NATE Hydronics Gas Service flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Sensible heat equation

Q (Btu/h) = 500 × gpm × ΔT for water at standard density. Use it to relate flow, temperature drop, and heat delivered in any hydronic loop.

Average water temperature

AWT = (supply temperature + return temperature) / 2. Emitter output tables are based on AWT, so lower supply temperatures on condensing systems reduce radiator and baseboard output.

Pickup allowance

Extra capacity added to the calculated heat loss to recover from setback and warm a cold building. Common residential values are 1.10 to 1.25 times the design load.

Design outdoor temperature

The winter dry-bulb temperature exceeded for 99% of annual hours at the site. Equipment and radiation are sized to this value, not to record low temperatures.

Combination gas valve

A single body containing two safety shutoff operators in series, a pressure regulator, and often a servo. Both operators must open and prove before gas reaches the burner.

Flame rectification

The control applies AC to the flame rod; the flame conducts current one direction only, producing a DC microamp signal that proves flame presence to the ignition control.

Hot-surface igniter

A silicon nitride or silicon carbide element that glows to ignite the burner. Its resistance changes with temperature, and the control times ignition rather than measuring igniter heat.

Condensing heat exchanger

A secondary stainless or aluminum exchanger that cools flue gas below the water dew point, recovering latent heat. Condensate must drain to a neutralizer or suitable receptor.

Stoichiometric air

The exact air volume needed for complete combustion of a fuel, about 10 cubic feet of air per cubic foot of natural gas. Real appliances add excess air above this ratio.

Excess air

Air supplied beyond stoichiometric requirement, usually 15-50% for atmospheric burners and 5-15% for power burners. Measured as flue O₂, typically 4-8%.

Category IV appliance

A condensing, positive-pressure appliance that vents with listed plastic pipe and must be sealed. It cannot share a common chimney with natural-draft appliances.

Combustion air openings

For a confined space using outdoor air, two openings are required: one near the ceiling and one near the floor, each sized 1 square inch per 1,000 Btu/h input.

Sediment trap

A tee with a capped drip leg installed at the appliance inlet so dirt and pipe scale collect below the gas valve instead of fouling the internal seats and orifices.

Manifold pressure

Gas pressure measured at the burner manifold while firing. Typical natural gas value is 3.5 inches water column; LP is usually 10 to 11 inches water column.

Leak test pressure

For new gas piping, test at not less than 3 psi or 1.5 times working pressure, whichever is greater, for at least 10 minutes, with the appliance isolated.

LP versus natural gas input

LP gas carries about 2,500 Btu per cubic foot versus about 1,030 for natural gas, so LP orifices are smaller and manifold pressures higher for the same rated input.

Low-water cutoff

A safety control that opens the burner circuit when boiler water falls below a safe level. It must be tested annually and mounted above the lowest safe water line.

High-limit aquastat

A safety temperature switch set above normal operating temperature, typically 180 to 200 degrees Fahrenheit. It opens the burner circuit to prevent overheating or steaming.

Trial for ignition

The timed period after the gas valve opens during which the control looks for a flame signal. No signal within that period causes lockout and valve closure.

Zone valve end switch

A switch inside the zone valve that closes when the valve opens fully, completing the circuit to start the boiler and circulator on a call for heat.

Closely spaced tees

Two tees piped no more than four pipe diameters apart that hydraulically separate the boiler loop from the distribution loop, allowing each to run at its own flow rate.

Point of no pressure change

The location where the expansion tank connects to the system, usually the pump suction. Pressure there stays constant, so the tank must connect at that point to work correctly.

Pump curve

A graph of a circulator's flow versus head. The actual operating point is where the pump curve crosses the system curve, which determines real gpm delivered.

Air separator

A device at the hottest, lowest-pressure point of the system that releases entrained air. It is paired with the expansion tank connection to keep air out of loops.

Combustion efficiency

Calculated from flue oxygen or carbon dioxide and stack temperature. Condensing boilers in condensing mode reach above 90% with stack temperatures near 100 to 130 degrees Fahrenheit.

Delta-T verification

Measured supply minus return temperature at design flow. Low delta-T with high flow suggests excess flow or a fouled exchanger; high delta-T suggests restricted flow.

High-fire and low-fire readings

Combustion measurements taken at both maximum and minimum modulation. A modulating boiler must stay within carbon monoxide and oxygen limits across the whole firing range.

Startup documentation

A record of inlet and manifold gas pressures, combustion readings, and safety-device test results left with the equipment so future service can compare against baseline values.

Flame signal microamps

The DC current a flame rod returns to the control, typically several microamps. Low or unstable readings point to a dirty rod, poor ground, or wrong flame position.

Rollout switch trip

A safety switch that opens when flame escapes the burner area. Causes include blocked flue, downdraft, or dirty burners; correct the cause before resetting.

Waterlogged expansion tank

A tank whose bladder has failed and filled with water, eliminating the air cushion. System pressure rises quickly when hot and relief valve may weep.

Short cycling

Rapid on-off burner operation from oversizing, low water volume, high limit set too low, trapped air, or zone flow mismatch. Check cycle time and boiler delta-T.