Photograph of  a modern oil-fired heating boilerHeating Boiler Operation Details
39 Steps in the sequence of operation of hydronic heating systems
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How does a heating boiler work - what are the steps in its operating sequence? This article describes how a hot water heating system (hydronic heat) actually works, step by step, to heat a building. An understading of the sequence of steps in the operation of a heating system, from the moment that a thermostat calls for heat until the moment that the thermostat stops calling for heat can help us diagnos and fix many heating system problems.

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How Heating Boilers Work & are Diagnosed: Heating Boiler Inspection by Sequence of Operation

Training in proper operation sequence of heating system equipment and in the function of its controls is a step towards technical correctness. If you do not understand how a mechanical system works you cannot reliably expect to observe missing or defective components. This discussion is an exercise using sequence of operation to work for completeness. It is not technically exhaustive, it focuses on a specific example: oil-fired hot water, zoned, heating system.

Examine the accessible parts of the heating system. Let your eye travel from component to component in the sequence of operation. Apply the inspection logic discussed earlier, at each step. Consider the implications should each component be missing, damaged, inoperative, leaky, noisy, sooty, repaired by an amateur, etc.

Think through the operating sequence as you examine each component in that order. The following are the steps in one common set-up. This list is lengthy and detailed. The actual visual examination may take only a few minutes.

How a Heating System Works - 39 Steps in the Operation of a Heating System

Photograph of  a modern oil-fired heating boilerWhat follows is a detailed, step by step description of how a heating boiler works. We name each heating system component and what it does, in the order that heating system components operate during the heating cycle.

Items shown in [brackets] are ones which may not be present on some heating systems. We include links to technical articles that explain the operation of various heating system components and parts.

The following steps in a heating boiler operating sequence are discussed as part of a complete heating system inspection procedure for hydronic or hot water heat beginning at HEATING BOILER INSPECTION GUIDE. For steam heating systems, details are at STEAM HEATING SYSTEMS. Similar information is provided for warm air heating systems at FURNACE OPERATION DETAILS.

  1. Room temperature drops, a condition sensed by one or more thermostats installed to control building temperature. - see HEAT LOSS in BUILDINGS
  2. Room thermostat senses the temperature drop and switches on - the thermostat is basically an "on-off" switch that calls for heat. ASHI 9.1.A.2 normal operating controls - see THERMOSTATS
  3. [If Zone valves are installed, each is controlled by an individual thermostat; the zone valve opens and] [If no zone valves are installed each thermostat controls one (or more) hot water circulators. In response to the thermostat (or zone valve end-switch) the hot water circulator [starts] [except in Canada where circulators may be set up to run continuously and where the thermostat directly turns on the oil burner] [Circulator usually located on cooler return-side of the distribution piping loop-longer life.] - ZONE VALVES, HEATING
  4. Boiler temperature and pressure are indicated on the Temperature/Pressure gauge and should show an increase in the boiler water, not to exceed normal operating limits (200 deg F or less and less than 30 psi) - Gauges on Heating Equipment
  5. Hot heating water leaves boiler, passing by the ...
  6. Air scoop, air separator, air purger (not always present; this component removes air in the hot water heat piping to stop hot water heating system noise and to avoid air-bound heating baseboards or radiators) - AIR SCOOPS SEPARATORS PURGERS
  7. Air vent or purge valve (Mounted on top of the air scoop, a brass fitting with a Schrader valve which permits air to escape. This component is not always present;these are often leaky or sealed off). Air purge valves, manual or automatic, may also be located at other spots on piping, baseboards, or hot water radiators themselves. -s and - AIR BLEEDER VALVES
  8. [Automatic] water feeder (normally the manual valve for water supply to boiler is "on", the automatic valve is closed unless the boiler pressure drops below 12-15 psi. This valve is often also a backflow-preventer.) - WATER FEEDER VALVES, HYDRONIC BOILER
  9. Expansion tank (if waterlogged this tank will cause hot water dumping at the relief valve)(is there an old expansion tank in the attic? if so the boiler system may have no relief valve and may rely on this attic tank and an overflow pipe which itself may flow outside or to a building drain. Modern systems which have a smaller expansion tank right at the boiler will also include a relief valve on (best) or close to the heating boiler itself and won't rely on a remote attic expansion tank. - EXPANSION TANKS
  10. [Zone valve] (not always used, shorter life if installed on the on the "hot" supply side of <->piping) - ZONE VALVES, HEATING
  11. Hot water from the boiler enters the heating distribution piping (watch for mineral salts indicating small clogged leaks)
  12. Hot water then passes through heating baseboards or radiators or wall convectors - which warm the room air by air convection and by heat radiation, and thus the ... ASHI 9.1.A.6 heat distribution systems including fans, pumps, ducts and piping with supports, dampers, insulation, air filters, registers, radiators, fan-coil units, convectors - RADIATORS and AIR BLEEDER VALVES
  13. Room thermostat senses the heat increase as hot water from the boiler passes through and radiates heat from heating devices nearby - THERMOSTATS
  14. Hot water continues passing through more distribution piping as it returns to heating boiler, flowing past
  15. a Zone service drain (drain valves installed on each heating zone piping, usually at or close to the boiler) and [flow balancing valves usually installed right at the zone drains] (are they leaky?) -
  16. [Circulator pump] (if it's not a convection system or "gravity hot water heating system" used on older houses) - Circulator Pumps & Relays
  17. The cooler hot water finally passes back into the boiler itself.
  18. Temperature in the boiler drops as cooler water returns and lowers temperature therein.
  19. Temperature sensor inserted into the boiler water and connected to the heating boiler primary control switch feels the temp drop and tells the ...
  20. Primary control or high-limit control senses that the temperature is falling inside the boiler, but nothing happens (in the U.S.) until ...
  21. Temperature drops about 15 deg F below the HI setting on the heating boiler Primary Control (such as a Honeywell R8182D boiler control). Then the
  22. Primary control turns on the oil burner (watch for drip/leak damage onto the control from above as that will damage this costly component) (Canadian systems: thermostat may activate burner directly.) - AQUASTAT CONTROL Functions or Stack Relay Switch
  23. the [oil fired] Oil Burner pumps oil from the tank through ... (did we see the tank? is there an oil filter?) [gas fired heating boilers open a gas valve and feed this fuel from an incoming LP or natural gas line] See OIL BURNER INSPECTION & REPAIR or GAS BURNER FLAME & NOISE DEFECTS
  24. [Copper] fuel line (and possibly sends excess back through a return line) - OIL TANK PIPING & PIPING DEFECTS or GAS PIPING, VALVES, CONTROLS depending on type of heating system fuel used.
  25. [Past one or more Fire-o-Matic safety valves] (fusible link used in some jurisdictions) through the pump unit, and [in good practice] through an oil filter canister, sending ...
  26. High pressure (100 to 120 psi) heating oil to burner nozzle for spray atomization into fire chamber (are the combustion chamber and chamber liner ok?)
  27. The oil burner Ignition Transformer (on oil burner systems) makes high voltage which is sent as a spark to ignite oil (tar ooze at transformer means the unit is failing, maybe from backpressure and overheating) (or on gas fired equipment the pilot light (or electrical igniter) permits a gas valve to open and a gas burner ignites) and ...
  28. Oil burner'sair intake blower (or gas burner air intake or power burner blower) unit sends combustion air into the fire chamber... (is there adequate combustion air? how about when the boiler room door is closed?)
  29. Oil (or LP or natural gas) begins to burn (rough noisy or smoky start or stumbling noisy poor shut-down of the burner, smoke, soot, odor, noise mean improper system operation) Safety controls assure successful combustion and shut down the system if the burner is not operating - CAD CELL RELAY SWITCH or Stack Relay Switch
  30. Hot gases from burning heating oil (or LP or natural gas) fuel pass through tubes (in steel boilers) or between sections (in cast iron boilers), heating that metal, thus sending heat back into the heating water through the heat exchanger. (Soot acts as insulation, slows heat transfer, increases temperatures in the flue, and increases heating costs -- was the boiler cleaned recently?)
  31. Hot combustion gases are collected at top of boiler and sent out through exhaust flue (metal pipe connecting the boiler to the chimney) ...
  32. Where the barometric damper (a draft regulator located on the flue pipe usually just above or close to the boiler - DRAFT REGULATORS - barometric dampers) - (or a draft hood on gas-fired heating equipment - SPILL SWITCH, FLUE GAS DETECTOR) assures proper and even draft; hot gases continue ...
  33. Up the chimney where combustion gases are vented safely outdoors. - ASHI 9.1.A.4 chimneys, flues, and vents (is the chimney improperly shared or vented to multiple floors?) ASHI 9.1.A.5 solid fuel heating devices [e.g. wood and coal stoves] - CHIMNEY INSPECTION DIAGNOSIS REPAIR
  34. Boiler temperature rises up to the "HI" limit as the burner continues to operate. (Thinking of High, this is a good time to take a look for a pressure relief valve and look for defects there: leaking, corroded, not piped to floor, reduced diameter piping.) - RELIEF VALVE, TP VALVE, BOILER
  35. A boiler water temperature sensor mounted inside the boiler water monitors temperature there and informs Primary Control when the "HI" limit is reached, causing the control to turn off the burner-(is there a noisy, rough, stumbling sloppy burner shut down? if so service is needed) (Circulator is continuing to run all during the time that the wall thermostat calls for heat) - AQUASTAT CONTROL Functions or LIMIT CONTROL, SINGLE
  36. The room is warm enough according to the thermostat so the ...
  37. Thermostat senses the temperature rise and opens its switch. (Special thermostat sophistication and functions excluded here) - THERMOSTATS
  38. Circulator pump stops (except in parts of Canada or other areas where for comfort and temperature evenness circulators are left running continuously) - Circulator Pumps & Relays
  39. The Oil Burner (or gas burner) will either stop then or continue until HI limit (AQUASTAT CONTROL Functions) is reached. - OIL BURNERS and OIL BURNER NOISE SMOKE ODORS. If a Low Water Cutoff Control is installed the boiler will be shut down if water drops to an unsafe level.

Comparing Operation of Heating Boilers in Canada and the U.S. - Continuous versus Intermittent Hot Water Circulation

On a typical oil fired heating boiler in the U.S. the wall thermostat is controlling the hot water circulator, turning it on or off. It is the temperature of the water circulating through the system (sensed at the primary control on the heating boiler) that actually turns the oil burner on or off to re-heat the water. That's why the wall thermostat is not an "accelerator" and that's why if the thermostat has been set to 60 °F., and the room temperature is at 60 °F., and we want to warm up to 68 deg.F., we just need to set the wall thermostat up to 68 deg.F. Setting the thermostat higher than that will not warm the room faster.

On a typical oil fired heating boiler in Canada where temperatures are cooler for more of the year, the circulator pump may be wired to run continuously all during the heating season, whenever power is turned on at the boiler. On these systems, the wall thermostat turns the oil burner on or off directly in response to room temperature.

This design tends to produce more even temperatures in the home, and it has an advantage which should be considered by anyone who owns an older home where drafts or poor insulation mean that there is a high risk of freezing heating pipes (freezing can occur in a heating distribution pipe, baseboard, or radiator when heat temperatures are set low and some corner or elbow or location of piping is exposed to very low temperatures.

If heating pipes freeze the result is loss of heat even if the boiler and circulator try to turn on, which in turn means there is risk of burst piping, water damage, mold contamination, or other costly problems. By forcing the water in the heating system to circulate continuously the risk of this freeze-up is greatly reduced.

This heating boiler operation article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.

The articles at this website also describe the components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs. We include product safety recall and other heating system hazards. The limitations of visual inspection of heating systems are described. We continue to add to and update this text as new details are provided. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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