaac Newton, a 23-year-old student there, returned to his family’s farm for the next 18 months.
A portion of the farm was an orchard, and it is widely believed that an apple fell on Newton’s head one day while he sat underneath a tree. From this observation, Newton developed his theory regarding gravity and its effect on objects. Specifically: “If object A has mass Ma and object B has mass Mb, then the force F on object A is directed toward object B and has magnitude.” He wrote like an engineer, didn’t he?
In our modern day times, no one wants to wait for anything, much less his hot water! Most of the means employed to shorten the wait for hot water use circulators with some form of time or temperature control. Another way to accomplish this task in residential or light commercial applications involves applying Newton’s law.
Let’s put Sir Isaac Newton’s formula to practical use. You can follow this on the drawing above. When heat energy is absorbed by water, the molecules expand and become less dense. Gravity (the force F) causes the denser, and therefore heavier, water molecules to seek out the lowest elevation inside the vessel.
All that is needed to set up circulation between the hotter (mass Ma) water at the top and the colder (mass Mb) water at the bottom is a loop that returns from wherever the demand is located to the lower connection of the storage tank or water heater. Gravity works its magic on the water and causes a thermal circulation flow that gently works its way out to the end of your loop and back through the return.
The entire loop must be well insulated to prevent wasting energy and short cycling.
You can easily set up a gravity loop by installing a 1/2-in. return connected to the hot water line at the point nearest the fixture that your customers identify as being the one where they want instant hot water; typically it’s the master bath shower. Run this line back to the mechanical room and connect it to the lowest port on the hot water tank.
New homes are a natural, but this works equally well in a retrofit application. If you can’t easily access the point of use on an upper floor, a connection at the base of the hot water riser will help minimize the wait for hot water on a long run.
Remove the boiler drain and install a brass nipple, 3/4-in.-by-1/2-in. tee and reinstall the boiler drain in the end of the tee. Install the tee with its outlet to the side and then a ball valve, swing check and a second ball valve laid level. I prefer using threaded brass fittings, but copper with sweat fittings could easily be substituted.
The brass swing check valve will need a hole drilled through the center of the swing check gate prior to installation. This hole doesn’t normally need to be larger than 1/4-in. and will work quite well with smaller drill sizes, provided you insulate the entire loop. Install this swing check so that the gate opens with flow toward your water heater return port. The second ball valve will give you the ability to access and service the swing check.
Remember this, though — the entire loop has now become an extended part of the vessel and its volume should be included in your calculations for the thermal expansion tank.
A number of years ago, we were installing the phc system in a huge home, something around 15,000 sq. ft. The plans called for a 120-gal. oil-fired water heater to serve the four bathrooms in the kids’ and guest wings.
A fair distance from the mechanical room was the second-floor master suite with a walk-in marble shower that had dual shower valves, first-floor kitchen with dual dishwashers, two double-bowl sinks and adjoining laundry with two clothes washers. These areas were to be served by a single 40-gal. electric water heater installed under the stairway with absolutely no access for future service! I’m not making this up.
A quick calculation showed that using the two high-volume shower valves alone would deplete the available hot water in less than five minutes. When I approached the architect and his mechanical engineer about installing a gravity hot water loop, they both told me it would never work. After much arguing, they grudgingly acknowledged that a small electric water heater wasn’t going to be an adequate source of hot water.
The mechanical engineer then drew up a fancy return loop rig for the oil-fired tank complete with a bronze circulator controlled by an aquastat. No amount of talking could persuade them to allow a simple gravity system.
After the job was completed, the owners moved in and the architect left. It was a simple matter of turning off the circulator and drilling a hole in the flow check. The owners never had to wait for hot water and they had a virtually inexhaustible supply from that hotel-sized 120-gal. oil-fired tank.