Boilers & Burners

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Glass is bouncing and the low water tripped twice, blow down the column and check the TDS before you reset it.

  TranslationThe water level in the gauge glass is surging and the low-water cutoff has shut the burner down twice, so clear the level connections and test dissolved solids before restarting.

• Hard light-off on number two, it's lighting above low fire, the low-fire switch on the mod motor is out of adjustment.

  TranslationBoiler two ignites with a bang because it lights at more than its minimum firing rate, since the switch on the modulating motor that proves low-fire position is set wrong.

• Pressuretrol's at 8 on a one-pipe job and the vents are spitting, knock it down to a pound and a half.

  TranslationThe steam pressure control is set far too high for a one-pipe steam system, causing water to spray from radiator vents, so lower the cut-out to 1.5 psi.

• Stack's up 80 since the tune-up and it's running rich, punch the tubes and dial it back in.

  TranslationFlue gas temperature has risen sharply and combustion is short of air, so clean the soot from the tubes and re-tune the burner with an analyzer.

• Pop-off's weeping every cycle, the ex tank is waterlogged, recharge it to 12 pounds.

  TranslationThe relief valve drips each time the boiler fires because the expansion tank has lost its air cushion, so restore its precharge to 12 psi.

• It's short cycling on the high limit, the mod-con's starved for flow, pipe it primary-secondary with its own pump.

  TranslationThe condensing boiler keeps shutting off on its temperature limit because too little water moves through its heat exchanger, so give it a dedicated pump on a circuit hydraulically separated from the system piping.

• Sulfite's zero and the DA's not venting, that's how we got oxygen pitting at the feed inlet.

  TranslationThere is no oxygen scavenger residual and the deaerator is not releasing gases, which explains the oxygen corrosion pits near where feedwater enters.

The main kinds of boilers, their key pressure parts, and the units used to rate them.

• Boiler horsepoweralso BHP, horse
  A boiler capacity unit equal to 33,475 Btu per hour, or evaporating 34.5 pounds of water per hour from and at 212 degrees F.
• Cast iron sectional boileralso sectional boiler, cast iron boiler
  A boiler built from cast iron sections joined in a row, used widely for residential and commercial heating.
• Condensing boileralso mod-con, condensing
  A boiler designed to cool flue gas below its dew point so water vapor condenses and gives up its latent heat.
• Drum
  A large cylindrical vessel in a watertube boiler where steam separates from water or where sediment collects.
• Firetube boileralso fire tube, shell boiler
  A boiler in which hot combustion gases run through tubes that are surrounded by the water being heated.
• Furnacealso firebox, furnace tube
  The part of a boiler where the fuel actually burns, such as the big flame tube of a firetube or the water-walled box of a watertube.
• Gauge glassalso water gauge glass, water glass
  The glass tube or window on a steam boiler that shows the operator the actual water level.
• Heating boileralso low-pressure boiler, Section IV boiler
  A low-pressure boiler limited to 15 psi steam, or to 160 psi and 250 degrees F for hot water.
• MAWPalso maximum allowable working pressure
  Maximum allowable working pressure: the highest pressure a boiler is approved to operate at, stamped on its nameplate.
• MBHalso MBtuh, MBtu/h
  Thousands of Btu per hour: the everyday unit for rating heating boilers, burners and heating loads.
• Passalso gas pass
  One run of combustion gases from end to end through a boiler before they turn or leave.
• Scotch marine boileralso Scotch, Scotch boiler
  A horizontal packaged firetube boiler with a large internal furnace tube and two to four gas passes.
• Try cocksalso gauge cocks, trycocks
  Small valves at set heights on a water column, opened by hand to check whether water or steam sits at each level.
• Tube sheetalso tubesheet, tube plate
  The thick flat plate at each end of a firetube boiler shell that the tube ends are rolled or welded into.
• Water columnalso column, water column assembly
  A vertical chamber piped to a steam boiler's steam and water spaces that carries the gauge glass and level controls.
• Watertube boileralso water tube, WT boiler
  A boiler in which water and steam circulate inside tubes while the fire and hot gases pass around the outside.
• Wetbackalso wet back
  Describes a Scotch marine boiler whose rear gas turnaround chamber is surrounded by water instead of lined with refractory.

Operating and limit controls, low-water cutoffs, flame safeguards, relief valves and the shutdowns they cause.

• Aquastatalso boiler aquastat
  Shop name for the immersion temperature control that fires and limits a hot water boiler. Shop talk
• BMSalso burner management, burner management system
  Burner management system: the safety controls that handle purge, ignition, flame proving and trips on larger boilers.
• Differentialalso diff, swing
  The gap between the point where a control turns the burner on and the point where it turns it off.
• Flame safeguardalso FSG, programmer
  The control that runs a burner's start sequence, watches the flame, and shuts the fuel off fast if the flame fails.
• Flame scanneralso scanner, flame detector
  The sensor that proves to the burner control that a flame is actually present.
• High limitalso high limit control, limit control
  A safety control that shuts down the burner if pressure or temperature rises above the normal operating range.
• Lockoutalso safety lockout, locked out
  A safety shutdown that holds the burner off until someone presses a manual reset.
• Low-water cutoffalso LWCO, low-water fuel cutoff
  A safety control that shuts the burner off when the boiler water level drops below a safe point.
• Nuisance tripalso nuisance lockout, false trip
  A safety shutdown or lockout caused by a faulty or marginal device, not by a real unsafe condition. Shop talk
• Operating controlalso operating limit
  The automatic-reset control that cycles or modulates the burner to keep steam pressure or water temperature in range.
• Outdoor resetalso ODR, reset
  A control strategy that lowers hot water supply temperature as outdoor temperature rises, following a set curve.
• Pigtailalso siphon, siphon loop
  A looped siphon pipe between a steam boiler and its pressure gauge or control, filled with condensate to block live steam. Shop talk
• Pop-offalso popoff, pop valve
  Shop slang for the safety valve on a steam boiler or the relief valve on a hot water boiler. Shop talk
• Pressuretrolalso pressure control, steam pressure control
  Shop name for the pressure switch that starts and stops a steam boiler's burner to hold pressure in a set range. Shop talk
• Purgealso purge cycle
  Blowing air through the furnace and flue before ignition or after shutdown to clear out unburned fuel.
• Relief valvealso RV, pressure relief valve
  The pressure-relief valve on a hot water boiler, rated in Btu per hour and set at or below the boiler's maximum pressure.
• Safety valvealso safety, steam safety
  The spring-loaded valve on a steam boiler that pops fully open to vent steam before pressure exceeds the boiler's rating.
• Trial for ignitionalso TFI, ignition trial
  The short, fixed time window during which the burner control must see a pilot or main flame before it locks out.

Burners, pilots, firing rate control and the sights and sounds of lighting and tuning a flame.

• Bumpalso bump it, jog
  To energize a motor, pump or burner for a second or two to check rotation or operation. Shop talk
• Burneralso burner assembly
  The assembly that mixes fuel with air and holds a stable flame to heat the boiler.
• Dial inalso tune, set up the burner
  To fine-tune a burner with a combustion analyzer until oxygen, CO and stack temperature land where they belong. Shop talk
• Dirt legalso drip leg, sediment trap
  A capped vertical stub of pipe below the gas inlet tee that traps scale, dirt and liquid before they reach the gas valve. Shop talk
• Flameoutalso flame out, flame failure
  A burner flame going out unexpectedly while the boiler is running. Shop talk
• Gas trainalso fuel train, gas valve train
  The string of valves, regulator and switches that controls and safeguards gas on its way to the burner.
• Jackshaftalso single-point positioning, jackshaft linkage
  The shaft and linkage system that lets one motor move a burner's fuel valve and air damper together.
• Lazy flame
  A long, soft, yellowish flame that drifts and licks the furnace walls instead of burning crisp and compact. Shop talk
• Light-offalso lightoff, light off
  The moment a burner ignites, and also the verb for lighting it. Shop talk
• Low-fire startalso low fire start, low-fire light-off
  The rule that a burner must light off at its minimum firing position, proven by a switch, before increasing rate.
• Mod motoralso modulating motor, modutrol
  The modulating motor that drives a burner's air damper and fuel valve through their range on command. Shop talk
• Modulationalso mod, modulating
  Varying a burner's firing rate continuously between low fire and high fire to match the load.
• Nozzlealso oil nozzle, atomizing nozzle
  The precision tip on an oil burner that atomizes oil into a fine cone-shaped spray, rated in gallons per hour.
• Pilotalso pilot light, pilot flame
  A small flame that lights the main burner and is often used to prove ignition before main fuel flows.
• Power burneralso forced draft burner
  A burner with its own fan that forces combustion air into the furnace, rather than relying on natural draft.
• Puffbackalso puff back
  A small explosion of oil vapor in the combustion chamber that blows soot and smoke out of the burner and into the room. Shop talk
• Rumblealso combustion rumble
  A low, throbbing combustion noise that sets the boiler and breeching vibrating, usually at one firing rate. Shop talk
• Turndownalso turndown ratio
  The ratio of a burner's highest to lowest firing rate, such as 4 to 1 or 10 to 1.

Excess air, flue gas, combustion testing, draft and venting, and what goes wrong when the mix is off.

• Air-free COalso CO air-free, COAF
  Carbon monoxide in flue gas corrected to zero excess oxygen, so dilution with air cannot hide a CO problem.
• Barometric damperalso barometric draft regulator, baro
  A weighted swinging gate on the flue pipe that lets room air in to hold chimney draft steady.
• Breechingalso flue connector, smoke pipe
  The duct that carries flue gas from a boiler's outlet to the chimney or stack.
• Combustion analysisalso combustion test, flue gas analysis
  Testing a burner's flue gas for oxygen, CO, CO2 and temperature to set it up safely and efficiently.
• Draftalso draught
  The pressure difference that pulls or pushes combustion gases through the boiler and out the chimney.
• Excess airalso EA
  Combustion air supplied beyond the exact amount needed to burn all the fuel, expressed as a percentage.
• Flue gasalso products of combustion, POC
  The hot exhaust that leaves the boiler after combustion, made up mostly of nitrogen, carbon dioxide, water vapor and leftover oxygen.
• O2 trimalso oxygen trim, trim control
  A control that measures flue gas oxygen and continuously adjusts the air-fuel ratio to hold it at target.
• Running richalso rich, fuel rich
  Burning with too much fuel for the air available, so combustion is incomplete. Shop talk
• Smoke spotalso smoke spot test, smoke test
  An oil burner test that pulls flue gas through filter paper and grades the dark spot from 0 to 9.
• Sooted upalso sooted
  Coated with soot on the fire side, from rich or poor combustion, so heat transfer and draft suffer. Shop talk
• Spillagealso spill, flue gas spillage
  Flue gas escaping into the room from a draft hood or damper instead of going up the chimney.
• Stack temperaturealso stack temp, flue gas temperature
  The temperature of the flue gas as it leaves the boiler, a direct measure of how much heat is going up the chimney.

Feedwater, condensate, traps, vents and the piping practice of steam and hot water heating.

• Air separatoralso air eliminator
  A hydronic fitting that pulls air bubbles out of circulating water and sends them out through an automatic vent.
• Bleedalso bleed the radiators, vent
  To let trapped air out of a hot water radiator, baseboard or high point by opening its small vent until water appears. Shop talk
• Condensate returnalso returns, condensate return system
  The piping and equipment that bring condensed steam back to the boiler room for reuse as feedwater.
• Dead menalso the Dead Men
  Steam heating slang for the long-gone engineers and fitters who designed and piped the steam systems that remain in service. Shop talk
• Deaeratoralso DA, DA tank
  A pressurized feedwater heater that strips dissolved oxygen and carbon dioxide out of boiler feedwater with steam.
• Delta Talso ΔT, temperature drop
  The temperature difference between supply and return water in a hydronic system. Shop talk
• EDRalso equivalent direct radiation, square feet of radiation
  Equivalent direct radiation: the square-foot rating of heating radiators, used to size steam boilers.
• Expansion tankalso ex tank, X-tank
  A closed tank with an air cushion that absorbs the expansion of heating water as it warms up.
• Feedwateralso boiler feed, feed
  The water pumped into a steam boiler to replace the water boiled off, a blend of condensate and treated makeup.
• Hartford loopalso Hartford connection, underwriters loop
  A residential steam boiler piping arrangement that stops a leaking return from draining the boiler below a safe level.
• Main ventalso main air vent, steam main vent
  A large-capacity air vent at the end of a steam main that lets air out fast so steam can fill the main.
• Makeup wateralso make-up water, makeup
  Fresh water added to a boiler system to replace water lost to leaks, blowdown, venting and unreturned condensate.
• Point of no pressure changealso PONPC
  The place in a closed hydronic loop, where the expansion tank connects, whose pressure the pump cannot change.
• Primary-secondaryalso P/S, primary/secondary
  Describes hydronic piping where each circuit has its own pump and connects to a main loop through two closely spaced tees.
• Pumping awayalso pump away, pump away from the tank
  Placing a hydronic circulator just downstream of the expansion tank so it pushes water away from it. Shop talk
• Radalso radiator
  Short for radiator, especially the cast iron column or tube radiators of steam and hot water systems. Shop talk
• Spittingalso spitting vent, spitting radiator
  Describes a steam radiator vent that throws out water and steam instead of just air. Shop talk
• Steam trapalso trap
  An automatic valve that drains condensate and air out of steam lines and equipment while holding steam back.
• Wet return
  A steam system return line run below the boiler water line, so it always stays full of water.

Blowdown, scale, corrosion, carryover and the chemistry that keeps boiler water in check.

• Blowdownalso blow down, BD
  Draining some boiler water on purpose to remove concentrated dissolved solids and settled sludge.
• Bouncing glassalso bouncing water line
  A gauge glass water level jumping up and down instead of holding steady, a warning sign of surging or dirty water. Shop talk
• Carryoveralso carry-over, entrainment
  Boiler water and the solids dissolved in it leaving with the steam instead of staying in the boiler.
• Cycles of concentrationalso cycles, COC
  How many times more concentrated dissolved solids are in the boiler water than in the feedwater.
• Foamingalso foam, frothing
  A stable layer of bubbles on the boiler water surface that makes level hard to read and carries water out with the steam.
• Lay-upalso layup
  Protecting an idle boiler from internal corrosion during an outage or the off season.
• Oxygen pittingalso O2 pitting, oxygen corrosion
  Deep localized corrosion pits in boiler steel caused by dissolved oxygen in the water.
• Oxygen scavengeralso scavenger, O2 scavenger
  A chemical fed to boiler feedwater that reacts with and removes the last traces of dissolved oxygen.
• Priming
  Sudden surges that throw slugs of boiler water out with the steam.
• Scalealso boiler scale, lime scale
  A hard mineral crust that forms on boiler heat transfer surfaces from calcium, magnesium and silica in the water.
• TDSalso total dissolved solids
  Total dissolved solids: the concentration of dissolved minerals and salts in boiler water, held in range by blowdown.
• Water softeneralso softener, zeolite softener
  An ion exchange unit that removes calcium and magnesium hardness from makeup water by swapping them for sodium.

Failure modes, noises and the routine work that keeps a boiler alive.

• Bagalso bulge
  A local bulge in a boiler tube, furnace or shell plate where overheated metal has been pushed out by pressure.
• Dry firealso dry firing
  To run the burner on a boiler that has lost its water, overheating and often destroying the heating surfaces.
• Firemanalso boiler tender, stationary fireman
  The traditional title for a boiler operator who tends fires, watches water and keeps steam up. Shop talk
• Firesidealso fire side, gas side
  The surfaces of a boiler exposed to flame and combustion gases, as opposed to the waterside surfaces in contact with water.
• Kettlingalso kettle noise
  A rumbling, crackling or whistling noise from a hot water boiler, like a kettle about to boil. Shop talk
• Punch the tubesalso brush the tubes, rod the tubes
  To clean soot from firetube boiler tubes by running a brush on a long rod through each one. Shop talk
• Short cyclingalso short-cycling, rapid cycling
  A burner starting and stopping far too often, with short run times that waste fuel and wear parts.
• Sweatingalso sweat, fireside condensation
  Flue gas moisture condensing on the cold fireside of a boiler during startup and dripping into the combustion chamber. Shop talk
• Thermal shockalso temperature shock
  Damaging stress from a sudden temperature change in boiler metal, such as cold water hitting hot iron.

Frequently asked questions

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