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This article describes the operating and safety controls on a Heating System Boiler - central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs. The photo above shows a modern cad cell relay on an oil fired heating boiler - one of the safety controls which we discuss in this article.

Here we describe the key operating and safety controls on heating boilers used to provide hot water heat through baseboards, convectors, radiators, and radiant heat flooring or ceilings. Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles: see BOILER PARTS LIST for a detailed list of heating boiler controls, other heating system components, parts such as circulator pumps & draft regulators. If your building uses steam heat see STEAM HEATING SYSTEMS. If your building uses warm air heat see FURNACE CONTROLS & SWITCHES. See ELECTRICAL POWER SWITCH FOR HEAT.

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Heating System Controls Inspection, Peripherals, Key Components

How to Identify, Reset, or Adjust Hot Air Heat Furnace Controls and Switches

While going through the detailed sequence in the operation of the heating boiler, watch for and inspect the condition of the heating boiler controls and safety devices (as required by ASHI 9.1.A.3 automatic safety controls). Here we provide a list of most of the switches and controls found on heating systems. Each item is defined and we provide links to one or more detailed articles about the inspection, testing, setting, and use of each of these devices.

• Combination Control High Limit: On a typical combination control, the "Hi Limit" is the cut-off temperature for the heating boiler on a call for heat. The cut-on temperature is hard wired in this control at about 15 degF below the "Hi Limit" setting.
• Combination Control Low Limit: On these controls the "Low Limit" is NOT the "cut on" point for heat but rather it is a setting which is intended to maintain heat inside the boiler in order to assure that the boiler can produce hot water when a tankless coil is installed.
• The "Low Limit" is normally set at least 20 degF below the "Hi Limit" to avoid a "lockout" condition on this control which we discuss just below. During warm months when the boiler is not being called-on to heat the building itself, the "Low Limit" keeps heat in the boiler for the tankless coil. So the "Low Limit" is actually a "low range" operating upper limit on boiler temperature that applies out of the heating season.
• Combination Control Differential: On this control, the "Diff" or differential control dial specifies the amount below the "Low Limit" to which boiler temperature can fall before the boiler should turn on to keep the boiler warm for making domestic hot water through the tankless coil.
• Details about the control settings of boiler aquastats and how to set these devices to save money on heat, remain safe, and function properly, can be read at Aquastat Functions
• For a discussion about individual High Limit Switches and Low Limit Switches and controls on hot water heating boilers see High Limit Controls and Low Limit Controls

• Boiler Cad Cell Relays Explained Modern heating boilers use a Cadmium Cell sensor, usually located inside the oil burner tube, to "see" the presence of flame and thus to assure that the oil burner assembly stops pumping oil into the combustion chamber if flame ignition is unsuccessful.
• The cad cell is wired to a relay switch (usually a gray box with a red "reset" button located on top of the oil burner) which switches the oil burner off when a flame is not established in the burner.
• Resetting the Oil Burner Cad Cell Relay: if the red button on the cad cell relay is sticking up and the oil burner has shut down, the homeowner is permitted to try ONCE to "reset" the system by pressing the red reset button. If the oil burner does not turn on and run normally and continuously (no smoke, no loud noises, etc.) for at least 5 or 10 minutes after resetting the relay or pressing the reset button, DO NOT keep resetting the system since doing so can flood the combustion chamber with un-burned heating oil - a dangerous condition.
• Details about Cad Cell relay switches, how to inspect, test, and reset them can be read at Cad Cell Relays
  
  

Pressure and Temperature gauge on heating boilers: this gauge displays the heating boiler internal pressure and temperature. Typical pressure for a residential boiler serving a two story home would show 12 psi cold, and less than 30 psi hot. Over 30 psi boiler pressure will cause the pressure relief valve to open. Typical hydronic heating boiler operating temperature settings are LO-120-160 HI-180-200 degF. At 200 degF. we'll see should see pressure under 30 psi. Typical operating temperature observed at the gauge will be below the high, and can be as low as nighttime room temperature in non-heating season if no tankless coil is in use. The temperature/pressure gauge may help in checking for normal conditions before and during boiler operation. However the gauge can be wrong!) See Pressure Gauges, Boilers and also GAUGES ON HEATING EQUIPMENT for more details about pressure gauges on hydronic (hot water) heating boilers. Pressure gauges for steam boilers are discussed at Steam Pressure Gauge.

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BOILERS, HEATING
  BOILER CONTROLS & SWITCHES
  Air Bleeder Valves
  Aquastat Functions
  BOILER LEAKS CORROSION STAINS
  BOILER OPERATING PROBLEMS
  BOILER PARTS LIST
  Boiler Pressure Controls & Settings
  Cad Cell Relay Switch Flame Sensors
  Circulator Pumps & Relays
  Draft Regulators, Dampers
  Expansion Tanks
  Gauges on Heating Equipment
  Limit Switches, Boilers
  Low Water Cutoff Valves, Boilers
  Mixing Valves
  Relief Valves - TP Valves
  Reset Switch - Primary Control
  Reset Switch - electric motors
  Spill Switches
  Stack Relay Switch
  Zone Valves
  BOILER LEAKS CORROSION STAINS
  BOILER NOISE SMOKE ODORS
  BOILER OPERATING PROBLEMS
  BOILER OPERATION DETAILS
  BOILER PARTS LIST
ELECTRICAL POWER SWITCH FOR HEAT

How to Identify, Reset, or Adjust Hot Water Heating Boiler or Steam Boiler Controls and Switches

For details about the controls, components and switches commonly found on hot water heating systems see the articles listed below in which we explain how to identify, set, re-set, repair, replace, or avoid problems with the components of a hot water heating system.

• Hot Water Heating Boiler Operating Details - 39 Steps to Hot Water Heating Success
• Air Bleeder Valves for Hot Water Heating Systems: Radiators, Baseboards, Convectors
• Aquastats Heating Boiler Primary Controls: A guide to Aquastatus for control of hot water heating boilers and tankless coils
• Cad Cell Relays on oil fired furnaces or boilers as flame sensors & safety devices
• Check Valves Guide to check valves and backflow preventers on hydronic (hot water) heating systems (gas or oil fired)
• Circulator Pumps Guide to circulator pumps and circulator relays on hydronic (hot water) heating systems (gas or oil fired)
• Draft Hoods on gas fired heating equipment, function and safety
• Draft Regulators & barometric dampers on oil fired heating equipment
• Electrical Power Switches: Where to Turn On or Off Heaters for Furnaces & Boilers, Heat Pumps or Electric Furnaces or Air Conditioners
• Expansion Tanks Guide to pressure control and expansion tanks on hydronic (hot water) heating systems (gas or oil fired)
• Flue Gas Spill Switch on gas fired heating equipment sense combustion and protect from blocked flues
• Gas Piping, Valves, Controls: Guide to LP and natural gas meters, valves, tanks, piping, gas leaks, gas regulator assembly & other gas controls
• Gauges for Temperature & Pressure: Guide Temperature & Pressure Gauges on Hot Water or Steam Heating Boilers
• Low Water Cutoff Controls: Guide to LWCOs on hot water heating systems
• Mixing Valves: Guide to mixing valves on hot water heating systems and radiant heat systems
• Oil Burners: Guide to Oil Burners for heating systems, boilers & Furnaces: basic parts, operation, maintenance, performance & money-saving tips
• Oil Tanks: Guide to Heating Oil Underground & Above ground Oil Storage Tank Leaks, Testing, Problems & Solutions, Home Buyer's / Home Owner's Guide
• Oil Piping: A guide to heating oil piping, valves, controls, leaks, repairs for heating systems
• Parts & Components Guide for Heating Boilers: How to Identify the Components of Hot Water Heating Boilers - A Guide to Hydronic Heating System Parts
• Relief Valves - TP Valves: A guide to temperature and pressure relief valves & safety controls on hot water and steam heating systems
• Stack Relay Switches: Guide to finding, resetting, maintaining stack relays on oil fired furnaces or boilers as flame sensors & safety devices
• Tankless Coils: A Guide to tankless coils used to produce domestic hot water on heating boilers: hot water quantity, safety, temperature control, leaks, corrosion, repairs
• Thermostats & Heat Controls for furnaces & boilers, oil & gas fired, heat pumps or electric furnaces or boilers
• Water Feed Valves: A guide to water-feeding/pressure-reducing valves on hot water and steam heating systems
• Zone Valves: A guide to zone valves for heating zone control on hot water heating systems

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building design, inspection, and repair, and about indoor environment testing, diagnosis, and cleanup are at the InspectAPedia Bookstore.
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More Information on Building Diagnostic Inspections and Repairs

• Carbon Dioxide Gas Toxicity
• Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
• Dust from HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
• Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
• Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
• Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
• Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
• Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
• Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
• Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
• Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
• Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
• National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
• The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
• Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
• The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
• Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
• "Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
• Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
• Installation Guide for Residential Hydronic Heating Systems
• Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
• The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)

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