Virginia Residential HVAC

K-value is the heat passing through one square foot of material ONE INCH thick in one hour per 1°F temperature 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.

The subcooling method is generally used to check charge on systems with thermostatic expansion valves (TXVs), while the superheat method is used on systems with fixed orifice metering devices such as capillary tubes.

An indoor reset control matches boiler water temperature to the load by monitoring cycle time (raising temperature on long cycles, lowering it on short cycling), while an outdoor reset control responds to the difference between indoor and outdoor temperatures.

In a wet-base boiler the water surrounds the combustion area for maximum heat transfer surface, while in a dry-base boiler the water is contained above the burner and the area under the combustion chamber is dry.

A shell-and-coil condenser's coiled water tubing cannot be accessed for mechanical cleaning and must be cleaned with chemicals, while a shell-and-tube condenser has removable end caps allowing mechanical cleaning of the straight water tubes.

Cast-iron boilers used in residential and light-commercial work usually hold 15–30 gallons of water and take longer to heat but maintain heat for extended periods, while low-mass copper-tube boilers hold only a few gallons.

Per the IECC, maximum duct leakage is 3 cfm per 100 ft² of conditioned space when tested before the air handler is installed, versus a maximum of 4 cfm per 100 ft² when tested with the air handler installed.

Duct air velocities above 700 fpm can cause excessive noise in the ductwork, while air velocities greater than 500 fpm at a supply register may produce drafts — two separate concerns.

A double-pole thermostat opens both legs of the 240 V circuit and is considered safest, while a single-pole thermostat opens only one leg, is generally less safe, and is used less frequently for baseboard heating.

A radial system runs individual branch ducts directly from the central plenum to each supply outlet with no trunk line, while a perimeter loop system is used with downflow furnaces in homes without basements, with ductwork laid in the concrete slab and branch ducts feeding a perimeter duct.

Using the 97.5% value instead of the 99% value results in an 11% decrease in recommended system size, appropriate only for buildings with large thermal capacity and small total window area; when in doubt choose the outdoor design temperature on the low side.

Yellow tips indicate insufficient primary air and incomplete combustion (unburned carbon particles), whereas orange outer edges are caused by dust particles and are not a sign of lack of primary air, and too much primary air causes a noisy flame that lifts above the burner.