Oklahoma HVACR Contractor Unlimited

A pictorial diagram shows all connections plus the approximate physical location of devices in the unit (and often wire colors), while a ladder diagram shows the order of component operation as horizontal rungs (individual circuits with loads and controls) between two vertical power lines, with devices near the top energized first.

K-value is heat passing through one square foot of material one inch thick in one hour per 1°F difference (Btu·in/hr·ft²·°F), while C-value is based on the material's actual measured thickness rather than a standardized 1-inch thickness.

BRI is a specifically diagnosable illness from airborne agents whose symptoms do NOT disappear when the occupant leaves, whereas SBS has no specific cause or identifiable illness and its symptoms disappear when occupants leave the building.

The subcooling method is used to check charge on systems with thermostatic expansion valves (low subcooling suggests undercharge, high suggests overcharge), while the superheat method is used on fixed-orifice systems such as capillary tubes.

An in-line ammeter must be wired in series with the load (in dc, black lead to negative and red lead to positive before energizing), while a clamp-on ammeter clamps around only a single conductor — clamping both hot and neutral makes the opposing magnetic fields cancel and the meter reads zero.

Turning the range screw clockwise raises both cut-in and cut-out an equal amount (differential unchanged), while the differential screw turned clockwise raises only the cut-out pressure and does not affect cut-in.

In an HSI system the glow coil only ignites and cannot sense flame; a separate flame rod proves flame by rectification (a dirty/oxidized rod makes the module close the gas valve seconds after ignition), and thermocouples are used in standing-pilot systems, not HSI.

Three-phase supply is already phase-split and its voltages peak at different times, so torque is produced without start windings or capacitors and power is essentially constant (never an instant when all voltages are zero), unlike single-phase, which needs phase-splitting components.

A current-limiting fuse uses temperature-sensitive resistors that open on overcurrent and close again when cooled (keeping current below rated locked rotor amperage), while a multipurpose fuse combines fast-acting and time-delay traits — riding through brief small overloads such as motor starting but blowing immediately above about 500% of rated current.

The code covers natural gas piping systems operating at 125 psig or less, but LP-gas piping systems only at 20 psig (140 kPa gauge) or less, except as provided in Section 402.7.

Per the IECC, maximum leakage is 3 cfm per 100 ft² of conditioned space when tested before the air handler is installed, versus 4 cfm per 100 ft² when tested with the air handler installed (ducts pressurized to +25 Pa WRT the conditioned space).

Per IMC 2024 §603.2, Manual D (or the manufacturer's instructions or other approved methods) is an accepted duct sizing method within a single dwelling unit, while Manual J is for load calculations and Manual S for equipment selection; other buildings use the ASHRAE Handbook of Fundamentals.