NATE Hydronics Gas Service cheat sheet

Sensible heat formula: Q (Btu/h) = 500 × gpm × ΔT for water; the constant 500 derives from 8.33 lb/gal × 60 min/h × 1.0 Btu/lb·°F.

Design ΔT for a standard residential boiler loop is typically 20°F (180°F supply / 160°F return); high-mass and condensing systems often run 10-30°F ΔT.

Heat loss by Manual J-style methods: transmission through envelope (U × A × ΔT) plus infiltration (1.08 × cfm × ΔT) plus duct losses; equipment sized to design load, not connected radiation.

Boiler output must cover design heat loss plus pickup allowance; typical residential pickup factor is 1.10-1.25 of calculated load.

Radiator/baseboard output ratings are published at 180°F average water temperature and 65-70°F room air; output falls roughly 2-3% per °F of reduced average water temperature.

Average water temperature (AWT) = (supply + return) / 2; use AWT, not supply temperature, when matching emitters to a condensing boiler's lower design temperatures.

Water expands about 4% from 40°F to 200°F; system volume in gallons ≈ Btu/h output ÷ 10,000 × 1.5 for rough sizing of expansion and buffer needs.

Design day conditions come from ASHRAE 99% winter design dry-bulb for the location; using the coldest recorded temperature oversizes equipment and shortens condensing boiler cycling.

Atmospheric (natural-draft) burners draw combustion air by draft; power burners use a blower and are typically fan-assisted or forced draft.

Gas train order from supply: manual shutoff valve, union, sediment trap, appliance regulator, manifold with main and redundant valves, then burner orifices.

Combination gas valve contains two safety shutoff operators in series plus a pressure regulator and servo; loss of either operator or the proving circuit prevents firing.

Hot-surface igniter (silicon nitride or carbide) draws high current at first; resistance rises as it heats, and the control proves ignition by flame rectification, not by igniter temperature.

Flame sensor is a rectification rod or flame rod; it must be in the flame envelope and clean, since a contaminated rod gives low microamp signal and nuisance lockout.

Condensing boilers add a secondary stainless or cast-aluminum heat exchanger; flue gas is cooled below dew point (~130°F for natural gas), recovering latent heat.

Primary/secondary piping with a hydraulic separator or closely spaced tees protects boiler flow and allows different flow rates between boiler loop and distribution loop.

Low-water cutoff, high-limit aquastat, and rollout/blocked-vent switches are safety devices wired in series with the gas valve circuit; any open switch stops firing.

Stoichiometric air for natural gas is about 10 ft³ air per ft³ gas (roughly 9.5:1 by volume); typical excess air is 15-50% for atmospheric and 5-15% for power burners.

Target flue CO₂ is about 8.5-9.5% for natural gas at reasonable excess air; O₂ of 4-8% in flue gas indicates acceptable excess air for most boilers.

Carbon monoxide above 100 ppm in undiluted flue gas, or rising CO with rising excess air, signals incomplete combustion or flame impingement.

Draft: Category I appliances vent into negative-pressure chimneys; Category III and IV are positive-pressure and require sealed, listed venting (Category IV condensing uses PVC/CPVC/PP).

Combustion air: two permanent openings, one within 12 in of the ceiling and one within 12 in of the floor; each opening sized 1 in² per 1,000 Btu/h for vertical ducts, 1 in² per 2,000 Btu/h for horizontal ducts to outdoors.

Single opening to outdoors sized 1 in² per 3,000 Btu/h; indoor air from a confined space requires two openings each 1 in² per 1,000 Btu/h of total input.

Vent connector slope is at least 1/4 in per foot rise toward the chimney; total vent connector length and elbow count must follow the appliance listing and code tables.

Test for spillage and draft at the draft hood after startup: a flame or smoke test should be drawn into the hood, and a draft gauge should read negative in the connector.

Natural gas heating value is about 1,030 Btu/ft³ (varies by utility); LP gas is about 2,500 Btu/ft³, so LP orifices are smaller for the same input.

Pipe sizing uses longest-length and total Btu/h tables (NFPA 54); add appliance inputs plus any future load, and account for each fitting as equivalent length.

Sediment trap (drip leg) is required at the appliance inlet, installed as a tee with a capped nipple below the gas valve so debris does not reach the controls.

Inlet pressure: natural gas manifold typically 3.5 in w.c.; supply pressure usually 6-7 in w.c. LP manifold is 10-11 in w.c. with supply around 11-13 in w.c.

Leak test: isolate the appliance and pressurize piping to at least 1.5 times working pressure but not less than 3 psi for a 10-minute test on new piping; never exceed the appliance valve rating.

Leak detection on an operating system uses soap bubbles or an electronic detector; a manometer drop test on a closed section is the accepted method for a new installation.

Manifold pressure is measured with a manometer at the valve outlet tap with the burner firing; adjust the regulator screw to the nameplate value.

Gas piping must be bonded to the grounding electrode system and never used as an electrical ground path; CSST requires listed bonding per manufacturer instructions.

Safety chain in series: high-limit aquastat, low-water cutoff, rollout switch, blocked-vent pressure switch, and air-proving switch all must be closed to permit firing.

Low-water cutoff must be installed above the lowest safe water line and tested annually; a float or probe type breaks the circuit when water drops below the safe level.

High-limit aquastat typically set 180-200°F; it is a manual-reset or auto-reset switch separate from the operating aquastat.

Thermostat wiring: R to W calls for heat, R to G runs the fan, R to Y calls cooling; a boiler uses R-W plus a separate circulator circuit (T-T or end switch).

Zone valves and circulators are wired through zone thermostats; end switches on zone valves start the boiler and pump on a call for heat.

24 V control circuits use a step-down transformer; a shorted or overloaded circuit blows the low-voltage fuse, so check transformer output and fuse before replacing the control.

Flame safeguard control has a trial-for-ignition period (often 15-90 seconds) and a lockout period; repeated lockouts usually mean no flame signal, not a bad control.

Line-voltage and low-voltage wiring must be separated in the enclosure; use listed cable, proper strain relief, and connect the equipment ground to the boiler jacket.

Head loss in pipe and fittings is estimated from equivalent length and flow tables; size circulators by required gpm at the calculated head, not by pipe size alone.

Pump curve intersection with system curve gives actual operating point; a pump selected far from its curve midpoint may run out or deadhead.

Primary/secondary piping with closely spaced tees (max 4 pipe diameters apart) decouples boiler and distribution flow and prevents boiler short-cycling.

Air elimination: an air scoop or separator at the hottest, lowest-pressure point plus an expansion tank connection at the same point removes entrained air.

Expansion tank sizing uses acceptance volume and precharge; bladder tanks are precharged to system fill pressure, and the tank connects at the point of no pressure change (pump suction).

Series loops have the same flow through all emitters and add head loss; parallel (home-run) manifolds with balancing valves give independent flow to each zone.

Balancing valves on each branch set design flow; measure ΔT across the loop and adjust to achieve design temperature drop rather than guessing at valve position.

Pump wiring: PSC and ECM circulators have different speed control; ECM pumps need a constant-speed or delta-P setting matched to the system, and wet-rotor pumps must never run dry.

Purge gas lines before first firing and confirm no air remains; light-off should be smooth with no delayed ignition or flame rollout.

Measure and record: inlet gas pressure, manifold pressure, flue O₂ or CO₂, flue CO, stack temperature, and draft or vent pressure at high fire.

Combustion efficiency from flue O₂ and stack temperature; a condensing boiler in condensing mode shows stack temperatures near 100-130°F and efficiency above 90%.

Verify ΔT across the boiler at design flow; a low ΔT with high flow means excess flow or a fouled heat exchanger, a high ΔT means low flow.

Check safety devices during startup: test low-water cutoff, high limit, rollout switch, and blocked-vent switch to confirm each opens the gas valve circuit.

Confirm thermostat and zone operation: each zone should call, open its valve, start the circulator, and satisfy without short cycling.

Record combustion readings at low fire and high fire on modulating boilers; a modulating burner must stay within CO and O₂ limits across the entire range.

Document the startup with gas pressures, combustion numbers, and safety test results, and leave the readings with the equipment for future comparison.

No heat call: verify thermostat, then check for 24 V at the control, safety chain continuity, and gas valve operation in that order.

Short cycling causes: oversized boiler, low water volume, high limit set too low, poor air elimination, or a zone valve/circulator mismatch; check ΔT and cycle time.

Nuisance lockout with good flame: clean the flame rod, check ground continuity, verify flame signal microamps, and confirm the rod is in the flame, not just near it.

Condensate trap and neutralizer must be clean and draining; a blocked trap trips a pressure switch or floods the secondary exchanger and causes lockout.

Annual service: clean burner and heat exchanger, inspect vent connector and combustion air openings, test safety controls, and recheck combustion after cleaning.

Rollout switch trip indicates flame rollout from blocked flue, downdraft, or dirty burners; find and correct the cause before resetting, never just reset repeatedly.

Water-side problems: air noise, low pressure, and poor heat often trace to a waterlogged expansion tank, stuck air vent, or fill valve set too low.

Use a combustion analyzer and manometer on every service call; visual inspection alone will not reveal excess air, CO, or gas pressure faults.