BPI Heating cheat sheet

Heat transfers by conduction, convection, and radiation; in buildings, infiltration and exfiltration add a mass-transfer load that conduction-only calculations miss.

Sensible heat: Btu/h = 1.08 × CFM × ΔT for standard air at sea level. Latent heat: Btu/h = 4.5 × CFM × Δgrains.

1 Btu raises 1 lb of water 1°F; 1 therm = 100,000 Btu; 1 kWh = 3,413 Btu.

Approximate higher heating values: natural gas 1,030 Btu/ft³, propane vapor 2,500 Btu/ft³, No. 2 fuel oil 140,000 Btu/gal.

AFUE is a seasonal rating that includes cycling, standby, and off-cycle losses, so it is always lower than the steady-state combustion efficiency measured in the flue.

R-value is thermal resistance and U-value = 1/R; assembly U-values are summed conductances, and thermal bridging through framing lowers whole-assembly R-value.

Stack effect drives warm air up and out; the neutral pressure plane sits where indoor and outdoor pressures are equal and shifts with envelope leakage and exhaust operation.

Vent categories: I = negative pressure, non-condensing; II = negative pressure, condensing; III = positive pressure, non-condensing; IV = positive pressure, condensing.

Design heat load = envelope transmission + infiltration + ventilation at the outdoor design temperature; oversizing increases cycling losses and temperature swing.

Complete combustion of natural gas needs roughly 10 ft³ of air per ft³ of gas (stoichiometric); atmospheric burners typically run 25–50% excess air.

Air-free CO = measured CO × 20.9 ÷ (20.9 − O2); it normalizes dilution so readings from different appliances can be compared against a single limit.

Commonly used action levels: ambient CO 9 ppm (8-hour), undiluted flue CO air-free 400 ppm; any sustained draft-hood spillage is a fail regardless of CO reading.

Natural-draft Category I appliances normally show −0.02 to −0.04 in. w.c. draft in the connector; zero, positive, or fluctuating draft indicates spillage, blockage, or depressurization.

Confined-space combustion air: 50 ft³ of volume per 1,000 Btu/h input, or two permanent openings each sized 1 in² free area per 1,000 Btu/h, one near the top and one near the bottom.

BPI's combustion safety protocol uses a worst-case CAZ depressurization limit of about −5 Pa; more negative pressure puts natural-draft appliances at risk of backdrafting.

Backdrafting drivers: bath and kitchen exhaust fans, clothes dryers, return-duct leakage in unconditioned space, closed doors starving returns, and tight envelopes without makeup air.

ASHRAE 62.2 whole-house ventilation: Qfan (cfm) = 0.01 × conditioned floor area + 7.5 × (bedrooms + 1).

Indoor relative humidity of 30–50% limits condensation and mold; unvented combustion appliances add water vapor, CO, and NOx and are unsuitable where ventilation is limited.

Nameplate data set the verification targets: input and output Btu/h, AFUE, certified temperature rise range, manifold pressure, vent category, and clearances.

Natural gas manifold pressure is typically 3.5 in. w.c. and propane 10–11 in. w.c.; appliance inlet pressure is nominally 7 in. w.c. for natural gas and 11–13 in. w.c. for propane.

Vent connectors rise at least 1/4 in. per foot toward the chimney, are as short and direct as practical, and are listed for the appliance vent category.

Category IV (90%+) appliances vent in PVC or CPVC, pitch back to the unit, and drain acidic condensate through a trap that may require a neutralizer.

Gas piping at the appliance needs a sediment trap (drip leg), a union, and a shutoff valve within reach; leak-test with bubble solution or a manometer, never with a flame.

Return air may not be drawn from bathrooms, kitchens, garages, or rooms containing combustion appliances; return ducts in unconditioned space must be sealed airtight.

Heat exchanger inspection combines a visual check of accessible areas with combustion analysis before and after cleaning; shifting O2 or CO readings after cleaning point to cracks or blockage.

Electrical review: dedicated circuit, correct overcurrent protection, bonded ground, and a service switch within sight of the unit.

Maintain label-listed clearances to combustibles on all sides and above the appliance; single-wall vent connectors need substantially more clearance than listed Type B vent.

Run combustion analysis at steady state (10–15 minutes): record O2, CO, CO2, stack temperature, and draft, then compute air-free CO and excess air.

Percent excess air ≈ O2 ÷ (20.9 − O2) × 100; a natural gas appliance reading 5% O2 is running about 31% excess air.

Temperature rise = supply air temperature − return air temperature, measured after steady operation with probes outside direct radiant view of the heat exchanger.

CFM = output Btu/h ÷ (1.08 × ΔT); a 60,000 Btu/h output furnace at a 40°F rise moves about 1,390 CFM.

Total external static pressure = supply static + return static (absolute values summed) read with a manometer, then compared with the blower fan curve; 0.5 in. w.c. is a typical design value.

Blower door: ACH50 = CFM50 × 60 ÷ conditioned volume; about 3 ACH50 is a common tightness target for new construction.

Duct leakage is measured at 25 Pa (CFM25) with a duct blaster; leakage above roughly 10% of system airflow justifies sealing, especially when ducts run outside the envelope.

Pressure unit conversions: 1 in. w.c. = 249 Pa (≈250 Pa); 1 Pa ≈ 0.004 in. w.c.

Zonal pressure diagnostics compare house-to-zone and zone-to-outside pressures to rank leakage paths and predict the depressurization a CAZ will see.

Manual D: available static pressure = blower rating minus component losses; friction rate = available static ÷ total effective length of the longest run.

Hydronic loop essentials: expansion tank, air elimination, fill/pressure-reducing valve, circulator, zone valves, and balancing valves.

Fan limit switch: blower on near 120–140°F, off near 90–100°F, high limit near 200°F; repeated limit trips point to restricted airflow or an oversized unit.

Sequence of operation: call for heat → inducer proves pressure switch → ignition → flame proven (µA or mV) → main valve stays open → blower starts at fan-on temperature.

Set the heat anticipator to the current rating printed on the gas valve (typically 0.2–0.8 A); a mismatch causes short cycling or temperature overshoot.

Flame rectification current normally runs 1–10 µA; below roughly 0.5 µA the control module shuts the gas valve.

A high total external static pressure reading indicates undersized ducts, a dirty filter or coil, or closed registers; verify against the fan table before blaming the equipment.

Documentation package: nameplate data, combustion and draft readings, temperature rise, static pressures, defects and derating, technician signature, and homeowner education.