Radiators are an efficient, cost-saving way of heating a house. Even after the furnace has finished its cycle and heated a room to its thermostatically controlled temperature, the radiator continues to emanate heat. Even delivery pipes are a source of heat--spreading out the warmth over a narrow but long area.
The benefit of any type of radiator--hot water or steam--is its efficiency because it does not deliver dry, heated air to the rooms, as does a forced-air HVAC system.
There are two basic types of water-based radiators: the hot water radiator and the steam radiator.
Hot Water Radiators
Hot water radiators have water that is heated from a central boiler continually running through them. This water either runs in a loop (one pipe system) or up and back down again (two pipe system).
Hot-water radiators can look either like the conventional, "stand-up" radiators or like the low, out-of-sight baseboard heaters.
Unlike steam radiators, hot water radiators do not have the added benefit of increasing the humidity in the rooms--often a necessity in dry winter months.
One or Two Pipe Systems
Hot water radiators can be either the one-pipe or two-pipe system.
- One Pipe: In the one-pipe system, hot water leaves the furnace and moves in a continuous loop, returning back to the furnace as cooler water. This cooler water is re-heated and sent out on its journey again.
- Two Pipe: The two-pipe system moves hot water to the radiators in one pipe and returns it to the boiler in another pipe.
If you have a pre-1950 house and it has original radiators, they are most likely steam-based radiators. As the name makes clear, the heating source that runs through the supply pipes and into the radiators is steam.
This steam is created in a furnace located in an area separate from the living spaces--usually a basement.
One Pipe System
In the one pipe radiator system, a single pipe runs from the furnace to each radiator. Steam is driven through that pipe, fills the radiators, and then condenses and runs back down that same pipe in the form of water. The water is recycled and used again in the next cycle.
Two Pipe System
In the two pipe radiator system, one pipe delivers steam to the radiators and a second pipe separately returns condensed water back to the furnace.
Steam radiators can be messy. Many older homes with steam radiators have warped floors around the radiator area, an almost unavoidable byproduct of having a pressurized steam unit inside your house. Furnaces generating steam do so under pressure, and while it is rare, it is possible for these furnaces to explode.
The Noise Factor
A house with steam radiators is never a quiet house. While you can take measures to mitigate the sounds, you will always experience some degree of clanking (of pipes) and hissing (of steam escaping from valves). This is usually considered the price--or benefit--of living in an old home, whatever your view on the matter.
Which One is Best?
One reason both hot water and steam radiators have slowly been phased out is because of the potential for leakage. No matter how much effort you put into maintaining a centrally based radiator system, leakage will occur at some point.
Hot water heaters are generally considered to be more efficient than steam radiators. This is because hot water heaters move the water through the system by means of a pump. This pump allows the water to move at a predictable rate.
However, both are quite efficient in that they can be zoned: radiators can be turned on or off in individual rooms.
It's also important to keep in mind that, while delivery pipes can act as "long, narrow" heaters within rooms, they also waste energy when they run through other areas of the house, such as between the floor and ceiling.