Heat storage allows a wood burning boiler to operate at more efficiently because the boiler does not need to smolder to avoid overheating the house. Instead it fires at full burn, storing excess heat in a large water tank. This also allows for easy integration of multiple heat sources such as conventional / fossil fuel burners, and solar arrays.
There are two main type of storage tanks: Pressurized (closed) and non-pressurized (open).
Pressurized pros / Open cons:
* Most hydronic heating systems are pressurized, heat exchangers are required to interface between pressurized and non pressurized systems.
* Heat exchangers required for interfacing systems are expensive, pressurized does not need them if all parts of the system are pressurized. In addition, heat exchangers usually require additional circulation pumps.
* Open systems have the potential for evaporation and increased corrosion issues.
Pressurized cons / Open pros:
* Pressurized tanks tend to be harder to build as pure home-brew units, and be more expensive, most common pressurized tanks tend to be used LP gas tanks. Open tanks are more easily home built, although both require non-trivial amounts of engineering to be safe.
* Pressurized requires expensive expansion tank
* With open system one can easily add an extra heat exchanger in the future for driveway snow melt, hot tub, etc.
* Pressurized can go "bang" if not properly installed.
There is not a clear advantage to either design, which might be more suitable depends on the details of each individual system and the preferences of the builder.
With an open (non-pressurized) storage tank one needs to use a heat exchanger to transfer the heat from the boiler water to the tank for storage and then from the tank to the heating zone water to heat the house. Typically the same heat exchanger is used for both functions. Some designs that have been used by do-it-yourselfers follow (Please add any others):
* Copper coils - this is the type of heat exchanger used in the heat storage systems sold by STSS
* PEX coils - while the heat conductivity of PEX is not as good as copper PEX is MUCH cheaper. Using 3 times the length of PEX instead of copper is probably cheaper.
* Flat Plate heat exchanger
* Sidearm heat exchanger (more commonly used for heating DHW)
Copper Coil Heat Exchanger
A copper heat exchanger can be built using hard or soft copper at considerable savings over purchased copper coil heat exchangers.