A hot water heating system, also called a hydronic heating system, warms your home by circulating heated water through a network of pipes to radiators, baseboard units, or tubing beneath your floors. Instead of blowing warm air through ducts like a furnace, it uses water as the heat-carrying medium. This makes it quieter, more even in its heat distribution, and free of the drafts and dust circulation that come with forced-air systems.
How the System Works
The basic cycle is straightforward. A boiler or other heat source warms water to a set temperature, then a circulator pump pushes that heated water through pipes to heat-emitting devices throughout the house. As the water releases its heat into each room, it cools and flows back to the boiler to be reheated. This loop runs continuously whenever your thermostat calls for heat.
Nearly all residential systems today use a closed-loop design, meaning the same water recirculates over and over rather than being drawn from an outside source and discharged. Closed-loop systems are more reliable, require less maintenance, carry a lower risk of bacterial contamination, and use less expensive fittings than open-loop alternatives. The water inside the loop is typically treated with an antifreeze solution in colder climates to prevent pipe damage.
The Five Core Components
Every hydronic system relies on the same basic set of parts working together.
The heat source is usually a boiler, located in a basement, utility closet, or mechanical room. Boilers come in several types: cast iron, fire tube, water tube, and power burner models. Modern condensing boilers are the most efficient option available today. The vast majority of ENERGY STAR certified boilers now achieve an annual fuel utilization efficiency (AFUE) of 95% or higher, meaning only about 5% of the energy in the fuel is lost. Older non-condensing boilers typically run in the 80% to 85% range, so an upgrade can meaningfully cut your heating bill.
The piping network connects the boiler to every heat-emitting device in the house. Copper and PEX tubing are the most common materials. The layout can be a simple series loop, where water passes through one unit after another, or a parallel arrangement where each unit gets its own supply line for more even heating.
The circulator pump is what moves hot water through the pipes. Without it, water would just sit in the boiler getting hot. The pump overcomes friction losses in the piping so that heat reaches every room at a consistent rate.
The expansion system handles a critical physics problem: water expands as it heats up, and that extra volume needs somewhere to go. This system consists of three parts: an expansion tank that absorbs the increased volume, an air eliminator that removes trapped air bubbles, and a pressure-reducing valve with a backflow preventer that maintains the right water pressure. In a typical two-story house, the system fills at around 12 psi. Taller buildings need higher fill pressures, roughly 0.43 psi for every additional foot of height, to keep positive pressure at the top of the system.
Terminal units are the devices that actually release heat into your rooms. These come in four main types, each with distinct characteristics.
Types of Terminal Units
Traditional radiators are the large cast iron or steel units you see in older homes. They work primarily through a combination of radiant heat (warming objects and people directly) and convection (warming the air around the unit). They hold a large volume of water, which means they stay warm for a while even after the system cycles off.
Baseboard units are slim, low-profile heaters mounted along the base of walls. They use metal fins to transfer heat from the water pipe to the surrounding air, which rises naturally through the unit. They’re less visually intrusive than radiators but are generally less efficient than radiant alternatives.
Radiant floor systems embed PEX tubing directly into or beneath the floor surface. Water circulates through these tubes, warming the floor itself, which then radiates heat upward into the room. According to the U.S. Department of Energy, radiant floor heating is more efficient than baseboard heating and typically more efficient than forced-air systems because there are no duct losses. The heat is also distributed more evenly, with the warmest air at floor level where people actually feel it, rather than collecting at the ceiling.
Fan coil units push air over a hot water coil using a small fan, similar to how a forced-air system feels but using hot water as the heat source. These are common in apartments and commercial buildings where space is limited.
Heat Sources Beyond Gas Boilers
While natural gas and oil boilers remain the most common heat sources for hydronic systems, air-to-water heat pumps are an increasingly popular alternative. These work by extracting heat from outdoor air and transferring it to the water in your hydronic loop. They can also reverse the process in summer, supplying chilled water for cooling.
The efficiency gains are significant. Air-to-water heat pumps can use up to 47% less energy than a high-efficiency condensing gas boiler and up to 70% less electricity than electric baseboard heating. Their efficiency is measured by a coefficient of performance (COP), which represents how many units of heat you get for each unit of electricity consumed. Top-performing models achieve a COP above 5.0, meaning they deliver five times more heating energy than the electricity they use. Modern cold-climate models can operate in temperatures as low as negative 22°F, making them viable in regions where heat pumps were once considered impractical.
Solar thermal panels and wood-fired boilers are other options, though less common. One advantage of hydronic systems is that you can swap the heat source without replacing the entire distribution network, so a home currently running on an oil boiler could switch to a heat pump while keeping the same piping and terminal units.
Safety Features
Hot water heating systems operate under pressure, which makes safety devices essential. The most important is the temperature and pressure (T&P) relief valve. If a control failure causes the boiler to overheat or over-pressurize, this valve opens automatically to release water and prevent a catastrophic failure of the vessel. Residential boilers typically use relief valves rated at 30 psi.
The T&P valve is often described as the last line of defense. In the event of a runaway firing condition where the boiler keeps heating without shutting off, it is the only device that will prevent the tank from failing. Despite its importance, it is one of the most neglected safety components in residential heating. You can test the valve’s mechanical movement by lifting its test lever, but a full evaluation requires confirming that the valve’s pressure rating does not exceed the maximum working pressure of the boiler and that its capacity matches the system’s heat input.
Maintenance and Troubleshooting
Hot water heating systems are relatively low-maintenance compared to forced-air systems, since there are no filters to change or ducts to clean. But they do need periodic attention.
The most common issue homeowners encounter is trapped air in the system. Air pockets prevent hot water from circulating fully through a radiator or baseboard unit, creating cold spots. The telltale signs include a cold patch at the top of a radiator while the bottom stays warm, an entire radiator that won’t heat up, gurgling or rattling noises from the unit, or unexplained damp patches and mold on nearby walls.
Fixing this requires “bleeding” the affected radiator. Turn off your heating first and let the system cool so you don’t risk a hot water spray. Locate the bleed valve, which sits at the top corner of the radiator and looks like a round hole with a square center. Place a cloth or small container below it to catch drips. Insert a radiator key into the valve and turn it counterclockwise. You’ll hear hissing as air escapes. Once the hissing stops and a steady stream of water appears, close the valve by turning the key clockwise. After bleeding any radiators, check your boiler’s pressure gauge, because releasing air can lower the system pressure and you may need to add water through the fill valve to bring it back to the correct level.
Beyond bleeding, annual professional servicing of the boiler keeps the system running safely and efficiently. This typically includes checking the burner, inspecting the T&P relief valve, testing the expansion tank’s air charge, and verifying that the circulator pump is operating correctly. Systems filled with an antifreeze solution should have the mixture tested periodically to ensure it still provides adequate freeze protection.
Pros and Cons at a Glance
Hydronic heating has real advantages over forced-air systems. It operates almost silently, distributes heat more evenly, doesn’t blow allergens and dust around the house, and allows room-by-room temperature control through zone valves. Radiant floor versions in particular provide a comfort level that forced air struggles to match.
The drawbacks are worth weighing, though. Installation costs are higher than forced-air systems, especially for radiant floor setups that require tubing to be embedded in the floor structure. The system heats only; it does not provide air conditioning unless paired with a separate cooling system or an air-to-water heat pump that handles both. Leaks, while uncommon in a well-installed system, can cause water damage that’s more costly to repair than a duct leak. And if you’re in a home that already has ductwork, adding a hydronic system means building an entirely separate distribution network rather than using what’s already there.