Hydronic heat is basically heated water pumped through pipes into baseboard radiators, free standing radiators,
or tubing in cement floors. The same water is reheated and circulated through the pipes.
The heat then radiates up and warms the room. It is a very constant, draft free heat. Hot water can also be
pumped through a heat exchanger, transfering the heat to a forced air system. The water is usually heated in a boiler
that is fueled by oil, propane, natural gas, wood, wood pellets, sunflower hulls, coal, or electricity. Some boilers even
burn hay bales.
Corn fired boilers have not worked out as well as the corn fired forced air furnaces. The residue from the corn plugs
up the boiler as the heat transfers into the water.
Tankless boilers still have a boiler tank of some sort, even though it may be very small. Boiler tanks vary
in volume from less than a gallon as in some electric and gas boilers, to over 100 gallons as in some wood fired boilers.
All "closed" systems need an expansion tank to relieve the pressure of the heated water. Most but not all boilers work
under a pressure of about 12 lbs.
A pressurized closed system is needed when using antifreeze/water mix in the boiler & pipes.