NATE Hydronics Oil key terms

The supply water temperature a hydronic system is engineered to deliver at design outdoor conditions, commonly 180°F for fin-tube baseboard. It sets pipe sizing, emitter selection, and boiler output requirements.

A room-by-room estimate of building heat loss through envelope surfaces and infiltration at design conditions. It determines required emitter output, water temperature, and boiler capacity before any piping is laid out.

The delivered heating capacity of an emitter or boiler under stated conditions. Emitter ratings are published per foot at a specific average water temperature and flow, so output must be corrected when actual conditions differ.

The mean of supply and return water temperatures at an emitter. Emitter output varies with this average, so a 180°F supply with 160°F return yields a 170°F average and lower output than nominal ratings.

The temperature difference between supply and return water. It indicates how much heat the system extracted per gallon and is used to calculate flow rate from required BTU delivery.

The volume of water circulated through the system per unit time, usually gallons per minute. Required flow equals BTU per hour divided by 500 times the delta T.

A piping arrangement where water passes through each emitter in sequence before returning to the boiler. Simple and inexpensive, but downstream rooms run cooler and individual zone control is impossible.

A piping layout with a single main loop where each emitter is connected by a pair of closely spaced tees or a diverter fitting. Water partially bypasses each emitter, allowing individual valve control.

A piping layout with separate supply and return mains, so each emitter receives full supply temperature water and returns independently. It supports direct-return or reverse-return balancing arrangements.

A two-pipe arrangement where the supply runs to the farthest emitter first and the return starts at the nearest, equalizing pipe length and pressure drop so branches self-balance without balancing valves.

A layout where a primary loop and one or more secondary loops connect through closely spaced tees. The tees hydraulically decouple the loops so circulators do not interfere with each other.

The decoupling of flow and pressure between two circuits so each circulator operates independently. Achieved with closely spaced tees, a hydraulic separator, or a low-loss header.

A pump that moves water through the hydronic loop, overcoming pipe, fitting, valve, and emitter pressure drop. Selection requires matching pump head and flow to the system curve.

A graph of a circulator's head pressure versus flow rate. The operating point is where the pump curve intersects the system resistance curve, determining actual delivered flow.

The loss of pressure as water travels through piping, fittings, valves, and emitters due to friction. It increases roughly with the square of flow and must be totaled to size the circulator.

Removal of entrained air from hydronic water using air scoops, separators, or automatic vents at high points. Air causes noise, poor circulation, corrosion, and pump cavitation.

A fitting that slows water velocity so entrained air bubbles rise and vent from the system. Installed at the boiler supply before the circulator, often combined with an expansion tank connection.

A vessel that absorbs the volume increase of water as it heats, limiting pressure rise in a closed system. It contains an air charge separated from system water by a bladder or diaphragm.

A safety device that opens at a set pressure, typically 30 psi for residential boilers, to prevent overpressurization. It must never be plugged, capped, or piped to a hidden discharge point.

A motorized valve that opens or closes water flow to a heating zone in response to a thermostat. It is typically wired as an end switch that starts the circulator and burner.

A temperature-actuated control that starts and stops the burner or circulator to hold boiler water within a set range. High-limit and low-limit functions are often combined in one control.

A safety aquastat that shuts down the burner if boiler water exceeds a set maximum, commonly 200°F. It prevents overheating and steam production in a hot water system.

A safety device that stops the burner when boiler water level falls below a safe point, preventing dry firing and overheating. It may be float, probe, or conductance type and requires periodic testing.

A control strategy that lowers supply water temperature as outdoor temperature rises, matching emitter output to load. It improves efficiency and comfort by reducing cycling and overshoot.

A device that blends hot boiler water with cooler return water to deliver a lower supply temperature, protecting radiant floors or matching outdoor reset curves.

A device that atomizes fuel oil, mixes it with air, ignites the mixture, and directs the flame into the combustion chamber. It includes the pump, nozzle, electrodes, and blower assembly.

A precision orifice that atomizes oil into a fine spray at a rated flow in gallons per hour and a specified spray angle and pattern. It must match the burner's rated input.

The oil pressure delivered to the nozzle by the fuel unit, commonly 100 psi for residential burners. Higher pressure reduces droplet size and increases flow above the nozzle's rated capacity.

The oil pump mounted on the burner that draws oil from the tank, pressurizes it to the nozzle, and returns excess oil to the tank. It may be a single-stage or two-stage design.

A pair of metal rods that create a high-voltage spark to ignite the oil-air mixture. Gap spacing and position relative to the nozzle are critical for reliable lightoff.

A step-up transformer supplying high voltage to the electrodes, typically 10,000 volts or more. It operates only during the trial-for-ignition period on burners with interrupted ignition.

The burner's safety control that supervises startup, opens the oil valve, monitors the flame, and locks out the burner if flame is not proven within the trial-for-ignition time.

A flame-sensing device that changes electrical resistance when exposed to light from the flame. The primary control uses this signal to prove combustion and keep the burner running.

A safety shutdown in which the primary control stops the burner and locks out when no flame is proven during trial for ignition. Manual reset is usually required before restart.

The percentage of fuel heat released that is usefully transferred, indicated by flue gas temperature and carbon dioxide or oxygen readings. High stack temperature and excess air lower efficiency.

A measurement of unburned carbon in flue gas using a pump and filter paper, rated from zero to nine. A trace to slight smoke reading confirms proper air-fuel adjustment.

The negative pressure that draws combustion air through the burner and carries flue gases up the chimney. It is measured in inches of water column and must be adequate but not excessive.

A hinged flap installed in the flue connector that opens to admit room air and stabilize draft. It prevents excessive draft from pulling heat up the chimney and disturbing the flame.

The boiler section that transfers heat from combustion gases to system water. It must be cleaned and inspected for soot, scale, and cracks that reduce efficiency or allow flue gas leakage.

A closed vessel that heats water for space heating or domestic hot water without necessarily boiling it. Residential hot water boilers operate below 250°F and 160 psi.

The air supplied to the burner for complete combustion of fuel oil. Insufficient combustion air causes smoke, soot, and carbon monoxide; tight modern homes may require outside air ducting.

The timed period during startup when the primary control allows oil and spark before proving flame. If no flame is sensed within this window, the control locks out for safety.

A valve with a calibrated position indicator used to adjust flow among branches so each zone receives its design flow. It corrects uneven heating in direct-return and multi-zone systems.