NATE Hydronics Gas Service key terms

The calculated rate of heat loss from a building at design outdoor temperature, used to size the boiler and distribution. It combines transmission losses through the envelope with infiltration and ventilation losses.

The coldest outdoor temperature a locality is expected to see at a stated frequency, typically the 99 percent winter design value. It sets the temperature difference used in heat loss calculations.

Extra capacity added to a boiler or terminal unit sizing calculation to recover from a deep setback or warm a cold building. It is a percentage above design heat load, not part of the load itself.

The average temperature of surrounding surfaces as felt by occupants. Cold walls or windows lower it and can cause discomfort even when air temperature is at setpoint.

A boiler in which water flows inside tubes surrounded by combustion gases. It responds quickly to load changes and holds little water volume, making it common in commercial hydronic systems.

A boiler in which hot combustion gases pass through tubes submerged in a surrounding water shell. It holds a large water volume and is typical of residential and light commercial hot water heating.

A temperature-actuated control that starts and stops the burner to hold boiler water within a set range. High-limit and low-limit functions are often built into the same device.

The aquastat or thermostat function that cycles the burner during normal demand. It is distinct from the safety high limit, which is set higher and acts only on failure.

A safety device that shuts off the burner if boiler water temperature exceeds a set maximum. It is set above the operating control and must not be used for normal cycling.

A safety control that stops burner operation when boiler water level falls below a safe point. It protects against overheating and dry firing in steam and some hot water boilers.

A diaphragm or solenoid-operated valve that opens to admit gas to the burner only when the ignition and safety circuit permits. It usually includes redundant operators and a pressure regulator.

A valve design with two operators in series so that a single failure cannot leave the valve stuck open. It is required on most automatic gas-fired appliances as a safety feature.

The gas pressure measured at the burner manifold, downstream of the appliance regulator. It is set to the manufacturer's value and verified with a manometer during startup and service.

The gas pressure delivered to the appliance inlet before any appliance regulator. It must fall within the range stamped on the rating plate for the appliance to operate safely.

The manifold pressure reading immediately after the gas valve closes and before the regulator opens again. It is slightly higher than flowing pressure and confirms the regulator seats properly.

Air supplied to the burner to mix with gas and support complete combustion. It may come from the room, a dedicated duct, or direct-vent piping depending on appliance category.

The percentage of air supplied above the theoretical amount needed for complete combustion. It ensures full burning but too much lowers efficiency by carrying heat up the flue.

The exact amount of air theoretically required to burn a given quantity of gas completely. Real appliances always supply more than this, measured as excess air.

A colorless, odorless gas produced by incomplete combustion. It is toxic and indicates a need to check air supply, gas pressure, and flue condition immediately.

The process of measuring flue gas oxygen, carbon monoxide, and temperature to calculate efficiency and confirm safe, complete combustion. Results guide air and gas adjustments at startup.

The temperature of flue gases leaving the boiler. Comparing it with supply water temperature gives the net stack temperature used to estimate heat lost up the chimney.

The negative pressure in a vent or chimney that draws combustion products out of the appliance. It may be natural, induced by a fan, or forced by a power burner.

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 protects the appliance.

An appliance that takes combustion air from outdoors through one pipe and discharges flue gases outdoors through another. It is sealed from room air and needs no indoor combustion air opening.

A boiler designed to recover latent heat by cooling flue gases below their dew point. It requires a corrosion-resistant heat exchanger and a drain for condensate.

A water seal in the condensate drain line of a condensing boiler that blocks flue gases from escaping into the drain. It must remain filled and be protected from freezing.

A pressure test of new or repaired gas piping using air or inert gas, held for a specified time and observed on a gauge. The test pressure and duration come from the applicable fuel gas code.

A dirt pocket or nipple installed at the bottom of the gas line near the appliance to catch debris before it reaches the gas valve. It must be installed so debris can be removed.

Another name for the sediment trap, a short vertical section of pipe below the gas connection that collects scale and dirt. It is required by code at most gas appliance connections.

A pump that moves hot water through the boiler and distribution piping. Its flow and head must match the system's pressure drop to deliver design heat to the terminals.

A graph of the flow and head a circulator can produce across its operating range. The point where it crosses the system curve is the actual operating point.

A plot of the pressure drop through the piping and terminals at various flow rates. Intersecting it with the pump curve shows the flow the circulator will actually deliver.

A piping arrangement in which the boiler loop and distribution loops are connected by closely spaced tees. It hydraulically separates the loops so each circulator sees only its own circuit.

A control that lowers the boiler supply water temperature as outdoor temperature rises. It matches heat output to load and improves efficiency by reducing cycling and standby losses.

A motorized valve that opens or closes to direct hot water to one heating zone. It is controlled by that zone's thermostat and usually has an end switch to start the boiler.

Removal of dissolved and entrained air from hydronic water using air scoops, separators, or vents. Air causes noise, corrosion, and poor circulation if left in the system.

A tank that absorbs the volume increase of water as it is heated, limiting pressure rise in a closed hydronic system. It may be plain steel or have a diaphragm separating air from water.

A flame-sensing method in which the flame conducts current between an electrode and ground, producing a small DC signal. The control interprets that signal as proof of flame.

The probe or rod that detects the presence of flame and signals the ignition control. Contamination or poor grounding causes nuisance lockouts and must be cleaned or replaced.

The electronic module that sequences the burner, proves flame, and locks out the gas valve on failure. It manages trial for ignition timing and safety shutdown.

A safety state in which the ignition control shuts the gas valve and stops trying to light after a failed trial. It usually requires manual reset or a power cycle to clear.

A rating equal to the evaporation of 34.5 pounds of water per hour at 212 degrees Fahrenheit. It is used for steam and larger hot water boilers and equals about 33,475 Btu per hour.

The upper section of a boiler where steam separates from water before leaving the boiler. Its water level is critical and is monitored by low-water and level controls.

A water and antifreeze mixture used in hydronic systems exposed to freezing. It changes heat transfer, viscosity, and pump head, so flow and boiler settings must be adjusted.