Hot Water Coil
From NEPA Crossroads Knowledge Base
A hot water coil is special loop of piping within a furnace that will heat a fresh water to supply domestic hot water.
A thermosiphon loop is the most simplistic way to tie a hot water coil into your existing hot water heater. The basics are quite simple, hot water likes to expand. The following diagram is the basic setup. As the water comes in from your inlet supply from the street it will pass through the first check valve. This first check valve will will prevent hot water from backing up into the cold water supply. It then meets a T and is forced into the coil.
The second check valve on the return from the hot water heater prevents the cold water supply from going into the hot water heater first forcing it first to go through the coil in the furnace to insure that you are heating all incoming water with the coil. This also creates a single way the water can flow for the loop.
After going through the coil it exits the furnace, this water is usually extremely hot. Be sure to heed the warning about using a thermo limiting mix valve. This outlet from the coil is connected to your hot water heater where you currently have the cold water supply connected. The return line to loop back to the coil is facilitated by the drain on the bottom of the hot water heater. This return line runs from the drain to the T where the cold water inlet from the street is. This completes the loop.
Since hot water wants to expand it will naturally flow from the coil into the top of the hot water heater forcing cold water out of the bottom back into the coil. This a natural occurrence and requires no pumps to get the water to flow. Because of the check valves it has only one way to go. Note that the hot water heater should be as high off the ground above the coil as possible and a very short distance from your furnace for best results. Longer distances hamper or prevent the thermo siphoning action.
A setup like this can provide almost an endless supply of hot water at a very low cost, most of the water will be heated by the coil because its already very hot when it enters the water heater. The thermal siphoning action will slowly replace the water in the tank. It also acts as buffer between the really hot water coming out of the coil and people. You will use some additional energy with hot water heater generally but it doesn't amount to much. The more water you use the more efficient this system could be, it may at that point just become a glorified storage tank rarely having to heat water at all.
If you use this design add a thermo limiting mix valve to control your domestic hot water temperature out to faucet; below code set point of 125 degrees. The design as outlined under a very efficient system could build domestic hot water over an extended thermo siphon cycle easily to boiler temperature. Boilers run at least 140 and far higher and the possibility of scalding water is very real.