NATE Hydronics Oil cheat sheet

Sensible heat formula: BTUH = 500 × GPM × ΔT for water (500 = 8.33 lb/gal × 60 min/hr × 1.0 Btu/lb·°F); use 485 for 30% glycol and 450–460 for 50% glycol.

Water expands roughly 4% from 40°F to 212°F; system fill pressure must exceed the highest point's static head by 4 psi minimum at the top of the loop.

Static pressure rise: 0.433 psi per foot of water column; a 20 ft rise above the fill point adds 8.66 psi.

Design ΔT commonly 20°F for baseboard, 10–20°F for cast iron radiators, 30–40°F for high-output fin tube or snow melt; lower ΔT raises flow and pump head.

Heat loss by degree-day method: BTUH = U × A × ΔT; whole-house sizing commonly uses 1.4 × design heat loss for boiler pickup allowance.

Series loop: full flow through every emitter, lowest cost, highest temperature drop at last unit; one-pipe: emitters in parallel with diverter tees; two-pipe direct return: shortest path; two-pipe reverse return: balanced by equal pipe length.

Air is the most common hydronic complaint; air separators, automatic air vents at high points, and proper fill pressure prevent air binding and pump cavitation.

BTU content of No. 2 fuel oil ≈ 138,000–140,000 Btu/gal; 1 gallon of oil ≈ 1.4 therms of natural gas for rough comparison.

Theoretical (stoichiometric) air for No. 2 oil ≈ 14.4 lb air/lb oil; typical practice supplies 12–15% excess air, giving 5–8% CO2 or roughly 9–11% O2 in flue gas.

Smoke number 0–1 with a No. 2 oil flame is the target; smoke above trace indicates incomplete combustion, poor atomization, or low draft.

Net stack temperature = flue temperature − combustion air temperature; steady-state efficiency ≈ 100 − (net stack temp ÷ 15) − (smoke number × 2) for No. 2 oil (rough field rule).

Nozzle ratings are in GPH at 100 psi; flow varies with the square root of pressure, so a 1.00 GPH nozzle at 140 psi flows about 1.18 GPH.

Electrode gap typically 1/8 in (0.125 in) with tips 1/16–1/8 in ahead of the nozzle face; incorrect gap or position causes delayed ignition and puffback.

Draft: natural draft oil boilers commonly need −0.02 to −0.04 in. w.c. overfire; power burners often −0.02 in. w.c. or per manufacturer.

Components: fuel pump with cutoff solenoid, nozzle, electrodes, transformer (10,000–15,000 V), cad cell flame detector, primary control, blower wheel, and air shutter/bands.

Cad cell resistance: below ~400–600 ohms = flame present; above ~1,600 ohms = no flame; primary controls lock out on sustained high resistance.

Aquastat settings: high limit typically 180–200°F, low limit (for tankless coils) 140–160°F with 10–20°F differential; circulator hold-off prevents cold-start condensation in oil-fired cast iron.

Oil primary control sequence: call for heat → blower and ignition → fuel valve opens → flame proven by cad cell → run; failure to prove flame within 15–45 s causes lockout.

Residential oil primary controls provide 15-second or 30-second lockout timing; manual reset is required after lockout on most modern primaries.

Circulator control options: aquastat switching, relay with end switch on zone valves, or outdoor reset; zone valve end switches must be rated for circulator load.

Low-water cutoff is required on steam boilers and on hot water boilers above 400,000 BTUH input or per code; probe-type and float-type are common.

Combustion air: oil appliances need roughly 50 cu ft per 1,000 BTUH input if the space is unconfined; confined spaces require two permanent openings sized per NFPA 31.

Flue gas CO2 and O2 are inverse: 0% O2 ≈ 15.4% CO2 theoretical for No. 2 oil; 6% CO2 corresponds to roughly 7% O2.

Limit circuit wiring: high limit and low-water cutoff are series safety devices; never bypass a safety in a control circuit.

Copper baseboard output ≈ 550–600 BTUH per linear foot at 180°F average water temperature with 4 GPM; output drops roughly 1% per 1°F drop in average water temperature.

Pump head formula: head (ft) = friction loss (ft per 100 ft) × total equivalent length ÷ 100; add 2–4 ft for each fitting, valve, or boiler.

Flow rate formula: GPM = BTUH ÷ (500 × ΔT); a 60,000 BTUH zone with 20°F ΔT needs 6 GPM.

Primary/secondary piping decouples boiler flow from system flow; primary loop flow must exceed secondary loop flow to prevent temperature blending at the tees.

Zone valve sizing: size to the zone's design GPM, not pipe size; a 3/4 in zone valve typically handles 3.5–8 GPM depending on CV.

Expansion tank: acceptance volume must cover system water volume × expansion factor; diaphragm tanks are precharged to fill pressure, bladder tanks are not.

Air separation: use a spirovent or air scoop at the boiler supply where water is hottest and pressure lowest; install with at least 12–18 in of straight pipe upstream.

Pump curves: operating point is where the pump curve intersects the system curve; throttling a valve moves the operating point up the curve, reducing flow and raising head.

Combustion test order: draft first, then smoke, then CO2/O2, then stack temperature; adjust the air shutter for 0–1 smoke and target CO2, then recheck draft.

No heat with burner running: check circulator, air lock, zone valve, aquastat setting, and low-water cutoff before assuming burner fault.

Puffback on startup is usually delayed ignition from a worn electrode, wrong gap, dirty nozzle, or too much oil before ignition; inspect nozzle and electrodes first.

Short cycling causes: oversized boiler, aquastat differential too narrow, circulator hold-off, low water flow, or a faulty thermostat anticipator.

Oil pump pressure: 100 psi standard for residential; 140 psi raises flow ~18%; a vacuum gauge reading over 10 in. Hg indicates a restricted oil line or clogged filter.

Annual service items: replace nozzle and oil filter, clean heat exchanger and flue, check electrodes, test cad cell, verify draft and combustion, inspect expansion tank charge and relief valve.

Relief valve discharge must terminate 6–12 in. above the floor or to a safe drain; never cap, plug, or reduce the relief valve outlet.

Flue gas CO above 100 ppm air-free indicates a combustion problem; check for blocked flue, soot, or improper draft before leaving the job.

Before startup: verify boiler is level, relief valve piped, fill valve open, expansion tank precharged, circulators free, and all air purged from the system.

Purge air by opening the fill valve and a drain at the highest point until steady water flows; then bring system to 12–15 psi cold for a typical two-story residence.

Commissioning combustion: confirm draft, nozzle size and angle match the firing rate, set air for trace smoke, and record CO2, O2, stack temperature, and efficiency on a combustion report.

Oil line installation: use only approved oil lines, a filter at the tank, and a two-pipe system if the tank is below the burner or lift exceeds 8 ft with a single line.

Leak diagnosis on a hot water system: isolate zones with valves, watch the fill valve and pressure gauge; a continuous pressure rise indicates a domestic coil or tankless coil leak.

Thermostat and zone wiring: R to W for heat, R to G for fan; on a zone panel, end switches from zone valves must be wired in parallel to the circulator or boiler T-T.

Winter freeze protection: glycol systems need a higher fill pressure and a larger expansion tank because glycol expands more and has lower specific heat.

Final documentation: leave combustion test results, nozzle size, pump pressure, aquastat settings, and a service tag on the boiler for the next technician.

Before startup: verify boiler is level, relief valve piped, fill valve open, expansion tank precharged, circulators free, and all air purged from the system.

Purge air by opening the fill valve and a drain at the highest point until steady water flows; then bring system to 12–15 psi cold for a typical two-story residence.

Commissioning combustion: confirm draft, nozzle size and angle match the firing rate, set air for trace smoke, and record CO2, O2, stack temperature, and efficiency on a combustion report.

Oil line installation: use only approved oil lines, a filter at the tank, and a two-pipe system if the tank is below the burner or lift exceeds 8 ft with a single line.

Leak diagnosis on a hot water system: isolate zones with valves, watch the fill valve and pressure gauge; a continuous pressure rise indicates a domestic coil or tankless coil leak.

Thermostat and zone wiring: R to W for heat, R to G for fan; on a zone panel, end switches from zone valves must be wired in parallel to the circulator or boiler T-T.

Winter freeze protection: glycol systems need a higher fill pressure and a larger expansion tank because glycol expands more and has lower specific heat.

Final documentation: leave combustion test results, nozzle size, pump pressure, aquastat settings, and a service tag on the boiler for the next technician.