Different Types of Radiator/ Boiler Systems

Next to forced-air heat, a boiler-and-radiator system is the most common type of heating system in cold-winter climates. All boiler systems use hot water to heat the home. There are two main types of boilers—steam and hot water (also known as hydronic)—and many more types of radiators or heating appliances, the units that transmit the heat into the rooms. In general, boiler heating systems are clean, effective and reliable, and the newest boilers can offer superb energy efficiency.

Central heating radiator and woman in striped socks
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Radiators offer gentle, even heating.

How to Tell If You Have Steam or Hot Water

If you live in an older home with radiators, you may have a steam or a hot water heating system, depending on the age of the home. There are a few easy ways to identify the system type. For starters, steam heat in U.S. homes was largely phased out in the 1930s, so if your home has radiators and was built after 1940 or so, it almost certainly has hot water heat, not steam. Another clue is to look at the radiators. If each radiator has only one pipe connected to it, you have a one-pipe steam system, the most common type of steam heat.

If your radiators have two pipes, you might have two-pipe steam or a hot-water system. To determine which one, check the boiler. Steam boilers typically have a _sight glass_—a clear glass tube or vial attached to the boiler in a visible location. The level of water in the sight glass tells you how much water is in the system. A hot-water boiler does not have a sight glass but it does have an _expansion tank—_a bulbous metal vessel connected to the boiler with piping. This is a safety device that absorbs pressure in the system created by the water heating cycle. Most hot-water boilers dating from the 1950s and later also have one or more electric water circulation pumps mounted on or near the boiler.

Boiler expansion tank.
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Boiler expansion tank is found on hot water boiler systems.

How Radiators Work

All radiator systems provide heat through two processes: heat exchange and natural convection. Heat exchange is the transfer of heat from one heat source or material to another. In a radiator system, the heat source—whether it's gas, oil or electricity—heats up a heat exchanger in the boiler. The heat exchanger, in turn, heats up the water in the boiler. From there, the water (or steam) travels to the radiators and transfers its heat to the radiator material, which is usually metal, a good conductor of heat.

The radiator then transfers its heat to the air, and that's where convection comes into play. Thermal convection is based on the natural law that hot air rises and cold air drops. As the radiator warms the air around it, that warmed air rises and flows into the room. At the same time, the cold room air drops to the floor and is drawn toward the bottom of the radiator, where it becomes heated. This hot-and-cold rotation naturally heats the room without the need for circulation fans or other mechanical means.

Steam Radiators

A lot of old apartment buildings still have steam radiators partly because steam systems heat multistory buildings effectively and efficiently—so effectively that apartments in upper floors often have too much heat and occupants have to open the windows for comfort.

One-pipe steam radiator
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One-pipe steam radiator.

In a classic one-pipe steam system, the boiler in the basement heats water until it becomes steam. The steam rises through the building's pipes and into each radiator, where it displaces the air inside the radiator, forcing it out through an air valve. The steam transfers its heat to the radiator and as is cools, it turns back into water (called condensate). The water trickles down the interior walls of the pipes via gravity and makes its way back to the boiler, where the process starts all over again.

Hot Water Radiators

Hot water radiators come in many shapes, sizes and configurations, but most can be grouped into one of two main types. Standard radiators are the familiar wall- or floor-mounted vessels that have evolved over the years from grill-like iron hulks to sleek metal panels and even elegant towel bars and artistic wall hangings. Baseboard, or convector, radiators are straight sections of copper pipe outfitted with numerous thin metal plates, called fins. The fins work just like the metal vessels of standard radiators but are only a few inches square. Convectors may be mounted to a wall near the floor, or they may be set into the floor and are covered by a metal grate.

Hot-water radiator.
credit: Heating Systems
Standard modern hot-water radiator. are flat, wall-hugging units.

All standard hot water radiators have an inflow pipe and an outflow pipe, and many have some kind of temperature control for adjusting the flow of hot water through the radiator and thus the heat it puts out. Convectors are simply continuous pipes with finned sections and typically do not have individual temperature controls; instead, they are controlled by a thermostat, usually as part of a multi-zone heating system.

Hot-water convector.
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Hot-water convector.

In-Floor Radiant Heat

In-floor radiant heat is a variation of radiator heat that uses a hot-water boiler and flexible tubing to distribute the heated water. The tubing is snaked back and forth over a floor base and then encased in concrete, or it is secured to the underside of wood subflooring. In this system, the floor itself acts as a radiator, creating gentle, even heat over the entire floor area that rises up through the room. It also lightly heats furniture and other objects, making the overall heating effect more uniform than with most other types of heating. Radiant tubing can also be installed in special wall panels for heating bathrooms and other small spaces.

Types of Hot Water Boilers

Like furnaces, hot water boilers come in standard types and high-efficiency types. By federal law, boilers now must be at least 80 percent efficient, meaning 80 percent of the energy they consume is used to heat water and 20 percent is wasted. Standard boilers typically range from 80 to 88 percent efficient. High-efficiency boilers are more than 88 percent efficient and typically offer 90 percent or higher efficiency. These numbers are for gas boilers; oil boilers are slightly less efficient overall.

The main difference between conventional boilers and high-efficiency boilers is how they deal with the exhausted gases from the main burner, which contain a lot of heat. With a conventional boiler, all of the exhaust from the boiler's burner goes up a metal flue (chimney) and out through the roof, and all of its heat is wasted. High-efficiency boilers recirculate the exhaust and use its heat to pre-heat cold water in the system as it returns to the boiler. In the process of recapturing this heat, the exhaust gases cool down enough to be vented through a plastic (not metal) flue.

High-efficiency condensing boiler.
credit: ACHR News
High-efficiency condensing boiler.

One byproduct of this recirculation process is a mildly acidic water that must be drained from the system, also via a plastic pipe. As the exhaust gases cool, water vapor (a natural byproduct of combustion) condenses and turns to liquid water. This is why high-efficiency boilers are often called condensing boilers.

Another efficiency feature of many high-efficiency boilers is sealed combustion. In a conventional boiler, air for combustion (running the gas burner) is drawn from the room air around the boiler, which is technically heated air. High-efficiency boilers with sealed combustion draw combustion air from the outdoors through an intake pipe. Heated indoor comes at an energy cost; outdoor air does not. Therefore, sealed combustion helps boost a boiler's overall efficiency.