- Air separator
- A device, typically installed on the supply side near the heat source where fluid temperature is highest, that creates a zone of reduced pressure causing dissolved gases in the fluid to form bubbles, which are then collected and vented automatically. Prevents air from causing noise, cold spots, or pump air-lock.
- Balance point
- The outdoor temperature below which a heat pump alone can no longer meet a building's full heating load, requiring supplemental or backup heat. Relevant to sizing air-to-water heat pumps and hybrid systems in cold climates.
- Boiler
- A heat source that burns fuel (natural gas, propane, or oil) to heat water or another fluid for a hydronic system. Modern condensing boilers extract additional heat from combustion exhaust for higher efficiency.
- Circulator
- The hydronic industry's term for the pump that moves fluid through a heating loop. Circulators are typically low-power and, in variable-speed (ECM) form, adjust flow automatically to match demand.
- Closely-spaced tees
- Two tee fittings placed close together on a main pipe run, used to create hydraulic separation so a branch (secondary) circuit doesn't disrupt flow in the main (primary) loop.
- Coefficient of Performance (COP)
- A measure of heat pump efficiency: the ratio of heat delivered to electrical energy consumed. A COP of 3 means the heat pump delivers three units of heat for every one unit of electricity used.
- Condensing boiler
- A high-efficiency boiler that extracts extra heat from flue gas by condensing water vapor out of the exhaust, most effective when operated at the lower return-water temperatures common in radiant systems.
- Delta-T
- The temperature difference between a loop's supply and return fluid, often written as ΔT. Used to estimate heat output and flow rate, and to help diagnose whether a system is delivering the expected amount of heat.
- Expansion tank
- A small tank containing a compressible air cushion (usually behind a rubber diaphragm) that absorbs the volume increase of heated fluid, keeping system pressure stable as temperature changes.
- Geothermal heat pump / ground-source heat pump
- A heat pump that exchanges heat with the relatively stable temperature of the earth via buried loops, rather than with outdoor air, typically yielding higher and more consistent efficiency than air-source systems.
- Glycol
- A non-toxic antifreeze (usually propylene glycol) blended with water in hydronic systems exposed to freezing temperatures, such as snowmelt loops or unheated spaces.
- Heat-loss calculation
- An engineering calculation of how much heat a building loses to the outdoors under design conditions, used to correctly size a heat source, distribution system, and heat emitters rather than guessing or relying on rules of thumb.
- Home-run piping
- A piping layout in which each zone or loop has its own dedicated supply and return line running back to a central manifold, allowing independent balancing and control of each loop.
- Hydronic
- Describing a heating (or cooling) system that uses water or a water-based fluid, rather than air, as the medium for transporting heat through a building.
- Manifold
- A distribution hub, typically in a mechanical room, where a supply header splits fluid flow into individual loops (often through flow meters) and a return header collects the cooled fluid.
- Mixing valve
- A valve, commonly 3-way or 4-way, that blends hot supply fluid with cooler return fluid to deliver water at a controlled, often lower, temperature — essential for protecting radiant floors and floor coverings from excess heat.
- Outdoor reset control
- A control strategy that automatically adjusts a hydronic system's supply water temperature based on outdoor temperature, delivering just enough heat rather than a fixed temperature regardless of conditions, which improves both comfort and efficiency.
- PEX (cross-linked polyethylene)
- Flexible plastic tubing widely used for hydronic distribution piping and radiant floor loops, valued for its freeze tolerance, corrosion resistance, and ease of installation compared to rigid copper piping.
- Primary/secondary piping
- A piping strategy in which a primary loop circulates continuously through or near the heat source, while secondary loops branch off through closely-spaced tees with their own circulators, providing hydraulic separation between circuits.
- Radiant heat
- Heat transferred directly via infrared radiation from a warm surface (such as a heated floor) to cooler surfaces and objects in a room, distinct from convective heat transfer through moving air.
- Zone valve
- A valve that opens or closes to control fluid flow to a specific zone or loop, allowing different areas of a building to be heated independently.
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