BPI Heating flashcards

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

Sensible heat formula

Btu/h equals 1.08 times CFM times the temperature difference for standard air; it converts measured airflow and temperature change into delivered heating capacity.

AFUE

Annual Fuel Utilization Efficiency: a seasonal efficiency rating that includes steady-state combustion losses plus cycling, standby, and off-cycle losses, so it is always lower than measured combustion efficiency.

Stack effect

Buoyancy of warm indoor air creates upward pressure in a building; the neutral pressure plane is where indoor and outdoor pressures match and shifts with leakage and exhaust operation.

Thermal bridge

A high-conductance path such as framing or a steel stud that bypasses insulation and lowers whole-assembly R-value; it appears as cold stripes on a thermal image.

Vent category I

A venting classification for appliances operating with negative vent pressure and non-condensing flue gases, typically natural-draft furnaces and boilers connected to a vertical chimney.

Design heat load

The calculated Btu/h needed to hold indoor design temperature at the outdoor design temperature, summing envelope transmission plus infiltration and ventilation loads for the whole house.

Stoichiometric air

The exact air quantity required for complete combustion with no oxygen left over; natural gas needs roughly ten cubic feet of air per cubic foot of gas.

Air-free CO

Measured CO multiplied by 20.9 and divided by (20.9 minus O2); this removes the effect of dilution so flue readings from different appliances can be compared to a single limit.

Draft hood spillage

Flue gases escaping the draft hood relief opening into the room rather than up the vent; spillage indicates depressurization, blockage, or an undersized vent and must be corrected.

Combustion air opening

A permanent opening supplying burner air; the rule is one square inch of free area per 1,000 Btu/h input, using high and low openings for confined spaces.

CAZ depressurization limit

BPI caps worst-case depressurization of a combustion appliance zone at about five pascals; more negative pressure risks backdrafting natural-draft appliances and requires mitigation.

ASHRAE 62.2 ventilation rate

Whole-house mechanical ventilation formula: 0.01 times conditioned floor area plus 7.5 times the number of bedrooms plus one, expressed in cubic feet per minute.

Manifold pressure, natural gas

The pressure delivered to the burners of a natural gas appliance, typically 3.5 inches water column; verify at the valve outlet with a manometer while the unit runs.

Vent connector slope

A single-wall or listed connector must rise at least one quarter inch per foot of run toward the chimney so flue gases travel upward and condensate does not pool.

Sediment trap

A drip leg installed at the appliance gas inlet that catches dirt, pipe scale, and moisture before they can reach the gas valve or burner orifices.

Condensate trap, Category IV

A 90%-plus furnace produces acidic condensate that drains through a trapped line pitched back to the unit; the trap also keeps flue gas from escaping into the airstream.

Temperature rise range

Nameplate specification giving the acceptable supply-minus-return air temperature difference; a reading outside the range signals wrong airflow, wrong gas pressure, or a fouled heat exchanger.

Return air prohibition

Return air cannot be drawn from bathrooms, kitchens, garages, or rooms with combustion appliances because it spreads moisture, odors, and carbon monoxide through the house.

Excess air calculation

Percent excess air approximates O2 divided by (20.9 minus O2), times 100 for natural gas; values near 25 to 50 percent suit atmospheric burners.

Temperature rise measurement

Supply temperature minus return temperature, read after 10 to 15 minutes of steady operation with probes placed out of direct radiant view of the heat exchanger.

Airflow from temperature rise

CFM equals output Btu/h divided by 1.08 times the temperature rise; use nameplate output and the measured rise to check delivered airflow.

Total external static pressure

Supply static plus return static, with signs treated as absolute values; compare against the blower fan curve to see whether the duct system is restricting airflow.

ACH50

Air changes per hour at 50 pascals, computed as CFM50 times 60 divided by conditioned volume; a common blower-door metric of envelope air tightness.

Duct leakage, CFM25

Duct blaster airflow needed to hold the duct system at 25 pascals; compare with system airflow, since leakage above about 10 percent deserves sealing.

Inch of water column

A pressure unit used for gas and air measurement; one inch of water column equals about 249 pascals, and 0.004 inches equals roughly one pascal.

Steady-state combustion efficiency

Efficiency calculated from flue oxygen, stack temperature, and ambient temperature; it ignores cycling and jacket losses and therefore overstates the seasonal AFUE rating.

Heat anticipator setting

A thermostat adjustment matched to the current draw of the gas valve, typically 0.2 to 0.8 amps; set too high it shortens cycles, set too low it causes overshoot.

Flame rectification current

The microamp signal a flame sensor returns to the control module, normally 1 to 10 microamps; below roughly half a microamp the gas valve closes.

Fan limit switch

A temperature-actuated control that starts the blower near 130°F, stops it near 100°F, and opens a high limit around 200°F to protect the heat exchanger.

Manual D friction rate

Available static pressure divided by the total effective length of the longest duct run; the resulting friction rate in inches of water per 100 feet sizes each duct section.

Commissioning documentation

A written record of nameplate data, combustion readings, temperature rise, static pressures, and defects found, dated and signed, then reviewed with the homeowner at completion.

Sequence of operation

A call for heat energizes the inducer, a pressure switch proves draft, ignition lights the burners, flame is proven, and the blower starts when the fan switch reaches temperature.