Hydronic Heating Systems

Heating systems that use water to carry heat—known as hydronic systems—have several advantages over forced air systems, and while they’ve been in use for many years, the green building movement in particular is rediscovering their benefits.

Hydronic systems distribute heat evenly throughout a room, so they offer greater thermal comfort. Second, because water conducts heat 20 times faster than air, they are more efficient. Third, because they don’t need bulky ductwork, just small pipes, they are more versatile—hot water can be piped through baseboard radiators, beneath floors, and around radiant walls and ceilings.

Finally, with the right components, a hydronic system is quiet, warming you without you even knowing it’s there. But for this, good system design is critical. It’s wise to ask your dealer, distributor, or installer if he or she has continuing education credits in hydronic design and whether he or she is familiar with the enhanced heating module, which combines essential components into one convenient package for home heating.

Xylem RCW has many decades experience engineering hydronic system solutions. The following is a list of some must-have components:

Relief Valve—The relief valve’s job is to protect the boiler should its pressure rise to the maximum working pressure, usually 30 psi for residential boilers. A modern relief valve uses a spring-loaded diaphragm to hold the valve closed. It’s important that the valve connect directly to the boiler and that it never drain to the outside, in case freezing blocks the pipe and inhibits the valve’s operation.

Pressure-Reducing or “Feed” Valve—The job of the “PRV” is to provide the system with adequate water pressure. It does this by reducing the pressure of mains water coming into a building (around 30 to 60 psi) to that needed to maintain good pressure throughout the building, which for a typical two-story house is 12 psi. In the old days, gravity did this job thanks to an open tank in the attic (sometimes topped up with a bucket!) Ask for corrosion-resistant brass feed valves—the best to use where cold and hot water meet and where minerals precipitate.

Circulator— A circulator is a small centrifugal pump that moves hot water around a hydronic system. Circulators do not provide system pressure—that’s the feed valve’s job—so the only forces they must overcome are friction caused by pipes and fittings, inertia, and turbulence. They work by creating centrifugal force with an impeller that pushes water out through a small exit channel with enough force to overcome friction, or “pump head.” A circulator should be placed away from the compression tank to make air removal easier.

Low Water Cut-Off—Low water protection: it’s not just for steam boilers! Hot water boilers also can be damaged badly if water drops too low, and with more systems using buried or hidden pipes, it’s a good idea to protect against an undiscovered system leak. Don’t rely on a PRV or relief valve for this job—a PRV should be shut off once system pressure is adequate and a relief valve can dump water very quickly. In many regions, building codes govern the use of low water cut-offs. Ask your dealer, distributor, or installer about these if you are unsure.

Air Separator—An air separator is necessary for quiet, efficient hydronic operation. Cold water carries dissolved air and heating it creates bubbles, which can cause noise and even stop flow. While vents take care of larger bubbles, only a separator can rid a system of air. Xylem RCW engineers two kinds of separator. Enhanced air separators use tiny metal brushes to trap bubbles, while inline air separators slow water down through a wide chamber, releasing bubbles. Air comes out of solution where temperature is highest and pressure is lowest, so place an air separator after the boiler and before the circulator.

Expansion and Compression Tanks—Water expands when heated, and if a closed system is completely filled, a small tank must be fitted to accept the 5% or so extra volume that can be created after heating. Expansion tanks are filled with air that is compressed by the extra water as the tank fills. Two kinds are common in domestic heating systems. Closed steel compression tanks mix about two parts water to one part air, while diaphragm tanks separate air from water with a rubber membrane.

Flow Control Valve— Without a flow control valve, if a circulator is off, it’s possible for water to move by gravity alone, causing uneven or unwanted heating. A flow control valve keeps water from leaving the boiler thanks to a weight that rises or drops when a circulator is on or off. When the weight is down, water cannot flow. By the way, if a circulator is not working, turning off the valve can provide some flow and heat in an emergency.

Electric Zone Valve—An alternative to zoning with a flow control valve is to use electric zone valves in combination with zone circulators. In this case, each valve acts is a “gatekeeper” to the zone it serves. They work using a highly engineered electric switch that is governed by a zone’s thermostat and linked to the circulator. Ask for an electric zone valve with a slow-closing action, to reduce the chance of water hammer.

Balancing Valve—Another type of flow control valve is the balancing valve. “Balance” refers to water that flows evenly through all pipes. Without a balancing valve, water can follow the path of least resistance—the shortest branch—resulting in some zones receiving no heat. Xylem RCW manufactures many kinds of balancing valves—ask your dealer, distributor, or installer which one is right for your needs.